Originally Posted by
Doremus Scudder
Oh boy, here we go...
Steven,
First things first. Don't go choosing this or that exposure and development system till you understand a bit about the principles involved. You likely do already, but I'll give you my take anyway, so bear with me. Possibly it will be useful.
Note, in the below I'm talking about black-and-white negative film. Color film, and especially color transparency film, have a slightly different approach.
Most important is to recognize that film needs a certain amount of exposure to render any kind of image at all. When this level is reached (call it the threshold), you don't automatically get all the tonal separation the film has to offer. Even pretty big variations at very low exposures aren't separated very well. As the exposure level increases, the separation of tones becomes greater till, at about two or three stops above the threshold, the film is at its best, separating tones as well as it can. This lasts up until about 10, or even 12, stops above the threshold, when the separation of tones gets reduced again, and even levels out to the point that differences in exposure make no difference on the film. What I'm describing here is the film's characteristic response curve.
Naturally, we want to have the range of tones in the images we take fall in the portion of the film that gives optimum separation. That's the part that's from two or three stops over the threshold to the 10 or so stops over point. In Zone System parlance, Zones II or III through Zone X or so (i.e., a nine-stop range). One refinement here: keep in mind that most images don't need all of that exposure latitude. Actually, an image that has important elements along the whole range is pretty difficult to print traditionally unless you take steps to reduce the contrast on the negative from what is considered "normal," because printing papers don't have as much latitude as the film. Digital has even less. Most "normal" scenes have their important values between Zones III and VIII or so (i.e., a six-stop range from lowest to highest).
So, as you can see, it's important where you place a low value and it's important where the high value falls, but there's a pretty big window to hit. Since most average scenes have brightness ranges that only take up part of the film's "capacity," so to speak, there are a lot of ways to get them in there. Below are a few exposure methods, from less to more accurate.
1. Sunny 16. This was a favorite of amateur photographers for years (even professionals) since you don't even need a meter. On a sunny day, set your film speed as the shutter speed and use f/16. 100 speed film? 1/100 sec. (or as close as you can get) and f/16. This puts a surprisingly large amount of scenes with different brightness ranges in the sweet spot on the film. Cloudy? no problem, open two stops. Bright sand or snow? Close a stop, etc. Every Kodak film box had these exposure guidelines printed on the inside for years. Sure, a certain percentage of shots were ruined with this method (backlit scenes, scenes with extreme brightness ranges, etc.), but it works for the majority of photos.
2. Average metering. A meter measures the average brightness from a wide angle and gives you a reading that gets interpreted into exposure information (f/16 at 1/125 sec, etc.). Average meters are great for scenes that have brightness ranges that aren't very wide and where the distribution of tones includes roughly equal amounts of both dark and bright. Average meters tell you where the middle of the brightness range is and, whether you know it or not, you're placing that in the middle of the film's response range. Such meters don't do well with polar bears in the snow or cats in coal bins; they'll underexpose the former and overexpose the latter. That's why there's exposure compensation on cameras with such meters; the photographer is supposed to recognize the scenes where the meter will fail and compensate. Matrix metering, etc., etc. are just refinements of the average meter.
3. Incident meters do a similar kind of averaging, but they measure the light falling on the subject, not the subject itself. Same problems.
4. Finally, on to your new spot meter! With a spot meter, the photographer can measure specific areas in the scene individually. You can measure shadows, highlights, mid-tones and see how they relate to each other. But, what do you base the exposure on? Science and common sense tell us that underexposing film is worse than overexposing it. Heck, a two or three stop overexposure on most films will make no appreciable difference in the resulting print, especially if you're using a big negative. Underexposure, however, will destroy your shadows; either they will lack tonal separation or they will disappear entirely. It makes sense, therefore, to base your exposure on an important shadow value. In the simplest spot-metering scenario, you choose a shadow value in your scene that you wish to see as a "detailed black" on your print and base your exposure on that by giving it an exposure that is about three stops over the film's threshold (remember that?). That's Zone III in ZS parlance. You don't need to meter anything else; one reading of a shadow, Zone III exposure and Bob's your uncle (as long as the film-speed setting on your meter is correct and your film-development is adequate - more about these later).
