View Full Version : BTZS, Zone System, Everyone Else... Film Speed ?
Please correct me if I am wrong:
According to Phil Davis's book Beyond the Zone System (pp. 112-113 of the 3rd Edition), the 18% gray card (and incident meters which give us an 18% reading) are off by 1 stop. 18% is too light: it's not middle gray. He states that a normal subject has a brightness range of 7 stops, which is log 2.1. The middle of that is log 1.05, which corresponds to 9% reflectance.
Therefore, according to Davis, using the manufacturer's box film speed and metering a normal subject in direct light (not the shadows) with a either standard gray card or incident meter, we under-expose by 1 f-stop.
The Zone System gets around this problem by determining that the "real" film speed is 1/2 the box speed. The result is that we give 1 f-stop more exposure for normal subjects, IE the correct exposure.
BTZS gets around this by using the box speed (or something rather close to it, determined by careful testing) but metering in open shadows. Open shadows are (normally) 1 f-stop darker than (real) middle-gray, so by metering them, we give 1 f-stop more exposure for normal subjects, IE the correct exposure.
According to some videos that I have seen, portrait photographers customarily place an incident meter "under the chin" of the subject, IE in the open shadows of normal lighting, and use that reading along with the manufacturer's box speed to determine exposure. Although they aren't necessarily BTZS adherents, they appear to arrive at the same basic destination with regards to exposure.
Note: I'm not claiming that these three approaches are equivalent. I'm just asking if what I stated - as far as it goes - is basically true.
Ken,
Consider the fact that people don't sell books (and make money) by saying that existing standards are correct.
Film speed is dependent on many factors. You need to determine the correct speed for your film in your process.
I've shot box speed with incident metering for well over 50 years, and never noticed any exposure error.
If faced with a difficult/high contrast situation I'll spot meter and make appropriate adjustments.
Film manufacturers spend huge amounts of money on test equipment, and on the personnel and infrastructure
needed to operate it correctly. Conveying accurate information to the consumer is how they make their money.
That's not true of self-styled experts.
- Leigh
If that were true, wouldn't all current digital cameras with meters corresponding to middle grey also then be off by a stop? And who cares where the middle of the scale is mathematically, doesn't middle grey correspond simply to what we see as "middle grey?"
Jay DeFehr
4-Mar-2012, 20:41
If that were true, wouldn't all current digital cameras with meters corresponding to middle grey also then be off by a stop? And who cares where the middle of the scale is mathematically, doesn't middle grey correspond simply to what we see as "middle grey?"
Word.
Mark Barendt
4-Mar-2012, 20:53
http://www.apug.org/forums/forum37/99967-what-relationship-between-film-speed-camera-exposure.html
ic-racer
4-Mar-2012, 21:15
Calibration and use of an incident meter is an art to be mastered. As anyone that has one can tell you as you move the dome around the reading changes. What is the correct way? Meter such that your reading exposes the lowest area of interest in the negative on the toe, at least to the point where the slope is 0.3 times the slope of the straight portion. There are many paths to a well exposed negative. Pill has laid out one path to achieve that goal. I don't recall ever reading Phil making mention of any 'existing standards' as being incorrect, but would appreciate any references.
As anyone that has one can tell you as you move the dome around the reading changes.
If you're "moving it around", you don't know how to use it. You point the dome straight at the camera lens.
I don't recall ever reading Phil making mention of any 'existing standards' as being incorrect, but would appreciate any references.
The "existing standards" are the film speed printed on the box, the manufacturer's processing recommendations, and the exposure meter calibration standards.
- Leigh
Kimberly Anderson
4-Mar-2012, 21:32
FWIW, I have been rating all of my negative film at 1/2 the recommended ISO for years. If I remember correctly I started doing this after I took Zone way back when...
Bill Burk
4-Mar-2012, 22:03
I found my L-758-DR spotmeter seems to be calibrated to about 12% gray by first metering by incident, then switching to spot and metering the different patches on the "calibration chart" they sell with it. I wonder what would have happened if Ansel Adams got to try one of those.
One thought I toyed with is that the 18% gray card was intended to be a reference "known value". You are supposed to "know it is 18%" and treat it like that (open up slightly). Like when you meter your palm. Everybody knows you don't meter your palm and take that reading, you open up. Maybe we know to open up from a palm reading because it "never" works otherwise. But with "18%" the actual reading is close enough to fool you.
Or maybe the safety factor had something to do with it (Maybe the card was published when there was still a safety factor... and when you meter off a gray card you don't "need" the safety factor because you are being accurate).
As to how Zone System got around it, I am not sure. Ansel Adams wrote that he removed the "K-Factor" from his metering. But I think he thought his reflective spot meters were calibrated to 18% and I think what he did in effect was to cancel out the difference from 18% gray card to what the meter was actually calibrated to... 12% (or maybe 9%, whichever proves to be right).
As to how BTZS got around it.... BTZS uses an incident meter. An incident meter doesn't use a gray card. The white dome is the gray the meter is calibrated to (the "C" constant). There's no mystery about whether we are talking 9, 12 or 18% with an incident meter.
