using a zone chart to determine PEI

[Tim Meisburger]

I think I may have described this poorly, and confused the issue by mentioning the zone system. I am not really interested in exposure, I am suggesting that a chart with 11 tones or shades ranging from black to white, each a stop apart, would be a better test target for testing film than would the "average" scene generally recommended. It happens that such charts are used to illustrate the zone system (here is an example: http://www.cabbagetownphoto.com/zone.html). Using a step chart would, in one exposure, tell you whether your rated film speed is correct (the middle tone on the chart would be the middle tone in the negative), or provide a quantitative indication of how many stops you need to shift your exposure from the rated film speed.

For example, if on your negative the middle tone was zone VII of the test target, you would know you needed to reduce film speed two stops to ensure zone V in the negative matched the Zone V in the test target (actually, if comparing the negative to the positive chart this would indicate a two stop increase, but you get what I mean). No need to shoot a lot of film at different stops.

Also, this test would tell you how many zones your film could cover, and would simplify determining normal, +1 and -1 development times.

I believe that printers (humans not machines) use step charts or wedges for test and wonder why photographers do not.

[percepts]

Because if you put any chart in bright sunlight and measure the difference between the black patch and the white patch, it won't be 10 stops. You'll probably only get 5 to 7 stops at most. So you have to manufacture a lighting source to do it and some people have. But why bother when you can just expose a zone 1 neg and a zone 9 neg and print them with same time to find if everything is OK.

If you meter a shadow subject and then meter a bright cloud the two parts of the scene are in different lighting so you can get a brighter range than 10 stops. But with your zone chart you would have to manufacture lighting exactly for the dark and the light areas to be sure you had a exactly a 10 stop range. So you'd need flash and a lot of messing about to do it. Why make things difficult.

[Lachlan 717]

I think I may have described this poorly, and confused the issue by mentioning the zone system. I am not really interested in exposure, I am suggesting that a chart with 11 tones or shades ranging from black to white, each a stop apart, would be a better test target for testing film than would the "average" scene generally recommended. It happens that such charts are used to illustrate the zone system (here is an example: http://www.cabbagetownphoto.com/zone.html). Using a step chart would, in one exposure, tell you whether your rated film speed is correct (the middle tone on the chart would be the middle tone in the negative), or provide a quantitative indication of how many stops you need to shift your exposure from the rated film speed.

For example, if on your negative the middle tone was zone VII of the test target, you would know you needed to reduce film speed two stops to ensure zone V in the negative matched the Zone V in the test target (actually, if comparing the negative to the positive chart this would indicate a two stop increase, but you get what I mean). No need to shoot a lot of film at different stops.

Also, this test would tell you how many zones your film could cover, and would simplify determining normal, +1 and -1 development times.

I believe that printers (humans not machines) use step charts or wedges for test and wonder why photographers do not.

Tim,

Have a look at http://www.stouffer.net/TransPage.htm.

These are "see thru" step wedges that allow you to compare your film test results to a defined one.

They might help.

The issue that you have (as I see it) is that the toe of the exposure curve varies from film to film, as it does from process times/strengths etc within the same film. What you are trying to find out is where that particular "event/curve" occurs. Going for mid grey/18% grey/zone V only tells you what is happening on a straight part of the curve.

You need to know the lowest usable zone so that you can work up (understanding that a true mid grey can be manipulated to most other zones in printing, but you can never get detail into black shadows). Similarly, you also need to know the top end of the zone so that you can process to expand/contact the highlights.

If you want to get a reasonable/simple/quick EI test, here is my suggestion:

Do you have access to a roll film camera (pref. 120 film), light table and a spot meter? I would suggest doing some tests using a roll of film, starting from your base+fog "blank" shot, running at 1/3 or even 1/2 stop increases, from indicated Zone 0 (or even Zone -1/2 if film permits) until the roll is done (recording each frame's exposure details). Once you have processed this roll in a controlled fashion, use your spot meter to find the frame that is 1 stop brighter than the unexposed frame (make sure that you mask each frame so that only light passing through the frame is visible).

