BTZS, Zone System, Everyone Else... Film Speed ?

[Ken Lee]

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.

[Leigh]

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

[Corran]

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]

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]

http://www.apug.org/forums/forum37/9...-exposure.html

[Ken Lee]

http://www.apug.org/forums/forum37/9...-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.

[Mark Barendt]

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.

[D. Bryant]

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.

[Ken Lee]

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.

[D. Bryant]

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.