Kimberly Anderson
4-Mar-2012, 22:09
I have used the 1/2 speed rating when using a spot meter and an incident meter also. It has not made much difference honestly.
The problem with gray cards is that it's difficult to use them correctly.
They're supposed to be pointed so a line perpendicular to the card bisects the angle
formed by the lines from the subject to the light source and to the camera lens.
It's not always easy to determine exactly what that orientation should be.
The reading can vary significantly depending on card orientation.
The meter reading is taken along the line collinear with the lens.
- Leigh
Bill Burk
4-Mar-2012, 22:19
I have used the 1/2 speed rating when using a spot meter and an incident meter also. It has not made much difference honestly.
I think when you perform Zone System calibration, you often "find" a rated speed that is half box speed. But I don't think the plan was originally to use half the rated speed. Half-speed makes an effective shortcut because black and white negative film gives satisfying results when slightly overexposed.
Brian Ellis
4-Mar-2012, 22:23
There's nothing in the zone system that says the "real" film speed is 1/2 the box speed. The film speed is determined by testing. It might end up being 1/2 the box speed or it may not.
Kimberly Anderson
4-Mar-2012, 22:26
Exactly. I have found through testing several different types of films that for me the rule has been that I use 1/2 the box speed. It has also proven to be helpful when shooting color negative film too. Trust me, I didn't come by this idea blindly.
Heroique
5-Mar-2012, 00:15
I’ve calibrated and tested my equipment being used like I use them.
Ends up that box speed is best for me.
One-half box speed would have worked fine too in most of my b/w shots.
Mark Barendt
5-Mar-2012, 03:35
If you're "moving it around", you don't know how to use it. You point the dome straight at the camera lens.
Actually one of the classic ways to use an incident meter is called duplexing. It's described in Dunn & Wakefield's Exposure Manual.
Duplexing uses readings from different directions and averages them to find exposure settings.
Tobias Key
5-Mar-2012, 04:00
I've been using an incident meter for years, I've always given negative film an extra 2/3 stop and rated slide film at box speed. Never liked black and white at box speed.
Found this article about grey cards "Meters don't see 18% Grey"
http://www.bythom.com/graycards.htm
"I don't recall ever reading Phil making mention of any 'existing standards' as being incorrect, but would appreciate any references."
See Beyond the Zone System (pp. 112-113 of the 3rd Edition).
He provides 2 possible historical reasons why the 18% standard became the standard.
"The origin of the 18% reflectance gray card standard is unclear, but it appears likely that it was intended to be used with artificial lighting in the studio and that the original instructions suggested increasing the indicated exposure by 1/2 stop when using the card outdoors. Alternately, it may have originated in the graphic arts, where the luminance range of typical copy subjects is approximately 1.5 in log terms. For subjects of this range, the gray card is in fact, a middle gray because its 18% reflectance is approximately equal to a log value of 0.75.
The normal range of an ordinary photographic subject, however, is generally considered to be 7 stops, or 2.1 in log terms. The middle gray of this range is 1.05, which translates to a reflectance of about 9%. If the geometric center of the normal range is considered to be middle gray (which can be debated) the standard gray card is a full stop too light in value."
http://www.apug.org/forums/forum37/99967-what-relationship-between-film-speed-camera-exposure.html
Could you please summarize that thread ? I'll have to read it several times to discover the "takeaway" of what Stephen Benskin has explained. It's going over my head at the moment.
If that were true, wouldn't all current digital cameras with meters corresponding to middle grey also then be off by a stop? And who cares where the middle of the scale is mathematically, doesn't middle grey correspond simply to what we see as "middle grey?"
On the one hand we propose that engineers have already solved the problem, because cameras already work correctly - which is a tautology, isn't it ?
On the other hand, we're suggesting that engineering doesn't matter in the end. But doesn't that negate the value of the first proposition ?
There's nothing in the zone system that says the "real" film speed is 1/2 the box speed. The film speed is determined by testing. It might end up being 1/2 the box speed or it may not.
True enough. But isn't it a rather unusual "coincidence" that after testing, most people arrive at the same basic adjustment ?
I learned the Zone System from Fred Picker around 1970. Ansel had tested Tri-X + HC-110 and determined a film speed of 250. Fred did his own tests, and determined a speed of 250. I did my own tests and determined a speed of 250. It makes sense after all: we were all testing the same film and developer combination, doing the same basic agitation, with a decent thermometer, etc.
Mark Barendt
5-Mar-2012, 05:16
Could you please summarize that thread ? I'll have to read it several times to discover the "takeaway" of what Stephen Benskin has explained. It's going over my head at the moment.
Sure, but very, very roughly.
It does appear very true that 18% gray is not what things are calibrated to.
Closer to 12% but there isn't an absolute value, there is simply a standard range contained in an ISO standard.
(For examples of people adjusting to this simply read the directions included with a Kodak gray card and you get an offset that corrects the meter reading to get the exposure setting. Ansel also complained about the K factor used by meter manufacturers.)