So, why do some people meter a highlight and place it in Zone VIII? That's because that will work too, for most scenes. It's kind of like averaging metering, except we choose a highlight that we want detail in and base our exposure on that. For any scene that doesn't have a brightness range greater than six stops or so, the shadows will fall nicely on the response curve of the film. More than six stops? Well then you're underexposing shadows. Sure, this method works for the majority of scenes, but basing your exposure on a shadow value is superior if you want to ensure that you don't underexpose.
So, why do Zone System practitioners and others spend so much time metering both shadows and highlights? That's because of the second half of the photographic process that I haven't dealt with yet: printing. Traditional photo papers have a pretty narrow range (even the VC ones) and won't easily accept all the information that film can contain along the length of its response curve. Average scenes in the six-stop range do fine in this regard (as long as film speed and development are correct, of course). Scenes with more contrast end up giving you negatives with a contrast range that's difficult to print straight. Either the shadows are good and the highlights are blown out or, if you print the highlights right, the shadows are too dark. You can dodge and burn and mask such negatives or even use low-contrast filtration on your printing paper or... You can develop them to less contrast than normal! If you meter shadows and highlights with your spot meter and find that your important low and high values are too far apart, you can adjust development time from "normal" to give you a negative with a smaller overall contrast range.
The opposite is true for flat, contrastless scenes which would print with no real black or white; you can develop them for longer and increase the contrast range of the negative. This "tailoring" of the negative so that it prints well is what the Zone System is all about. The advantage it has over other systems is it allows you to see in your mind's eye what your results could be so that you can make informed decisions about exposure and development before exposing the negative.
Here's the Zone System in a nutshell:
1. You base your exposure on a shadow value, choosing a value that you want rendered as a detailed black in your print (or whatever; a luminous shadow or a jet-black void) and place it in the proper place on the film's scale.
2. You check the highlights to see where they fill end up given the shadow placement you've chosen. If you like where they fall, then all is well, just make the exposure as indicated on the meter. This is the Zone System normal, or "N." This, of course, assumes your film speed and development times are correct, more later.
3. If the highlights fall too high on the scale for your taste, the scene is too contrasty. N- development is indicated. One-stop too contrasty? N-1. Two stops? N-2, etc. You'll need to reduce development to get the extra contrast to fit in the right contrast range on the negative. But, since reducing development slows film speed a bit, you'll have to give a bit more exposure too. More later.
4. If the highlights fall too low on the scale, the scene is too flat. N+ development is indicated. More development actually speeds the film up infinitesimally, but so little that we don't have to worry about it usually.
That's basically all there is. The question is, where to start. Here are my recommendations:
1. Rate your chosen film at 2/3-stop slower than the ISO and save yourself the Zone System film-speed calibration. This is in line with 99% of Zone System testing and gives you a bit of a buffer so you don't underexpose. FWIW all the films I use and have carefully calibrated end up being 1/3-2/3-stop slower than box speed.
2. Use a common developer and the manufacturer's recommended time for developing your film. This will be your "N."
3. Plan tentative N-1 and N+1 development schemes as follows: for N-1 increase you exposure 2/3-stop and reduce the development time by 15% from N. For N+1 increase the development time 15% from N; no exposure compensation is needed.
Now, go out and make images, keep notes and print. For each development scheme be ready to adjust things as follows: If your shadow detail is too little for your taste, rate your film a bit slower (this will likely never happen, since you've got a 2/3-stop buffer already). If there is really a lot more shadow detail than you want (could happen with N+ development), rate the film slower. If you find yourself printing consistently with a higher contrast setting (all those N+ negative need a no. 5 filter?) then increase development time another bit, say 10-15%. If the opposite happens, you find yourself using a lower contrast setting consistently, then decrease development by 10 or 15%.
That's really all you need; no densitometers or step-tablets needed.
Sorry this got so long! Go have fun with your new spot-meter.
Best,
Doremus
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