Well, if you want to have a standard with which to make comparisons, it's good to have something simple and constant. It's the geek thing to do if you want reproducible results, such as when you want actually to measure image intensities, or if you're working with film and a wet darkroom and are trying to achieve personal control over your materials rather than randomly unpredictable good results, which anyone can do. Control is possible only by being able to reproduce the same conditions from one day to the next
It happens that photographic exposure is based on a geometric progression -- each step or "stop" of incremental exposure is a doubling or halving of brightness or intensity in the subject. So 9% reflectance is one stop darker and 38% reflectance is one stop and 76% is two stops lighter than middle gray. Where are we going here? To point out that we won't get to 100% reflectance in even "stops." If we work down from 100%, we get 50% - 25% - 12.5%, meaning that "middle" gray is about 2.5 stops below absolute white.
This means that Ansel Adams' original prints defining the zone system compressed subject intensities, as he put a full five stops of subject luminance above the middle gray of his prints, up to seven stops with transparency materials.
His definitions were widely accepted, not because Adams became a Photography God, but because in creating them he successfully described what was required technically to create an image that had universally pleasing and "realistic" characteristics.
On the other hand, if you want to detect Zones II and VIII -- the lowest and highest in which textural values are apparent -- you'll need a target with texture. This is one reason why I created my 2-zone board. The line dividing the zones is a proxy for texture. Or you can put a dark or light sticker on your 18% card.
A simple way to approximate this with paint is to mix a titanium-dioxide (the standard white) base with a carbon pigment (a neutral black).
The advantages of a large, painted target are that it won't blow over in a breeze, it's easy to fill the frame and to focus, and you can make it the way you want it.
The disadvantages of a large target are that you have to store it someplace and it dominates the landscape. A tiny middle-gray target, such as the 2-inch squares that were supplied with my spot meter, is that you can hide them in the scene and later use them in software for color and exposure correction.
My standard target is a board that I created by drawing a line down the middle and then painted it with neutral gray paint such that one half is exactly twice as bright as the other. The fact that the halves are exactly one stop different was convenient for zone system calibration, and was achieved fortuitously: In 1968 I went to a paint store, showed the owner an 18% card, and said I'd like a quart of neutral paint about that density. He took some white paint base (titanium dioxide) and mixed a whole lot of carbon black into it. Then, on a hunch, I asked him to make another quart of paint with half as much carbon pigment. Bingo. When the paint dried, I had a 1-zone test board.
I went to my local paint store with my spot meter, picked out a bunch of gray color swatches from the rack, and went outdoors. I measured them with my spot meter, and compared each with my 18% gray card many times. I used late-afternoon December sunlight. After about a half hour, the 20-degree temperature made precision seem less and less important, and it was impossible in the wind to keep the swatches at a constant angle to the sunlight to get reliable readings from each. So I retreated into the store and finished under bright fluorescent lighting. Sigh.
Because I do not have either a hand-held colorimeter or a standard light source, I asked for the pigment formulas of several candidate swatches, and picked the most neutral (base + black) for the middle gray. I repeated all the metering over and over again, noting that a 0.1-stop difference between successive readings was about equal to no change. I then had 3 quarts of custom paints mixed. The owner made swatches of the mixed paint, dried them. Under the flourescents, they were about 1.2 stops apart; outside, under the light of pure blue cloudless winter sky, they were precisely 1.0 stops apart. I forgot to measure them when we had some sun 3 days ago, but hey, they're close enough for gov'ment work. All we want to do is to draw a reproducible curve and be resonably close to the standard.
The paints are:
Benjamin Moore quarts of flat paint:
- Zone VI: #2120-50 "Silver Spring" = 1 qt of 215-1B base with (22 BK) & (1xOG)
- Zone V: #2126-30 "Anchor Gray" = 1 qt of 215-3B base with (1x30 BK) & (1x4 WH)
- Zone IV: #2126-10 "Black Tar"=1 qt of 215-4B base with (2x27 BK) & (4 WH) & (1 OY).
This one's pretty dark. The problem with dark paint is that any "polish" on its surface creates specular reflections that increase reflectance locally.
Paint this onto something sturdy such as a board -- chipboard works fine if it's 3/4 inch thick -- plywood of 3/8 inch shouldn't be too prone to warping. You may want more than one coat to cover well and to permit you to clean it with fine sandpaper. Sand the final coat with very fine emery paper to remove any gloss.
If you do this, you will want access to a densitometer. These are expensive. Hospital xray departments have them. It's part of their quality control process. I would suggest that you clean up, dress professionally, and call on your local hospital xray (radiology or imaging) department and ask for the head technician. A weekday afternoon between 1 and 3 is probably the best time because mornings are typically crushed with work; otherwise, telephone first and ask. Bring some test film with you, just in case. This facility will own a densitometer unless they have gone all-digital (the big ones may have) -- in which case you may be able to buy a used densitometer very, very cheaply.
You may need to say, "I just have a question. I have nothing to sell, and I'm not asking for any work to be done." This will make you interesting and signal that you're not going to be annoying.
When the technician appears, explain, "I'm a photographer, and I'm doing some technical testing of film and development. To do this work right, I need to measure the film with a densitometer. Do you have one that I could access when you're not using it?"
This approach is likely to result in a quick agreement that you can come in and use it at some time. In fact, a reason to bring some film with you is in case you're given immediate access to it. Another, of course, is to prove you're not kidding. And of course be ready to make special arrangements if the tech says, "Oh, we've gone digital. We don't use densitometers any more." They may still have the old one around some place, and if you are really lucky, friendly, and polite, you might be able to help it find a good home. Yours.
If you do film work, search for the works of Richard Zakia. He was professor in the Rochester, NY, School of Photographic Arts until about 1993, when he retired. He's now living in North Carolina. The best listing of his books that I could find are at CampusI.
The best how-to-do-it reference for studying characteristic curves with simple tools and techniques is in his The New Zone System Manual Revised Edition, available cheaply in used bookstores.
