Here's how I would define "Film Latitude" with a strong B&W bias....
It's the Log H value (which is exposure, and can be translated to f-stops) over which the exposure of a given subject can vary without "critical" loss of available tonal separation in the low or high values.
In other words, it's "what you can get away, exposure-wise with for a specific subject".
Now for the explanation:
- "Critical" certainly would certainly vary by photographer. A LF landscape photographer practicing the Zone System or BTZS would likely be striving to avoid shadow compression. A street photographer shooting a 35mm Rangefinder can probably live with some loss of spearation in the shadows.
- The "loss of availble tonal separation" is often characterized by the "toe" region (low values) and "shoulder" (high values) on the D&H curve.
Practically speaking, most of us only see the "film shoulder" for one of two reasons:
a) Gross error in over-exposure (rare)
b) Specular or polarized reflections (common)
So we either reach the film shoulder by accident or we simply can't help it(*). In my mind, this makes the shoulder far less significant than the toe when discussing latitude.
As a result, I believe many people would consider "Film Latitude" to be - more or less - the length (in F-stops) of the film's toe region.
This definition is also at the root of the wide-spread belief (and, in my mind, a true one) that a film like Kodak TMY can be said to have little film latitude. Yes, a short toe means you can have nice shadow separation - but if the exposure of subject elements in deep shadows falls below a critical Log H threshold - then that detail is irrevocably lost and unprintable.
You'll notice a sudden switch in my terminology from "low values" to "deep shadows". That's intentional. "Deep shadows" are the "low values" on the density range that your negative uses. And it's important to realize that this is a relative concern. It's not what your eye interprets as "dark" that constitutes a shadow - it's a region of the negative that receives little exposure because the corresponding region of the subject exhibited low luminance resulting in less exposure relative to other regions in the negative.
And, as many have pointed out, ultimately you have a finite density range to work with in your negative that can be translated to the available exposure range for the printing paper. Venture beyond that and you'll be (minimally) burning and/or dodging.
Obviously, exposure is governed by the EI we choose for our exposure. To discuss this in terms of Kodak TMY, 400TX, and TXP:
- When shooting TMY in constrasty light, many of us feel compelled to shoot it at (at least!) an EI of one stop below its rated ISO speed. This, in effect, is a decision based on the need for an exposure "safety factor" rather than achieving high shadow separation.
- When shooting TXP in contrasty light, many of us feel compelled to shoot it at an EI of about one stop below its rated ISO speed. But the reasoning here is different than for TMY. TXP is a very long-toed film and the decision to give it greater exposure often reflects a desire to achieve greater shadow separation by lifting the density range in the negative off of the film toe.
- When shooting 400TX in flat light, lots of photographer wouldn't think twice about shooting the film at EI 400 (box speed) or even higher! In flat lighting a bit of shadow compression can give a somewhat "richer" appearance to the negative.
For my part, here's how I expose the afore-mentioned films...
TMY: EI 160-200 (depending on developer). I never use TMY in anything in other contrasty light and never use it when I know an exposure could be rushed; I feel it's a poor choice for street photography. I don't use it that much, really, and when I do its for landscape/architecture shots where I want good shadow separation.
400TX: Anywhere from EI 200-640 depending on lighting. I use it merrily in all sorts of lighting conditions for all sorts of subjects. Depending on the subject and lighting, strong shadow separation may be desired. Or not.
320TXP: I've never used it in flat light but would shoot it at EI 125-160 in contrasty light. I don't often use it. I use it under the same lighting/subject conditions as TMY. So, again, good shadow separation is important to me.
(*) Alternative process photographers who do contract printing with UV light often need a very dense and contrasty negative. They are the exception, and Kodak TMY is a very popular emulsion with these folks because the film does not shoulder off until very high densities.