The thread at the link goes through a lot of why and how the standard ended up where it is today. Changes from ASA to ISO, safety factors, placement in relation to speed point...
The big take away I got was that believing your meter at box speed and following the directions when developing is a very reliable way to get good negatives.
Another take away is that shooting at alternative speeds, is simply a personal placement choice. If one likes more shadow detail 1/2 box speed can get you that. This also avoids underexposure mistakes, it provides a bigger safety factor.
A third take away, for me, is that personal film testing to get a personal EI is exactly that, personal. We don't all shoot the same way. Mid-tones and highlights are more important to me than shadow, box speed gives me plenty of shadow. That's good for me but others have different priorities.
Mark Barendt
5-Mar-2012, 05:22
True enough. But isn't it a rather unusual "coincidence" that after testing, most people arrive at the same basic adjustment ?
I didn't.
I'd also suggest that most people don't test and can truly do fine work using the boring old published numbers from Ilford, Kodak, Fuji...
Mark Barendt
5-Mar-2012, 05:22
Dupe
Renato Tonelli
5-Mar-2012, 06:56
There's nothing in the zone system that says the "real" film speed is 1/2 the box speed. The film speed is determined by testing. It might end up being 1/2 the box speed or it may not.
I learned the Zone System from Fred Picker around 1970. Ansel had tested Tri-X + HC-110 and determined a film speed of 250. Fred did his own tests, and determined a speed of 250. I did my own tests and determined a speed of 250. It makes sense after all: we were all testing the same film and developer combination, doing the same basic agitation, with a decent thermometer, etc.
This is my experience exactly. After reading Fred picker's book (which finally de-mystifyed the Zone system for me), I tested Tri-X with HC110 (B solution) and came up with an EI of 250.
Recently, I tested with X-TOL 1:1 and the result was twice box speed: confounded, I tested again and got the same results; pictorial negs at the new speed look like the have been exposed correctly.
D. Bryant
5-Mar-2012, 07:42
Could you please summarize that thread ? I'll have to read it several times to discover the "takeaway" of what Stephen Benskin has explained. It's going over my head at the moment.
Just visit http://btzs.org/.
Phil lays it all out.
Using the BTZS software is also very helpful.
Just visit http://btzs.org/....Phil lays it all out.
In post number 18, I've already quoted verbatim from his book, listing the actual page number. In post number 1, I summarized his points. I have already laid out - twice - what he lays out.
What he has laid out, is what I'm asking about. :)
Brian Ellis
5-Mar-2012, 08:14
There's nothing in the zone system that says the "real" film speed is 1/2 the box speed. The film speed is determined by testing. It might end up being 1/2 the box speed or it may not.
True enough. But isn't it a rather unusual "coincidence" that after testing, most people arrive at the same basic adjustment ?
I learned the Zone System from Fred Picker around 1970. Ansel had tested Tri-X + HC-110 and determined a film speed of 250. Fred did his own tests, and determined a speed of 250. I did my own tests and determined a speed of 250. It makes sense after all: we were all testing the same film and developer combination, doing the same basic agitation, with a decent thermometer, etc.
I have no idea whether after adequate testing "most people" arrive at 1/2 the box speed or not. A lot of people use that without testing and it certainly would be a good speed to use for starters if someone doesn't want to bother with testing. And maybe "most people" end up there even after testing, I really don't know. But it wouldn't be surprising, after all how big a variation can there be from the box speed? The likely range of differences is pretty limited.
But in standard zone system methodology there's nothing to my knowledge that says to ignore testing for film speed and just use 1/2 the box speed. Which I thought was your original question.
Users of large format cameras have tended to rate their film at about 1/2 of box speed. The main reason is that there seems to be a preference among view camera users to get good detail and contrast at the very bottom of the toe in the open shadows, and the only way to do that is to increase exposure so that Zone II details falls on the straight line section of the curve. This practice was wide spread with Tri-X 320, which with most developers has a notoriously long toe.
Some of the modern T-grain films produce by their nature a much more linear curve and can be used a box speed and still give good detail in the shadows.
Assuming the same type of metering practice this issue is, strictly speaking, not a BTZS issue but one that relates to the characteristic curve type of films/developers.
Sandy
Mark Barendt
5-Mar-2012, 08:59
Just visit http://btzs.org/.
Phil lays it all out.
Using the BTZS software is also very helpful.
That is not the same info or a summary of what the thread I linked to offers.
Mark Barendt
5-Mar-2012, 09:03
Users of large format cameras have tended to double the rate their film at about 1/2 of box speed. The main reason is that there seems to be a preference among view camera users to get good detail and contrast at the very bottom of the toe in the open shadows, and the only way to do that is to increase exposure so that Zone II details falls on the straight line section of the curve. This practice was wide spread with Tri-X 320, which with most developers has a notoriously long toe.
Some of the modern T-grain films produce by their nature a much more linear curve and can be used a box speed and still give good detail in the shadows.
Assuming the same type of metering practice this issue is, strictly speaking, not a BTZS issue but one that relates to the characteristic curve type of films/developers.
Sandy
Yep, agree fully.
