Personalizing Your Film Speed, Part 2

Copyright 1995, Tom R. Halfhill


This photo was taken on ISO 400 film at the rated speed and developed using the recommended time and temperature. But the correct film-speed setting for this film, developer, and camera should be 200, not 400. The photo is a full stop underexposed, resulting in a weak print with no true blacks. Proof can be seen in the sprocket holes below the image—the holes are blacker than the surrounding clear film, demonstrating that the paper is capable of a deeper black than exists anywhere in the picture. If the print were given additional exposure in the enlarger to produce pure black through the clear film, the image would be much too dark.

In Part 1 of this series, we described how to shoot a 36-exposure test roll as a first step toward personalizing the speed of your favorite black-and-white film. In this month's conclusion, you'll take your test roll into the darkroom and run some experiments which will reveal the optimum film speed for your equipment and procedures.

When the processed film is dry, examine the negatives closely. Many of the frames on the latter part of the roll will appear blank (clear, unexposed film). Those are the shots taken at four f/stops below the indicated meter reading for each ISO. Look for one of these frames that isn't quite blank, in which a small amount of density is beginning to emerge from the clear film. Check your notes to see which ISO setting was used for that shot. That's the ISO which is probably your true film speed, or close to it. Don't be surprised if it's lower than the manufacturer's standard ISO.

You'll know much more after making some prints. Set up your enlarger for a 5x7 or 8x10 and choose a normal contrast paper or variable-contrast filter. Most people consider a #2 contrast paper or filter to be normal, but others prefer #3. It's not unusual to find slight differences from one type of paper to another, and variable-contrast papers tend to change in contrast as they age. These are all good reasons why it's so important to establish a personalized film speed, and why the results of these tests can't be applied to other people's equipment and techniques.

Making Test Strips

Before making any full-size prints, you must first make a few test strips. However, these test strips will probably be a little different from any test strips you've made before. The object is to determine the minimum amount of exposure required to produce maximum black on your print paper.

Let's talk about this for a moment. Every kind of print paper has its own maximum black, also known as D-max (maximum density). This is the blackest black the paper can produce. For this test, your goal is to find the shortest enlarger exposure that will yield the D-max. No amount of additional exposure—even three hours of raw sunlight—would make the paper any blacker.

To run this test, find a blank frame of film. Be sure the frame is truly blank (completely clear) and from the same test roll of film you've just developed. The unexposed part of the leader or trailer will work fine, as will any blank frame within the roll. Place the blank negative in your enlarger just as if you were making a 5x7 or 8x10 print. Focus the enlarger and close down the lens to f/8 or so. (To focus, you may have to substitute another negative, then switch back to the blank negative.)

Now make a test strip. Place a strip of print paper on the easel, cover up all but a small portion with a piece of cardboard, and expose it for five seconds. Slide back the cardboard to reveal a little more of the test strip and expose for another five seconds. Repeat until the entire strip has been exposed. Try to fit at least six exposures on the strip.

Develop the test strip as you'd normally develop any print. Resin-coated papers should be developed for at least 60 seconds, and fiber-based papers for at least 90 seconds. Rinse the strip in stop bath, place it in the fixer tray, and switch on the white lights.

Remove the test strip from the fixer and allow it to dry. Wet paper always has a darker D-max, so don't base your observations on a wet test strip. (Resin-coated papers start drying immediately, but fiber-based papers take a little longer. Famed photographer Ansel Adams sometimes used a microwave oven to speed things along. Hair dryers work, too.)

Examine the dry test strip very carefully. If you see a distinct band of dark gray for each exposure you made, you probably haven't reached D-max. The darkest band may look black, but appearances are deceiving. Open up the lens on your enlarger by one f/stop and make another test strip.

If your test strip has a few bands of very dark gray leading to a longer black area, you've reached D-max. The longer black area proves that increasing the exposure past a certain point doesn't make the paper any blacker. Now here's the important part: Find the first  band that's as black as the paper gets. This will be the band immediately following the darkest gray. This band represents the minimum exposure for maximum black. If it's the fourth band on the test strip and you exposed each band for five seconds, then 20 seconds is the minimum exposure for D-max.

Write down this exposure time. However, keep in mind that it applies only to this particular kind of print paper when exposed and developed in the same way you exposed and developed your test strip. If you switch papers, change filters, readjust the enlarger, use a different print developer, alter the development time, make another test strip tomorrow, or change anything else, the minimum exposure for maximum black will most likely change as well. So for now, don't change a thing.

Assessing Film Speed

The next test strip you're going to make will bring you very close to your film's true speed. Remember the nearly blank frame on your test roll which showed a slight amount of density emerging from the clear film? It's one of the pictures taken at four stops less than the meter reading. If you have access to a densitometer—an expensive instrument designed for measuring film densities—that negative should measure about .10 above the background density of the clear film. But since you probably don't have a densitometer, I'm going to show you another way to approximate that density. In some respects it's more practical, and the results will be quite close to what you'd get with a densitometer.

