Film Speed examplesgreenspun.com : LUSENET : B&W Photo - Film & Processing : One Thread
I've found the discussions of film development in this forum to be very interesting and useful. However, because I don't know the jargon I am a little confused about 1 or 2 points. First when someone talks about speed when comparing development times and developers what are they really talking about physically. I thought film speed defines the way the silver grains react with photons and was a result of it's manufacture and developing would mainly affect contrast. Also it seems that many people state what the 'real' speed of a film is although there seems to be alot of discrepency between them. Unless someone is sure that their meter, shutter, ASA and aperatures are perfectly calibrated against set standards isn't it irrelevant? And finally does anyone know of a site or book that actually gives examples of the above concepts and development procedures?
-- andy laycock (email@example.com), December 06, 1998
The basic speed of the film is set at the time of manufacture. This means that a certain amount of light will produce a certain amount of density, given a certain amount of time in a certain developer.
From there, things get interesting. Ansel Adams admonished in his books that the photographer should always experiment with films and papers to find out what works for the individual. This means that what you use isn't what may work for your neighbor.
Let's take an unusual but definitely illustrative case: Infrared films.
Infrared film was first developed by Kodak in the 1940's for use in spotting enemy camoflage installations. The properties of infrared light cause it to be absorbed by dark aritificial structures, but reflected back brilliantly by dark folliage. This means that plants show up white, and factories or barracks show up black.
The development times given by the IR film manufacturers give a very high contrast index. This is fine for spotting cammoflage or police photography, but it's lousy for delicate and subtle artwork. Also, the Kodak and Konica films aren't even given an ASA rating. They give a starting spot, but then you must adjust it from there.
So what to do? You have to make a series of test exposures. Just take it and shoot up a few rolls, and carefully note down the conditions, the amount of light, shutter speed, and aperture value. Maybe even do things like pre-exposure to reduce contrast.
Now, the IR film starting points from the manufacturers give extremely high-contrast images. I found that out quite quickly. So I reduced the time I was giving for HC-110 dil. B. That was OK, but still the images had too much contrast. Now I have switched from HC- 110 to Xtol. The images are much better.
Then the image needs to be printed. So what contrast goes best with which paper? That's another thing which needs to be answered.
So with relentless and scientific experimentation, a photographer will be able to figure out what works the best. An remember, it's all for the individual, which means you. Or me.
I recommend that you read the Ansel Adams series, "The Negative", and "The Print". There are also many other fine books out there. The best thing to do, though, is just experiment and keep very good notes.
-- Brian C. Miller (firstname.lastname@example.org), December 06, 1998.
ISO have defined a standard for film speeds. Approximately, it defines the exposure you need to get Zone I at a density of 0.1 above film base plus fog, for a given contrast.
This is nice. There is an absolute, rock-solid definition of film speed. Anyone who quotes an ISO film speed can be taken to task if they are wrong.
So what do the manufacturers do? They don't tell us the ISO speed of their films. For example, I am not aware that Ilford have published the ISO speed of their new film "Delta 3200 Professional". Because they won't tell us, we have to do our own tests, with our non- standard equipment. For what it's worth, I think it is about 800, the same as "T-Max 3200". Although my equipment and methods are non- standard, they are well within one stop.
It is important to get these results, otherwise we may be fooled by the name into under-exposing by 2 stops.
As far as I am concerned, the ISO definition may be imperfect, but it is the only *real* speed.
One nice aspect of the ISO definition is that although it refers to only one contrast, if we develop for longer (or shorter), we will get greater (or lesser) contrast, but Zone I will stay at roughly 0.1 above fb+f. Hence the expression "expose for the shadows, develop for the highlights".
The other "definition" of film speed is the Exposure Index (EI). There is no standard definition for the EI. Indeed, this is the whole point of it. The EI you use should be whatever you set your meter to, in order to get the results you want (which includes variables like contrast, agitation techniques, thermometer accuracy, water quality, and so on). So if a manufacturer tells me I can use an EI of 3200, or 25000, they mean they can set *their* meters to that value and get the results that *they* want, which may not coincide with what *I* want. To be fair, they usually add disclaimers, like '...if you don't care about the shadows' and 'use these as a basis for your own experiments'.
Sometimes, EI means the exposure that places Zone V at a given density (say, 0.9). At greater contrasts, this gives a higher number than the ISO definition. But it really can mean anything at all. Iflord's definition is 'the speed at which the film can be used to produce excellent images.' Well, thanks for the precision, guys.
So, if someone tells you the "real" speed of a film, ask them if they mean ISO or EI. If they mean ISO, you can use that information directly. If they mean EI, you will only be able to use the information if you happen to use the same metering technique, etc.
Don't get me wrong, the concept of EI is great. When I know my own definition of EI for a film/developer/time/etc, I can set my meter and fire away. But because everyone's definition of EI is diferent, don't expect them to agree.
