Film speed is measured in terms of shadow detail. In general terms, the question is--what is the minimum amount of exposure needed to produce a visible increase in density over unexposed film (film base+fog). In Zone system terms, one measures film speed by seeing how much exposure is needed to produce a Zone I density of 0.1 (though you may decide that with a certain film/developer combination, a higher Zone I density produces better shadow separation). The ISO film speed is measured in a similar way, using a specific developer (that you would probably never want to use for normal photography) under very controlled conditions.
If you increase development time with your normal developer, the highlights will increase in density, but Zone I will stay in about the same place. You might get a slight increase in speed, but generally not enough to rate a 100 speed film at 200 or 400 or 800. You will get an increase in contrast though, and you might have a printable negative, but it would be inaccurate to say that you really achieved a speed of 200 with a 100 speed film by extending development.
It is possible to increase the speed of a film by using a different developer (like Microphen or Acufine), or a different development technique like stand development, or with certain other techniques like hypersensitizing the emulsion with ammonia vapors or pre-exposure, but you can't just increase the time and get more shadow density that way.
For a more a detailed understanding of how film reacts in developer you need to get a book or find a website that explains it in detail. Get Ansel Adams's "The Negative" or similar work that deals with the Zone System (ZS) as you need to understand this stuff to understand the ZS. You don't have to use the ZS, just understand where it is coming from. This includes the concept of Exposure Index (EI) which is your personal film speed for that film in that developer at that agitation, temperature and time. It is often 1/3 to 1 stop less than the manufactuter's marked speed. Comes as a bit of a shock at first!
You do not need densitometers and such like. There are several methods of finding your EI without one: mostly by looking at prints exposed for the minimum-time-for-maximum-black exposure. You can then use Les's rules of thumb for N+/N- development. I have a few PDFs I made from articles on the late Barry Thornton's website that detail finding personal film speed etc without densitometry. As far as I know these may be copyright of his estate so I will not post them, but the original articles are still available at archive.org here: http://web.archive.org/web/200409232...hornton.co.uk/ - scroll down to the bottom. Well worth a read.
Haris, since you are using roll film, it will be easy to see what happens. Do a shot with your "normal" exposure and development, then change your film speed. Write down the shot number and what you did to change exposure (more light, less light).
Once you have taken the full roll, do a "normal" development and then print what you have on a contact sheet. You will see a difference in the shots. Take a little time to study what you have in the print. If you change things by a full stop, you will see how it works. Find out what you need for shadows which have enough detail and look right to you.
You can do the same thing with development, once you have decided which exposure works best for you. It might be 100, but it might be 50 or even 25. There is no rule for this, because everyone's equipment is different; cameras, lenses, meter, developer, paper, it all goes into what works for you. Once you have the correct exposure for your needs, development can be changed to make contrast different. Good luck. tim
Thank you all very much for all advices and answers.
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Thank you all again.
Originally Posted by haris
First of all, you can't change a film's speed. A film in a particular developer at a particular dilution (and even with your particular development methods) has only 1 speed. The only caveat at to that is with tungsten light, as you noticed. Film has slightly less sensitivity to tungsten light, that's all. But, other than that, it has only 1 speed.
What you're talking about is "pushing," where you deliberately underexpose the film and then overdevelop it to get usable results. Many people think that you are increasing film speed - turning an ISO 100 film into an ISO 200 film. You are not. You are _shooting_ it at a higher EI (exposure index), but the actual, scientific film speed has not changed.
The key word in pushing is "usable." If you take Efke 100 and shoot it at EI 200, it won't look "normal" (as defined by the look you get when shooting at EI 100) no matter what you do with developers, etc. But if you're willing to give up shadow detail, accept more grain and contrast, then you can overdevelop (push) in order to get the midtones in the right spot so that you can still print the film. Again, it's about usable results, not "normal" ones.
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One important qualification - when you are considering setting exposure so that a shadow area will print in Zone III or Zone IV, you need to understand that what is being discussed are shadows which include detail you wish to be able to see in the print.
Many, if not most photographs include areas that are intended to be so dark as to reveal no detail. In Zone system terms, those areas are essentially ignored at time of determining exposure on the film - they are expected to fall into Zone 0.
If you set exposure to put those deepest darkest shadows in Zone III or Zone IV, they and everything else will be too dense in the negative (too light in the print), your highlights will likely block up and lose detail, and you will not be making best use of the capacity of the film and/or developer.
In my mind, the determination of which shadow to place into Zone III or Zone IV is the Zone System skill that improves the most with experience.
Originally Posted by haris
EFKE 100 has a non-linear response to the spectrum. Well - all films do that, but EFKE films are a bit more non-linear than most. That means that EFKE 100 has reduced sensitivity to red light, which there is a lot of in tungsten light. BTW, EFKE 50 and 25 are even more non-linear, and almost red blind.
So in warm light, EFKE 100 is less sensitive than the light meter would indicate and needs one stop more exposure. Ilford FP4+ is just the opposite, and can be used with half a stop less exposure.
So film speed is based on a standard spectral sensitivity, which few films follow. That means we may have to adjust exposure with changing light colour.
Next is the "personal speed" thingy:
Many photographers find that exposing the film at some other speed than the box value gives them better results. This could be a consequence of their metering methods, their light meter, the developer of choise, variations in the emulsions, or just plain personal taste. This is not ISO, which is strictly defined, but EI (for Exposure Index).
ISO film speed is defined from the shadows - how much exposure it takes to give a certain density. Density, BTW, is "above film base density + base fog" or fb+f for short. That only means "usable density".
So "Expose for the shadows" only means "expose enough to get some usable detail in the shadows - unless you think you want something else".
"Develop for the highlights" was a very good advice back in the days when all development was by inspection. It still is, for those of us who still do development by inspection!
What the zone system is, is an attempt to give the same level of contrast control when developing by time&temperature.
When you "push" a film you will get higher contrast, but the shadows won't gain much in density. So even if you get the midtones up by 2 steps (say from mid grey at ISO 100 to mid gey at EI 400), you still won't have increased the detail in the shadows. So developing longer in the same developer does not (significantly) incrase film speed. Using a different developer could; there are just as many speed-increasing developers as there are speed-losing developers.
Do I make sense at all?
-- Ole Tjugen, Luddite Elitist
In addition to Ansel's books, I always found Zone VI Workshop, by Fred Picker, to be eminently useful. YMMV.
The whole idea is to establish the workflow for a film/developer combination that is "normal", then you can vary factors from there.
Basically, KISS ... Keep It Simple, Stupid.
Honey, I promise no more searching eBay for cameras.
The easiest way to think about it is:
Exposure determines the density of the negative.
Development controls the contrast.
Yup. And specific developer defines tonal rendition. D-76 varies from Rodinal which varies from FG-7, which varies from Amidol, etc., etc.
Originally Posted by Gerald Koch
Honey, I promise no more searching eBay for cameras.