D. Bryant
5-Mar-2012, 09:18
That is not the same info or a summary of what the thread I linked to offers.
Quite!
D. Bryant
5-Mar-2012, 09:26
Just visit http://btzs.org/....Phil lays it all out.
In post number 18, I've already quoted verbatim from his book, listing the actual page number. In post number 1, I summarized his points. I have already laid out - twice - what he lays out.
What he has laid out, is what I'm asking about. :)
Ken my point is that based on your question the exposure scale of the print material needs to be accounted for and that's what Phil explains. I never half my film speed for TMAX films. I'm guessing you maybe using HP5. BTZS testing doesn't always lead to half film speed. Are you using BTZS software? Using that and having Phils database is very helpful.
If we are just simply going to scan negatives then you need to process your film to match the capability of the scanner you are using. Most modern consumer flat beds have limitations about how well they will scan highlight areas. So in a sense the scanner becomes your digital paper.
Scan a step tablet to discovery what your scanner really can't do.
Thank you all, for your patience and helpful answers. I have confused a number of ideas that are in fact only loosely coupled.
Sandy, thank you for dispelling my confusion for the Nth time.
Don, you're right: I have been using HP5+, and the BTZS test results which Fred Newman just performed for me (D-23 developer) gives an Effective Film Speed of around 270 for an SBR of 7.
I will scan a step tablet.
Peter Yeti
5-Mar-2012, 12:41
Inspired by this interesting thread, I wanted to know for sure and set up a test that is independent of subjective impression. Of course we all need to know effective film speed of our b/w material but that's certainly not a single number. It depends on the entire work flow. We all know that film speed is different for N or N-1 or N+1 development, it depends on the developer, on the enlarger head and so on. So I think that using b/w film or C-41 for testing meter calibration, zone V for middle grey, and reflectance of a grey card is a dead-end.
Here is my test set-up: Two Elinchrom flash heads with equal soft boxes set at 25 Ws each, outer edges 2m apart. Kodak 8x10'' grey card at 2.5m distance in the middle, resulting in extremly uniform illumination. Measurements were done with a Gossen Variosix F and repeated for 10 times each.
Results:
1.) Incident metering with flash
Repetition accuracy within +/- 1/10 f-stop
2.) Incident metering w/o flash, only modelling light at 100%
Repetition accuracy without measurable deviation
3.) Reflectance metering with flash
Repetition accuracy within +/- 1/10 f-stop. Depending on distance between meter and grey card there is a deviation of up to 2/10 when getting too close to the card because of the shadow of the meter.
4.) Reflectance metering w/o flash, only modelling light at 100%
Repetition accuracy without measurable deviation. Depending on distance between meter and grey card there is a deviation of up to 2/10 when getting too close to the card because of the shadow of the meter.
Comparison between 1.) and 3.) +/- 2/10 f-stop at most, neglegible on average
Comparison between 2.) and 4.) +/- 2/10 f-stop at most, neglegible on average
Well, if Kodak tells the truth about their grey card, then at least my Variosix F is definitely calibrated on 18% reflectance.
By the way, I shot thousands of chromes using this meter and incident measuring and got very reliable results. Since E-6 is standardized and chromes are quite unforgiving, this may give further confidence that the 18% calibration is used and reliable. And I never had problems to adjust my standard N development for various b/w films to obtain the specified speed.
Peter
Heroique
5-Mar-2012, 13:27
I hear it approaching like a thunder storm – a “precision vs. accuracy” discussion.
Precision versus accuracy indeed.
Well, if Kodak tells the truth about their grey card, then at least my Variosix F is definitely calibrated on 18% reflectance.
How do you know that it's not 17%, or 7 % for that matter ? All you know is that you got the same value from your light meter, over and over, no ? That just tells you that your meter is consistent, which is what we'd expect from a decent tool anyhow.
You could have metered any object with a matte finish, and unless your equipment were faulty, gotten consistent readings. It could be a sheet of white polystyrene packing material, for example.
Peter Yeti
5-Mar-2012, 14:01
If incident and reflective measurements give the same value, as they do in this experiment, then you found the reference. In this case a Kodak grey card for which Kodak gives a reflectance of 18%. Measurements are within about 10%. So, 17% reflectance would be in the error bounds, 7% wouldn't.
"If incident and reflective measurements give the same value, as they do in this experiment, then you found the reference. In this case a Kodak grey card for which Kodak gives a reflectance of 18%. Measurements are within about 10%. So, 17% reflectance would be in the error bounds, 7% wouldn't."
If your meters agree, that doesn't mean that they are both calibrated to the 18% standard, only that they are calibrated to the same standard as one another.
If your meters agree, it doesn't mean that 18% is the correct value to use for establishing film speed and other sensitometric concerns. It only means that your meters agree with one another.
The question entertained by this thread, is whether the 18% standard is an appropriate value.
Larry Gebhardt
5-Mar-2012, 14:45
"If incident and reflective measurements give the same value, as they do in this experiment, then you found the reference. In this case a Kodak grey card for which Kodak gives a reflectance of 18%. Measurements are within about 10%. So, 17% reflectance would be in the error bounds, 7% wouldn't."
