Different Exposure index

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Greetings, What are the advantatges to exposing film at a lower EI than the manufacturer's specifications(for example, HP5+ @ EI 200)? It seems that this goes beyond finding a personal EI for a specific film/camera/developer combination. Regards, Ken

-- Ken Bruno (salmon70@hotmail.com), April 08, 2002



I'll try to keep this short if I can. There are two things going on here. The first is that manufacturers determine their film speeds using very unique controlled exposure and development parameters. Usually the film is developed to a higher contrast index than most of us like to use, and with a developer that is not commonly used. Therefore, one must do personal tests to arrive at the proper film- speed for one's own developer and contrast requirements. This is usually slower since developing to the desired lower contrast usually means less developing time which effectively lowers film speed. The one-stop example that you cite is not uncommon, especially when a long-scale negative is desired.

The second reason to rate film slower than the manufacturer's EI is to intentionally overexpose it. This can be extremely helpful when separation in the shadows is very important and the film being used has a long toe. The overexposure moves the important shadow details up onto the straight (or straighter) line portion of the characteristic curve, thus increasing shadow detail. Of course, this sends the high values higher by the same overexposure. If the film used has a curve that does not shoulder to soon, then there is still adequate separations in the high values. A general increase in grain is the down-side to this practice. For small film users, it may not be worth the extra shadow separation. For large-format photographers the increase in grain may be insignificant.

Hope this answers your question satisfactorily.

Regards, ;^D)

-- Doremus Scudder (ScudderLandreth@compuserve.com), April 08, 2002.

Doremus, I question your comment that overexposing by a stop is going to increase grain size. An increase in grain size is a function of development time. The longer in the developer the more the grain can grow and clump. The initial exposure shouldn't have an effect.

-- r (ricardospanks1@yahoo.com), April 08, 2002.

Shadow detail (as Doremus has so well described) and the use of developers that shall offer better tonal range but the use of which makes the film lose some light sensitivity. Many of us believe that the manufacturer's given speed of films is not real, but created in the lab (by using specific combinations of exposure and development) to "boost" the numbers higher (for commercial reasons). So, the best EI for HP5 might easily be 200 and exposing the film at that amount of light may help it to give its best in terms of quality.

-- George Papantoniou (papanton@hol.gr), April 09, 2002.

Large-format photographers (like us) tend to prize a long tonal scale and shadow detail, which requires more exposure. "Exposure controls density, development controls contrast". So we tend to expose more (use a lower EI) and develop less than "normal" to control contrast. The film manufacturers sell their film on speed, so their recommendations are biased that way. The important thing is to find an exposure index/process time combination that gives you negatives that make prints you like easy. Emphasis on "you like". EI/process choices are personal and specific to the work you do. That's the core of the Zone System.

-- Mark Sampson (MSampson45@aol.com), April 09, 2002.

There's no underhanded 'fiddling' of the ISO speed by devious film manufacturers. It's just the way that ISO film speed is measured that gives a slightly optimistic number.
What else do you expect from a specification 'designed by committee'? Like most other standards bodies, I'm afraid they don't appear to know which part of their collective anatomy is for bending their arm, and which part is for sitting on.

It's when the box doesn't give an ISO speed at all that you want to get suspicious. For example; TMZ3200 isn't 3200 ISO, or anything like it. Nor would the ISO speed of Delta3200 or Neopan1600 be anywhere near to their implied EI ratings, if they were tested according to the ISO methodology.

-- Pete Andrews (p.l.andrews@bham.ac.uk), April 09, 2002.

"Doremus, I question your comment that overexposing by a stop is going to increase grain size."

Overexposing silver image B&W film increases grain. Do a test to see for yourself. Take two shots of the same subject, one over a couple of stops, and print. High density areas of a negative have more silver grains closer together, the easier to clump together.

-- Tim Brown (brownt@flash.net), April 10, 2002.

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