Then work out how many stops (or parts thereof) worth of frames there are between these 2 frames, compare this to your starting exposure value (eg. Zone 0) and you should have your compensation factor for the specific film/process you used.

This might not be as accurate as the techniques in some books (and no doubt some Forum Members' techniques), but it is close enough to get you started!

[Tim Meisburger]

Yes Lachlan. I have been looking at the Stouffer site, and have ordered one of their zone system charts. The way they use step charts to set exposure for printing in the graphic arts is the model I am suggesting, but I have not seen anywhere on their site or elsewhere a description of using this technique in normal (as opposed to copy) photography.

[percepts]

film curves are not usually straight from 0 to X so your theory goes right out the window.
Some films have a low zone V and some higher. How are you going to handle that one?

[Tim Meisburger]

Okay. I think I have it figured out now. I searched "step wedge" instead of zone chart and got some information on using these to test film and paper. Typically, they use a transparent 4x5 step wedge calibrated in half stops instead of full stops (like a zone chart) and either contact print it on paper or insert it over the negative in a 4x5 holder and photograph a blank wall. This is essentially the same process as photographing the chart itself. This is apparently explained in the book BTZS with analysis done based on densitometer readings, but it seems to me that if you can test film by visually evaluating a standard scene, then you could do the same visually evaluating a step wedge.

Anyway, I printed out a zone system chart and will give this a try.

[Chuck P.]

doesn't matter what kind of card you point your light meter at. The result will be the same which is an exposure which will make the result middle grey.
Problem is you think a "Zone Card" is a true middle grey but they never are. I assume you are thinking of a kodak 18% grey card. Well 18% is not the middle of a 10 zone system and your light meter is not calibrated to 18% even if 18% was the middle of a 10 zone system where each zone is 1 stop.

It would be the middle of an 11 step zone scale from zone 0 to zone X. Zone 0 is full black and Zone X is paper white, Zone V then becomes the geometric midpoint between full black and paper white. With the geometric midpoint at V, there are four distinct steps of gray on each side of it. So, an 18% gray card in ZS terms is the geometric midpoint of the zone scale.

[Lachlan 717]

film curves are not usually straight from 0 to X so your theory goes right out the window.
Some films have a low zone V and some higher. How are you going to handle that one?

Percepts,

Can you please explain what you're writing about? Whose theory? Perhaps quote the part that you're referring to...

[Chuck P.]

But why bother when you can just expose a zone 1 neg and a zone 9 neg and print them with same time to find if everything is OK.

This is fine, but the enlarging exposure time is critical if your're to evaluate the efficiency of your Zone I and IX exposures, it must be based on a time used to print a middle gray value. To do that, a Zone V exposure is made, then printed so that its print value is matches the gray card. Use that enlarging time to print Zone I and the Zone IX negatives. This method is best if the target is a textured target, such as screen tightly secured to a uniform toned card.

[percepts]

This is fine, but the enlarging exposure time is critical if your're to evaluate the efficiency of your Zone I and IX exposures, it must be based on a time used to print a middle gray value. To do that, a Zone V exposure is made, then printed so that its print value is matches the gray card. Use that enlarging time to print Zone I and the Zone IX negatives. This method is best if the target is a textured target, such as screen tightly secured to a uniform toned card.

No it does NOT have to be based on a zone V exposure. You print zone 1 test strip to a give a tone just perceptibly lighter than max black and then use that time to print your zone IX neg to give a tone just perceptibly darker than paper base.
If the zone IX print tone isn't correct, then adjust adjust development. This of course assumes you want 10 zone system and not 8 or 9 or 7 or something else.

Doing it the way you suggest is saying a zone V must be 0.7 print density which takes a very lteral view of AA writings. But it doesn't take any account of the fact that film curves are not straight and that for most films you can never achieve what you are suggesting and get a full range of tones from 0 thru X. i.e. Zone V print density is not fixed in law.