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.
Re: BTZS, Zone System, Everyone Else... Film Speed ?
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
Re: BTZS, Zone System, Everyone Else... Film Speed ?
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?"
Re: BTZS, Zone System, Everyone Else... Film Speed ?
Quote:
Originally Posted by
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?"
Word.
Re: BTZS, Zone System, Everyone Else... Film Speed ?
Re: BTZS, Zone System, Everyone Else... Film Speed ?
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.
Re: BTZS, Zone System, Everyone Else... Film Speed ?
Quote:
Originally Posted by
ic-racer
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.
Quote:
Originally Posted by
ic-racer
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
Re: BTZS, Zone System, Everyone Else... Film Speed ?
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...
Re: BTZS, Zone System, Everyone Else... Film Speed ?
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.
Re: BTZS, Zone System, Everyone Else... Film Speed ?
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.