Begin by putting the almost-blank negative in the enlarger. The enlarger should already be in focus, so don't tinker with any settings.

Place a fresh strip of print paper on the easel and give it the minimum exposure for maximum black you've just determined. Then cover half of the test strip with a piece of cardboard and repeat the exposure.

After you've developed the test strip and allowed it to dry, examine it carefully under white light. Half of the strip (the part exposed twice) should be maximum black, but the other half should be a very dark gray, just barely lighter than maximum black. Look closely, because the difference might be very subtle.

If the whole strip is maximum black, make another test strip using the almost-blank negative that was shot at the next lower ISO. For example, if the almost-blank negative shot at ISO 400 resulted in a completely black test strip, try the almost-blank negative shot at ISO 320. That negative should have just a little more density emerging from the clear film.

You shouldn't have to make more than two or three test strips before getting the result we're after: a strip that's maximum black on one half, and very close to maximum black on the other half. The negative which yields that result is very close to your film's true speed. In the future, pictures taken at that film speed will capture gray tones all the way down to four stops below the normal meter reading.

Don't be surprised if your personalized film speed is lower than the manufacturer's standard ISO. In fact, it may be much lower. If your camera's shutter and meter are accurate, it's not unusual for the true film speed to be half the manufacturer's rating—say, 200 for a film normally rated at 400. I've been running these tests for more than 15 years with many different kinds of cameras, meters, films, and developers, and I have yet to discover a black-and-white film that delivers optimum results at its full rated speed. Films such as Kodak Tri-X, T-Max 400, and Ilford HP5 usually work best at ISO 125 to 200. Kodak Plus-X, T-Max 100, and Ilford FP4 usually come in at ISO 40 to 64.

I know my equipment is accurate. For one thing, I've obtained nearly identical results using many different cameras and meters over the years. I've used professional-quality cameras whose shutters were recently tuned up by repair shops. And when I use the same equipment to shoot color slide film—which has virtually no latitude for under- or overexposure—I get perfect results at the manufacturer's normal film speed. Black-and-white film, however, invariably calls for half the rated film speed or less.

Even the film manufacturers sometimes acknowledge their b&w film speeds are a little optimistic. For instance, Kodak's book Advanced Black-and-White Photography (Eastman Kodak Company, 1985) describes a film-speed test similar to the one outlined in this article. "Generally, most people find they need a number slightly lower than the film's rated speed, and sometimes your number will only be half that of the rated speed," says Kodak. "A few people will need higher speeds, because of inaccurate shutter speeds or a meter bias."

From experience, I know that many people will resist that conclusion. They don't want to shoot a 400-speed film at 250 or 200 or 160 because they don't want to lose any film speed.

But it's important to understand you aren't really "losing" any film speed by running this test. The film's true speed was determined when it was made at the factory, and setting the ISO dial at a different number isn't going to change it. If these tests reveal your favorite 400-speed film works best at 200, then persisting to shoot it at 400 will only result in lower quality. If the light is weak and you can't shoot a picture with the film rated at 200, then go back to the 400 rating. The picture will be underexposed by a stop, but the quality won't be any worse than what you've grown accustomed to over the years. On the other hand, pictures taken at your personalized film speed of 200 will be much better than what you're used to.

Confirming Your Results

Another simple test can prove this. Go to the beginning of your test roll and find the picture made at the film's standard ISO. (This is the picture of the typical outdoor or indoor scene.) Place that negative in the enlarger. Again, don't tinker with any controls—the enlarger should still be in focus and positioned for a 5x7 or 8x10 print. Now make a print at the minimum exposure for maximum black as determined by your first test strip. Develop the print exactly as you developed your test strip.

When you examine your finished print, how does it look? Is it too dark? Too light? Or just right?

If it's too dark, your first reaction may be to make another print with a shorter enlarger exposure. That's how most people make prints. But remember, you've already exposed the print for the minimum time that produces maximum black. If you shorten the exposure any further, the print will have no true blacks—only very dark grays where the blacks should be. Your print will be like a stereo system with weak bass. You will have cheated yourself of the full range of tones your print paper was designed to deliver.

This photo is almost identical to the one at the beginning of the article. It was made on the same roll of ISO 400 film and developed normally. But it was taken at my personalized film-speed rating of 200, not ISO 400. The ISO 400 shot is a full stop underexposed, resulting in a weak print with no true blacks. The ISO 200 shot is properly exposed and has strong blacks. Proof can be seen in the sprocket holes below the image—they are no blacker than the surrounding clear film. The print was given the minimum enlarger exposure required for maximum black through the clear film.

If you switch to a higher paper grade or a higher-contrast filter, you may be able to shorten the exposure and still make a print that achieves D-max. The higher contrast paper will shove the darkest gray downward into maximum black. However, it's important to realize that this effect won't be limited to the blacks. Increasing the contrast will also shove the very lightest gray tones at the other end of the scale toward featureless white, and delicately separated middle grays throughout the entire print will be merged into single gray tones. If you carry this process far enough, the result is a high-contrast print with only two tones—pure black and pure white—with no grays at all.