If your meters agree, that doesn't mean that they are both calibrated to the 18% standard, only that they are calibrated to the same standard as one another.
If your meters agree, it doesn't mean that 18% is the correct value to use for establishing film speed and other sensitometric concerns. It only means that your meters agree with one another.
The question entertained by this thread, is whether the 18% standard is an appropriate value.
Ken, I think you are missing that Peter measured the grey card with reflectance and also using the incidence meter got the same results. We know the incidence meter is supposed to give a middle grey exposure, so for his meter it also means that the reflectance meter is calibrated to give middle grey based on an 18% card (assuming his card is 18% grey as Kodak says).
Peter Yeti
5-Mar-2012, 15:02
Well, not quite. Since the incident measurement can't know anything about the grey card used in the reflective measurement, it needs an absolute intrinsic reference value. If both measurements agree, it means that the reference for reflective and incident metering are the same. If the reflectance of the grey card is 18% then this is the common reference - at least for this meter. By the way, this is only one meter allowing for both types of measurements.
Now, whether the 18% standard is appropriate to putting it on zone V is another question. This is much more subjective in practice and has little to do with the densitometry. At least for me it works fine.
Ken, I think you are missing that Peter measured the grey card with reflectance and also using the incidence meter got the same results. We know the incidence meter is supposed to give a middle grey exposure, so for his meter it also means that the reflectance meter is calibrated to give middle grey based on an 18% card (assuming his card is 18% grey as Kodak says).
Thank you for the explanation. I see now.
From what I've read, not all meters are calibrated for 18%. His two are, and that's helpful.
That being said, the fact that something is customary, does not make it right, and the quotations I gave earlier expressed concern about the validity of the number.
As the original poster, I am grateful and satisfied with the answers I got to my original question. Because I am also fortunate enough to be a moderator here, I will take this opportunity to close the thread.
Ken Lee
22-Dec-2012, 17:37
I re-opened this thread because I finally found the quote I was looking for:
Beyond the Zone System, 4th Edition, "Metering for the Incident System" pp 134:
"Notice that these film speeds seem exaggerated; they are, in fact, just double the normal speeds. As explained earlier, this is done deliberately to compensate for the 1-stop overexposure that normally results when the camera settings are based on the low-light incident reading."
In other words, the recommended technique is to take an incident reading in the shadow area of the scene, but don't use it directly: underexpose by 1 stop, by using twice the film speed.
What I don't understand is how the doubled film speed relates to the Effective Film Speed derived by densitometric testing. Do BTZS practitioners routinely determine an effective film speed with sensitometry, but then double the speed in the field in order to compensate for having metered the shadows ?
I had Fred Newman perform a BTZS test for me - with HP5+ and D-23 - and the effective film speed for normal scenes was 250. I subsequently performed my own densitometric tests and got the same results. In practice (and given a scene of normal brightness range) would I meter the shadows, and use a film speed of 500 ?
BetterSense
22-Dec-2012, 17:40
Commercial films have the ISO speed printed prominently on the box. Conducting tests to see if their meters match your meters is tilting at windmills. I have better things to do.
Here's a hint: If you are not getting enough shadow detail, expose more. Can I have a book deal now?
sanking
22-Dec-2012, 17:58
What I don't understand is how the doubled film speed relates to the Effective Film Speed derived by densitometric testing. Do BTZS practitioners routinely determine an effective film speed with sensitometry, but then double the speed in the field in order to compensate for having metered the shadows ?
Ken,
You can determine a personal speed point reference if you like, but I always run my tests with Winplotter with the SPR set to 2.4, which as I recall is referenced to Delta 100 in Xtol. Use of the SPR of 2.4 gives you a useful anchor by which to evaluate all film/developer combinations. So in essence I determine a standard EFS with sensitometery, then adjust in the field for the method of metering. I am pretty sure Fred would have used an SPR of 2.4.
You might be interested in this article by Phil Davis. http://www.btzs.org/Articles/PSP.htm
Sandy
Bill Burk
22-Dec-2012, 18:34
"Notice that these film speeds seem exaggerated; they are, in fact, just double the normal speeds. As explained earlier, this is done deliberately to compensate for the 1-stop overexposure that normally results when the camera settings are based on the low-light incident reading."
In other words, the recommended technique is to take an incident reading in the shadow area of the scene, but don't use it directly: underexpose by 1 stop, by using twice the film speed.
The quote seems like plain English, exactly as you summed. Take the film speed results from Fred Newman's sensitometry, which are reliably accurate film speeds. Since you chose to use the shadow incident reading, set the meter EI at double the real speed to keep from overexposing by one stop.
You wouldn't use double the film speed results for other kinds of metering. For simple examples, in Zone System you still might use about half film speed, for averaging meters you would still use the whole film speed.
Chuck P.
22-Dec-2012, 18:58
There's nothing in the zone system that says the "real" film speed is 1/2 the box speed. The film speed is determined by testing. It might end up being 1/2 the box speed or it may not.
