Understanding the relationships between emulsions, exposure, and development.

Poohblah · Dec 9, 2008

I've started reading AA's "The Negative" and I skimmed most of the parts about the Zone System, but one part caught my eye and totally threw off everything I thought I knew about film. It's the diagram for over- or under-developing (N+0, N+1, N-1, etc.). After reading that, and then the characteristic curves later on, I understand that under-developing simply reduces the D-max and makes the characteristic curve of the film less steep (i.e., less contrast or less gamma). Obviously over-developing raises the D-max, etc.

Now comes the part where I am confused. Through my elementary knowledge of b/w emulsions, I learned that every emulsion has a sensitivity level (ASA). I also learned that under-exposing and over-developing effectively raises the ASA (and the contrast and grain) and vice versa. However, based on the characteristic curves I see now, over-developing simply raises the D-max and gamma, not filmbase+fog, meaning that in the final print, the shadow details are sacrificed and the shadows are just as dark as they would have been if not for under-exposing and over-developing.

So this means that the ASA, as I understand it, is simply the sensitivity the manufacturer desires the photographer to use in order to obtain "optimal" gamma. If the film is over-exposed and under-developed, then gamma decreases, and there comes a point when development is so little that D-max and gamma are too low to obtain an acceptable print.

Yes?

Now, assuming I am correct, I have a question about the implication of these relationships. Recently, before I read "The Negative", I was shooting with my Holga. Using my experience as a guide, I knew that I would need a 400 speed film, but I only had 200. So I shot the roll and over-developed the film. The midtone and shadow details turned out very well, but my highlights were blown out - even the lowest grade paper I had couldn't bring everything into the dynamic range of the paper. Looking back, would it have been better to develop normally to avoid blowing out the highlights, rather than developing for the midtones? Isn't this how the saying goes, "Expose for the shadows, develop for the highlights"? Did I just demonstrate what not to do, thus validating the saying?

Photo Engineer · Dec 9, 2008

You are almost right.

Overdevelopment also sharpens the toe of the characteristic curve giving less latitude, but more speed, and overdevelopment also increases fog as a general rule.

As for your example, remember that nothing can restore what is not there! So, in a normal negative with 5 stops or more of latitude, you can usually take a 200 speed film, shoot it at 400 and get very good negatives with no change in development. And, you lose less due to the increase in development would cause.

PE

Poohblah · Dec 9, 2008

Okay, looking at the characteristic curves I understand what you are saying. I think I'm finally getting this.

Chuck_P · Dec 9, 2008

Poohblah said:
However, based on the characteristic curves I see now, over-developing simply raises the D-max and gamma, not filmbase+fog

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Increased development absolutely does increase the fb+f density. This is well documented in The Negative. Fb+f at normal dev is .04 for my test (d-761+1 /tmx) and .06 at N+2 dev. When fog levels increase as a result of increased development, an additional decrease in exposure can be made so that the expansion you require is true. If I place a shadow on zIII and plan N+2 dev, my tests tell me that I need to reduce exposure another 1/3 stop for a true +2 expansion (with t-max films, a 1/3 stop is significant). This is because the fog level increases with +2 development, not making the adjustment means I'm not getting a complete +2 expansion of the negative.

So this means that the ASA, as I understand it, is simply the sensitivity the manufacturer desires the photographer to use in order to obtain "optimal" gamma.

Click to expand...

ASA/ISO as the manufacturer states is the threshhold point of exposure that yields the first usable density above the fb+f level. Manufacturer's speed ratings are really based on highly controlled laboratory settings and their own equipment; this is why folks do their own testing because how we use the film is not in the same setting or same equipment.

If the film is over-exposed and under-developed, then gamma decreases, and there comes a point when development is so little that D-max and gamma are too low to obtain an acceptable print.

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Yes, if the density of the negative is too flat, obtaining a good print (one that you may have visualized) can be hard. The point is to develop the film to the same negative density range given any exposure and development (-1,-2, N, +1,+2) you give the film----this optimizes the negative for printing.

I knew that I would need a 400 speed film, but I only had 200. So I shot the roll and over-developed the film. The midtone and shadow details turned out very well, but my highlights were blown out - even the lowest grade paper I had couldn't bring everything into the dynamic range of the paper. Looking back, would it have been better to develop normally to avoid blowing out the highlights, rather than developing for the midtones? Isn't this how the saying goes, "Expose for the shadows, develop for the highlights"? Did I just demonstrate what not to do, thus validating the saying?

Click to expand...

Yes, expose for the shadows and develop for the highlights. You could have rated the film at 200 (providing more exposure to the shadows) then reduced development by 20-25% or so to control the highlights as well. Then resorted to other printing controls for desired contrast.

removed account4 · Dec 9, 2008

different developers give different characteristics as well.
i read somewhere that glycin based developers don't fog film as much
as some others, so with all zs stuff, you have to test your film, all your film
with all your developers to get your baseline and know where to start from.

Poohblah · Dec 9, 2008

Yes, judging by all those graphs out there, different developers give different results, hence the test test test mantra Though I think i'll just stick to HC-110 for now.

CPorter said:
Increased development absolutely does increase the fb+f density. This is well documented in The Negative.

Yes, expose for the shadows and develop for the highlights. You could have rated the film at 200 (providing more exposure to the shadows) then reduced development by 20-25% or so to control the highlights as well. Then resorted to other printing controls for desired contrast.

Thank you, between what you and PE said, and a more careful reading of The Negative, I understand that over-development increases the fog.

P.S. I was shooting with a Holga that day, so "rating" the film was a moot point

Christopher Walrath · Dec 9, 2008

In my opinion you chose the correct book to dive into. I use HC110 right now. Of course I've only been processing my own negs since February and I am just now trying different timing and such during development to play with the final negs. Take your time. Ask questions here. Read and, above all else, DO. Practice makes perfect.

Photo Engineer · Dec 9, 2008

Someday, I think I really must do a workshop on this process. It is soooooooo misunderstood. I think I might insist that it be a free workshop (IE nothing for me, but a fee to the sponsoring institution) to save money.

Sorry and apologies to all. This is given as a nearly year long course to new people at Kodak.

PE

nworth · Dec 9, 2008

The ISO speed of a film is determined by the exposure needed to produce a certain density (0.1 above base and fog) in the toe of the characteristic curve when the film is developed under some strictly specified conditions. Look at the characteristic curves of a film developed for different times. The illustration in the AA book will do, but the curves in the technical data sheets on the Kodak web site may be more clear. You will notice that the density at the left hand edge, where there is essentially no exposure, increases with development. This is the increase in fog noted above. Now look at the exposure (Zone) where each curve rises 0.1 units of density above its base level. You should note that it takes less exposure to do this with increased development. But there is not a whole lot of change. Depending on the film and developer, you may see a one Zone change, equivalent to one stop, between normal development and a hard push (N+2 development). But also note that the steepness of the curves increases dramatically with increased development. That is the increase in contrast mentioned above. It means that negatives exposed for normal subjects will be very hard to print when push processed. In many cases when push processed, the film may not be able to handle the normal light range in a linear fashion, and the highlights will not show adequate separation between tones (they will be "blocked"). Different developers will affect the curves differently, but they all will follow the same pattern. Despite claims to the contrary, there is no magic soup that will allow you to push a film to three or four stops more speed. Push processing can be very valuable, however, when the subject has low contrast or where blocked highlights are not a concern.

Understanding the relationships between emulsions, exposure, and development.

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