Switching to a higher contrast grade is fine if it's done for expressive purposes, or to compensate for unusually flat lighting, or to salvage negatives you couldn't help underexposing. But you shouldn't have to resort to higher contrast grades to print negatives which are supposed to be normal. A normal negative should print almost effortlessly on normal paper. If printing is difficult and frustrating for you, it's probably your negatives which are bad, not your printing techniques.

When minimum exposure for maximum black yields a print that's too dark, it means either your film was underexposed in the camera or underdeveloped in the darkroom. When minimum exposure for maximum black yields a print that's too light, it means either your film was overexposed in the camera or overdeveloped in the darkroom. A print that's too light is slightly preferable to a print that's too dark, because you can increase the printing exposure without sacrificing true black. Ideally, however, you want to bring both exposure and development into balance so your negatives are as close to normal as possible.

Assessing Development Time

How do you figure out if your problem is exposure or development? You guessed it—run another test.

Go back to your test roll. Remember the pictures you took of the blank surface at three stops over the meter reading? On the negatives, these pictures appear as very dark frames with heavy density. Referring to your notes, find the frame which matches the personalized film speed as determined by your previous tests. For instance, if the above tests revealed a personalized film speed of ISO 200, locate the dark frame which was also taken at ISO 200.

Place this negative in your enlarger. Again, don't change any controls or anything else. Put a fresh strip of paper on the easel and cover half of it with cardboard. Give it the minimum exposure for maximum black, then develop normally. When it's dry, examine it closely under white light.

Half of the strip should be blank white—the half that was covered by the cardboard and therefore received no exposure. This is the whitest white your print paper can produce. (By the way, it's called D-min, the opposite of D-max.) The other half of the test strip should be very light gray, just a shade away from pure white. If the entire strip is pure white, your film was probably overdeveloped. It means that details in your pictures which fall three stops above the normal meter reading won't be printable without burning in.

On the other hand, if the gray half of the test strip is a little too dark, your film was probably underdeveloped.

You might wonder why we're blaming development this time instead of exposure. If the entire test strip is blank white, why doesn't it indicate the film was overexposed instead of overdeveloped?

Actually, there might be some overexposure too, but overdevelopment is more likely. The reason has to do with the basic characteristics of black-and-white film. Generally, exposure has relatively more effect on the lower print densities, from near-black to medium gray. Development has relatively more effect on the higher print densities, from medium gray to near-white. You've probably heard the expression, "Expose for the shadows and develop for the highlights." It may have seemed too vague before, but now you know exactly what it means.

The earlier test strips helped you determine the proper amount of exposure required by your film—the true personalized film speed. The last test strip helps you determine the correct amount of development required by your film. If your entire test strip is blank white, you need to shoot another test roll and develop it a little less. A good starting point is to cut the development time about ten percent. For example, if you developed the first test roll for 7 minutes, that's 420 seconds (7 x 60). Ten percent would be 42 seconds, so you'd develop the next roll for 6 minutes and 15 seconds (rounded off).

It may take some juggling to fine-tune both your exposure and development. If you shorten or lengthen the developing time, you may discover that the film speed will vary a little. Generally, though, the film speed as determined by your first tests will be accurate to within one-third or two-thirds of a stop.

Condenser Heads

If your enlarger has a condenser head instead of a diffusion or cold-light head, you may have a bit more trouble bringing your exposure and development into balance. Condenser enlargers tend to print the highlights a little lighter, which means you have to cut the film development time to preserve the highlight detail. But the reduced development time may depress the lower tones, so you have to reduce the film speed to recover detail in the shadows. This in turn might throw the highlights out of whack again.

Switching to a lower paper grade is another alternative, but remember, the effect won't be limited to the highlights. Contrast will be reduced throughout the entire print, even though the higher-contrast effect of the condenser head is largely limited to the highlights. You can prove this to yourself by comparing a condenser enlargement of a negative to a contact print of the same negative. The highlights won't look the same.

(The condenser-versus-diffusion debate has been raging for decades, and I don't want to reopen it here. However, I will say that I have years of experience with both types of enlargers, and I'll never use another condenser head as long as I live.)

The tests described in this article may take some time. There's no easy shortcut. However, when you finally achieve perfect balance between your personal film speed and development, your rewards will be great. Not only will your negatives be as good as the film can deliver, but they'll also be a joy to print. They'll contain the maximum amount of printable detail across the full range of tones, from near-white to near-black. Tones which fall above and below these thresholds will be rendered in the whitest whites and the blackest blacks your paper can produce, lending almost three-dimensional depth to your prints. Manipulations such as burning-in and dodging will be most often used for creative purposes, not to rescue poor negatives. And even if you under- or overexpose some of your pictures, at least the errors will be distributed around the film's true speed, not a speed that's artificially low or high.

Once you're liberated from nagging technical problems, you'll be free to do what you want most—make good photographs.


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