Just my own 2 cents, but--------exactly, my ZS tests using TMX with both d-76 1:1 and xtol 1:1 proved to yield an EI of exactly the box speed---ZS makes no claim what the real speed should be, only that the effective speed is determined by testing. It is true, though, as has widely been found, that the tested speeds are usually 2/3 to 1 stop less the box speed, but there is no gaurantee.
Mark Barendt
23-Dec-2012, 05:49
What I don't understand is how the doubled film speed relates to the Effective Film Speed derived by densitometric testing. Do BTZS practitioners routinely determine an effective film speed with sensitometry, but then double the speed in the field in order to compensate for having metered the shadows ?
I guess my first question would be what does BTZS calibrate to? Do they peg normally to "open shade" or what?
Once the "peg" is known then the offset they are suggesting may make more sense.
For example, if "normal" zone system and spot metering techniques were applied and you had decided you always want to place the shadow point say 2-stops down, then instead of always doing the math in your head you could simply reset the spot meter's ISO setting.
So for example if you were shooting film that tested at 125 you could reset the meter to 500, spot meter your shadow point, and the meter would read the camera setting directly.
The film rating didn't change to 500 we just applied an offset to force the meter to give us the camera setting.
Brian Ellis
23-Dec-2012, 07:00
It's been too many years since I used the BTZS system to comment on the accuracy of the statements related to it. But as for the "traditional" zone system, I'm not aware that it gets around anything by reducing box film speed in half. That's probably a better practice for someone who's unwilling to test to determine their film speed than blindly using the box speed but ideally one tests to determine a film speed when using the zone system.
In many cases it may not matter as michael slade suggests. Sometimes two mistakes can cancel each other out (e.g. underexposing and overdeveloping). And other times the range in the scene is well within either exposure. But it does or can make a difference in a situation where the shadows are important and the box speed would give insufficient density/detail to the shadow areas.
Ken Lee
23-Dec-2012, 07:06
What Bill said in post 46 makes sense to me, and matches my testing. I just wanted to make sure I wasn't missing something.
When I performed BTZS testing with step-wedges and a calibrated sensitometer, I got the same results as Fred Newman. Using a spot meter and the Zone System, I use that film-speed as-is. When metering BTZS style (in the open shadows with an incident meter) I derive the same camera settings by subsequently increasing exposure by one stop - equivalent to Phil Davis' recommended short-cut of doubling film speed.
Chuck P.
23-Dec-2012, 07:48
I meter in spot fashion and employ the ZS technique, I can read any shadow from the camera's position along the lens axis------what does a BTZS user do when he can't directly measure a shadow area because he is unable to reach it? If you can reach the shadow with your incident meter and take a reading, it may not provide the same luminance value while "in" the shadow as it may from the camera position, in fact it could be quite different.
sanking
23-Dec-2012, 08:27
I meter in spot fashion and employ the ZS technique, I can read any shadow from the camera's position along the lens axis------what does a BTZS user do when he can't directly measure a shadow area because he is unable to reach it? If you can reach the shadow with your incident meter and take a reading, it may not provide the same luminance value while "in" the shadow as it may from the camera position, in fact it could be quite different.
Let me clarify a couple of points.
First, any system of metering, either for film or digital camera, requires some type of creative interpretation.
Second, BTZS is not based exclusively on either incident meter readings or reflective metering with a spot meter. The testing provided data that allows field use of either incident metering or a traditional zone system type spot metering.
Third, even if only incident metering is used a single shadow value reading is not the only viable method. One could 1) take a single shadow value reading and double the EFS, 2) take a shadow value reading and a highlight value reading and average the two, or 3) take a single highlight reading and halve the EFS.
Whatever method you use the critical issue is one of choice, i.e. determining which area of the subject contains the appropriate shadow values for your creative interpretation.
Finally, to directly address the question, one can nearly always find shadow areas near the camera position that approximate the luminance values of shadow areas in the subject that can not be reached with the meter. If not, then one can always resort to a spot reading.
BTZS is not an either/or, brain/no brain system. Rather, it is based on the concept that the photographer understands his/her materials and equipment, and knows how to use both creatively to achieve a desired end.
Sandy
Chuck P.
23-Dec-2012, 08:51
........... to directly address the question, one can nearly always find shadow areas near the camera position that approximate the luminance values of shadow areas in the subject that can not be reached with the meter. If not, then one can always resort to a spot reading.
Thanks for that answer. Now, I wander, do most BTZS users carry both a spot meter and incident meter with them, I figure most use a nearby shadow.
The other points you made are, of course, understood, except I always thought (quite erroneously it appears) that BTZS was incident meter oriented because that always seems to be the context with which it is discussed.
Mark Barendt
23-Dec-2012, 09:02
I meter in spot fashion and employ the ZS technique, I can read any shadow from the camera's position along the lens axis------what does a BTZS user do when he can't directly measure a shadow area because he is unable to reach it? If you can reach the shadow with your incident meter and take a reading, it may not provide the same luminance value while "in" the shadow as it may from the camera position, in fact it could be quite different.