And this is really the point of experimentation. I agree with Andy and Brian, in as far as the most important thing about film speed is to know what works for you, rather than what the standards say.
Incidentally, my own definition of EI is "the speed that gives me good detail in Zone III, which generally means Zone I is 0.1 above fb+f". This definition works for the way I meter, but would *not* work for high contrast films, where Zone I might be 0.1, but Zone III might be totally blocked out.
As for literature, "round up the usual suspects": A. Adams, "Beyond the Zone System" by Phil Davies, and the Focal Encyclopedia of Photography.
-- Alan Gibson (Alan.Gibson@technologist.com), December 06, 1998.
Thanks for the great responses to my questions. I have read all of Adams books (including his autobiography) but it was so long ago that I had forgotten the definitions. I guess coming from a pure science background I get frustrated when I don't have exact, statistically significant data to help me understand but I guess that's the nature of the beast in photography. There seems to be no alternative but to experiment myself, which I find extrememly tedious. I like the ISO definition because it gives me a better known starting point. I am still a little unsure of the usage of the word 'speed' though, as in a previous posting someone described the use of a certain dilution of developer as giving 'good speed' to the film. I have also seen it used to describe paper and paper developer. Thanks again.
-- andy laycock (email@example.com), December 07, 1998.
I agree, this whole speed thing is a mess, and I would like it to be cleared up. Manufacturers used to quote ISO speeds (well, back then, it was ASA). Since the concept of EI became popular, the manufacturers seem to say 'oh good, we don't need to stick to the standards any more, and can quote whatever numbers we like'. In fact, perhaps it was the manufacturers who poularised EI in the first place?
There is an ISO speed definition for paper, and this is used (and quoted by Ilford!) for their Multigrade paper. I can't remember what the definition is, because it isn't important to me: in the darkroom, I use test strips rather than meters.
By the way, I rapped Ilford on this issue, but the other manufacturers are just as bad, and I do like Ilford films. Preliminary results suggest that I may prefer Delta 3200 to T-Max 3200.
-- Alan Gibson (Alan.Gibson@technologist.com), December 07, 1998.
Gee whiz Andy, a scientist who finds experimentation tedious? :) Better give it all up and get yourself an APS camera! >D
Yeah, the only way to get statistically significant data is from running your own tests, and doing it with at least two or three different developers. AA warned about checking the manufacturer's info on every new film package you open so you won't get caught by suprise by "improvements".
I have as yet to test my film. I did buy a grey scale and color card, and I just need to get off my behind and make up a little still life and run the tests.
-- Brian C. Miller (firstname.lastname@example.org), December 08, 1998.
Yup it's a sad fact that as you get older the bench holds little charm for most scientists. The fun part is devising the experiments and asking the right questions, then you make your underlings do the dirty work. This way no challenge is too great and no experiment is too complex...or pointless. If I could find someone willing to do the tests (and pay for them) I could keep them busy for years.
On a serious note I was wondering if using the method of printing the sprocket holes until they just disappear and then picking the exposure that looks the best was a good way to find your camera's correct ASA setting for a given film etc.
-- andy laycock (email@example.com), December 08, 1998.
I suppose my equivalent is a technologist who spends more time in here talking about film than actually shooting the stuff. Never mind, I'm off to Goa in a few days, shooting 5x4, what fun.
>> "On a serious note I was wondering if using the method of printing the sprocket holes until they just disappear and then picking the exposure that looks the best was a good way to find your camera's correct ASA setting for a given film etc."
No, because it makes all sorts of horrible assumptions about the density range on the negative and contrast of the paper, and your way of metering, and so on. It *is* a reasonable method for finding the minimum printing time of a negative (or, indeed, entire roll of film).
A better way of getting a very rough EI, with my definition of EI, is: put a card (it doesn't have to be an 18% card, but it shouldn't be coloured) in a position with constant illumination. Meter it. Photograph it at 5 stops more, then 4, 3, 2, and 1. Look at the negatives. Pick the one that shows the grey card just off film-bass- plus-fog (i.e. density about 0.1). If this is the "+4 stops" negative, you guessed the correct EI. If it is the "+3 stops" negative, you should halve your guess. And so on for the other negatives.
You can adapt this method to find your own EI, if your definition is different to mine.
-- Alan Gibson (Alan.Gibson@technologist.com), December 09, 1998.
Off to Goa! I'm not sure where that is but it sounds sunny and warm and considering El Nino has given Vancouver amost 2 months of cold rain I envy you.
Thanks for the advice re:EI. I might even get ambitious enough to try it this time. I think that I am finally ready to take the plunge and try to improve my exposure and development abilities. Wish me luck.
-- andy laycock (firstname.lastname@example.org), December 09, 1998.