Actually, as i understand it, the distance itself doesn't matter as long as the spot meter reads only the target. The luminance of a point source doesn't change with distance, just it's significance/relative size within the scene. Haze and flare are a separate issue.
A spot meter measuring a known target (say a grey card or some such thing at 10 meters) and an incident meter used at the same place the card is placed will lead you to the same camera setting. (Dunn & Wakefield, Exposure Manual)
One problem for spot meters is that as distance increases the sample size for the spot meter increases, at 100 meters you would need a pretty big card, at 1000 meters you might need the short side of a barn. Even a 1-degree spot starts becoming an averaging meter as distances become significant.
A second problem for spot meters is that the targets in the scene aren't always "knowns" like a grey card is. Many times the target, especially at a significant distance, is a muddled mess.
Looking out my front door right now at 100 meters I would be hard pressed to get a 1-degree reading that didn't include both dark green trees and snow. How much of each...? Who knows?
Incident meters, on the other hand only require getting into or reading some portion of the same lighting the subject is in.
For an open shade reading this is really, really, simple in most any situation; if you are inside or under something go outside where the meter can see the sky, dome extended/out, point the meter away from the light source, and take the reading, apply the offset and set the camera. This works whether or not you can see the light source directly.
Bill Burk
23-Dec-2012, 09:03
Sandy's post is worth reading twice.
BTZS is a comprehensive book with chapters on spotmetering and sensitometry. It "goes beyond" Zone System by teaching duplex incident metering. But you can spotmeter too.
Back when the book was written, you could simulate scene shadows by bending over and holding the meter a certain way in the shadow of your puffy down jacket. I don't think anyone wears jackets like those anymore, so I don't know if it's possible to re-create the effect. Certainly the subject will refuse to wear glasses that big. (Sorry couldn't resist poking fun at the 70's look that had me giggling the whole way through that book - I see no fault with the teaching).
Traditional Zone System testing comes up with Exposure Indexes that disagree with rated speed because of the way the testing is done. Stephen Benskin has ideas what is behind that: Partly it's due to flare, partly due to using 7 stops instead of 7 2/3 stops. Partly from using a different statement of quality - traditional standard was visual match looking at prints.
sanking
23-Dec-2012, 09:09
Thanks for that answer. Now, I wander, do most BTZS users carry both a spot meter and incident meter with them, I figure most use a nearby shadow.
The other points you made are, of course, understood, except I always thought (quite erroneously it appears) that BTZS was incident meter oriented because that always seems to be the context with which it is discussed.
I believe your assumption that BTZS is incident oriented is correct. My point is simply that it is not limited to incident metering, and certainly not to the one kind of incident metering being discussed, i.e. single shadow value reading.
My main meter is a Sekonic L-558, which can be used either for incident readings or as a 1º spot meter. Newer models of the Sekonic also offer either incident or spot metering.
Sandy
Bill Burk
23-Dec-2012, 09:15
A spot meter measuring a known target (say a grey card or some such thing at 10 meters) and an incident meter used at the same place the card is placed will lead you to the same camera setting. (Dunn & Wakefield, Exposure Manual).
That compares "Spot Meter" and "Incident Meter"
But I get different results when comparing "Spot Meter" and "Reflected Meter" where the reflected light reading is close to the subject, and spotmeter is taken from camera position.
Simplest proof is to take spotmeter reading of the shadow under a car. Then walk up to the car, bend down and take another spotmeter reading. The close-up reading will indicate the shade is deeper/darker.
I believe the primary issue in this case is flare. You touch on the problem Mark, when you look out your front door and see grass and snow.
Faced with your scene, I would use Zone System... Spot the darkest thing I think will be worth seeing in the picture and decide to place it on Zone II or III depending... I would not hesitate to turn the knob to incident mode for a quick sanity check of the reading.
BetterSense
23-Dec-2012, 09:43
Simplest proof is to take spotmeter reading of the shadow under a car. Then walk up to the car, bend down and take another spotmeter reading. The close-up reading will indicate the shade is deeper/darker.
But doesn't the same situation hold for the camera? if its lighter for the meter, it stands to reason that it will be lighter to the camera as well, no?
Mark Barendt
23-Dec-2012, 09:44
That compares "Spot Meter" and "Incident Meter"
But I get different results when comparing "Spot Meter" and "Reflected Meter" where the reflected light reading is close to the subject, and spotmeter is taken from camera position.
Simplest proof is to take spotmeter reading of the shadow under a car. Then walk up to the car, bend down and take another spotmeter reading. The close-up reading will indicate the shade is deeper/darker.
I believe the primary issue in this case is flare. You touch on the problem Mark, when you look out your front door and see grass and snow.
Faced with your scene, I would use Zone System... Spot the darkest thing I think will be worth seeing in the picture and decide to place it on Zone II or III depending... I would not hesitate to turn the knob to incident mode for a quick sanity check of the reading.
I agree that haze and flare, as well as sample size, affect spot readings and you actually are helping me make my case, as is Sandy.
What I'm getting at is that the thought that "spot meters are better when it isn't easy to get close to the subject" just isn't real.
I'm not dissing spot metering here, they do have certain creative advantages like easily tying any specific tone in the scene to the film curve at a specific point regardless of the overall lighting. It's not that you can't get there with an incident meter, just that the thought process is more direct with a spot meter.
Mark Barendt
23-Dec-2012, 09:56
But doesn't the same situation hold for the camera? if its lighter for the meter, it stands to reason that it will be lighter to the camera as well, no?
Yes, I would suspect more so even. I would guess that a 1-degree spot meter should be less affected than most lenses just because of the field of view. That's pretty much why a 1-degree spot is used, basically to exclude the influence of the rest of the scene.
Chuck P.
23-Dec-2012, 16:13
A spot meter measuring a known target (say a grey card or some such thing at 10 meters) and an incident meter used at the same place the card is placed will lead you to the same camera setting. (Dunn & Wakefield, Exposure Manual)
Taking the gray card out of the discussion------I don't put one in the important shadowed area before reading the luminance from it. The luminance of a particular surface may very well be darker than a gray card in the same shadow, on the surface anyway, I question it. It's an interesting point though as I see it, I'm going put this to some practical experiment the next time I'm photographing.
Captain_joe6
23-Dec-2012, 16:49
I feel like we're missing the part where someone says something to the effect of "ISO can only be accurately and repeatably determined under exacting scientific standards in a laboratory setting, as per ISO guidelines, which may differ greatly from field conditions with various manufacturers' equipment and calibrations of a multitude of other instruments and chemicals in the photosensitive-optical construct" followed by several threads where others say "Yes, except...," and finishing no closer to where we started, with all confused, many frustrated, and nobody confident of their technological skills anymore.
Did I miss that part somewhere?
I've had a couple of beers today. ;)
Mark Barendt
23-Dec-2012, 20:36
Taking the gray card out of the discussion------I don't put one in the important shadowed area before reading the luminance from it. The luminance of a particular surface may very well be darker than a gray card in the same shadow, on the surface anyway, I question it. It's an interesting point though as I see it, I'm going put this to some practical experiment the next time I'm photographing.
The magic is having a/any "known" target not necessarily a gray card; grass is a reasonable substitute, concrete, granite...
If the relationship of the target to the film speed is known it works.
Bill Burk
23-Dec-2012, 20:53
But doesn't the same situation hold for the camera? if its lighter for the meter, it stands to reason that it will be lighter to the camera as well, no?
Yes. And you can see it with your eyes too.
Bill Burk
23-Dec-2012, 21:12
I agree that haze and flare, as well as sample size, affect spot readings and you actually are helping me make my case, as is Sandy.
What I'm getting at is that the thought that "spot meters are better when it isn't easy to get close to the subject" just isn't real.
Without getting at the concept of "better"... Spot meters are good at estimating what will reach the film because the optics leading to the sensor are similar to the optics leading to the film.
I use spot readings and do sanity check with incident readings. So I know I can trust both.
Mark Barendt
23-Dec-2012, 21:17
...the optics leading to the sensor are similar to the optics leading to the film.
How so? What optics?
Bill Burk
23-Dec-2012, 21:21
I feel like we're missing the part where someone says something to the effect of "ISO can only be accurately and repeatably determined under exacting scientific standards in a laboratory setting."
Ah yes. I've given up trying to achieve ISO conditions. I have repeatability. But my accuracy is only within 2/3 stop. So that leaves me with theoretical discussions, occasional demonstration of concepts. And the best part... Developing film and making prints.
You haven't seen recent work by me because I used up my supplies... but I've been shaking the boxes under the tree and one of them feels like paper.
Bill Burk
23-Dec-2012, 21:21
How so? What optics?
Just the spotmeter lens, similar to the camera lens.
Chuck P.
23-Dec-2012, 22:03
Without getting at the concept of "better"... Spot meters are good at estimating what will reach the film because the optics leading to the sensor are similar to the optics leading to the film.
I use spot readings and do sanity check with incident readings. So I know I can trust both.
I do spot readings too, but the sanity check is confirmed with the negative, happy to report I am still sane after several years.;)
I am reading Ansel Adams Examples and was amused to read
"He (Edward Weston) relied for his first exposure on his personal approach to the use of the exposure meter, usually doubling the indicated exposure.
If Weston considered that film speeds should be halved, who am I to say different.
Mark Barendt
24-Dec-2012, 10:58
I am reading Ansel Adams Examples and was amused to read
"He (Edward Weston) relied for his first exposure on his personal approach to the use of the exposure meter, usually doubling the indicated exposure.
If Weston considered that film speeds should be halved, who am I to say different.
Well I assume you are the guy who has to print the negatives. That's a pretty important person who should say different if need be.
Also, are you using the same film, lighting, subjects, and chemicals?
How about the same printing materials, chemicals, and methods as Weston?
If not Weston's choices aren't necessarily right or even relevant to you. (They aren't necessarily wrong either.)
Chuck P.
24-Dec-2012, 12:43
If Weston considered that film speeds should be halved, who am I to say different.
Come now, you have to start thinking for yourself-----I can say that because I used to be someone who didn't. :)
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