Personalizing Your Film Speed, Part 1

Copyright 1994, Tom R. Halfhill

[ CAMERA AIMED AT GRAY CARD ]

Use a setup like this for the second part of your film-speed test.


How fast is an ISO 400 film?

If that sounds like an obvious question -- such as "Who's buried in Grant's tomb?" -- think again. The real answer isn't quite so obvious. An ISO 400 film might well have a true film speed of 400. But it might also be 640 or 320 or 200.

Experienced photographers know that many films yield better results when used at nonstandard ratings. Even the film manufacturers advise that ISO ratings should be regarded as starting points for your own experiments. No standardized film-speed rating can account for the inevitable variations in equipment and personal technique.

For example, some photographers routinely shoot their color slide film at one-third stop under the manufacturer's rating to achieve richer color saturation. Others shoot color print film at one-half to a full stop over the standard rating for finer grain and more shadow detail.

But color films aren't nearly as flexible in this regard as black-and-white films. B&W offers more opportunity for personalizing your film speed because you have much more control over the development process. You can alter the developing time and temperature, increase or decrease the dilution of the developer, switch to an entirely different developer, and experiment with various styles of agitation. All of these things will make measurable differences, and you don't have to worry about color shifts and other problems that go along with the nonstandard development of color films.

Let's clear up one point right away. This article isn't about "pushing" or "pulling" -- the deliberate attempts to either speed up or slow down a film's effective speed by manipulating exposure and development. Instead, we're talking about finding the optimum "normal" speed of a film when exposed and developed with your equipment and procedures. This is the speed at which the film delivers the best possible combination of sensitivity, contrast, gradation, grain, and sharpness. Pushing and pulling are specialized techniques that come later. In fact, until you've determined your film's true normal speed, you won't have an intelligent starting point for manipulations such as pushing and pulling.

If you're blindly shooting and developing any black-and-white film according to the manufacturer's standard instructions, there's a fairly good chance you're not getting optimum results. Film manufacturers have to generalize because they aren't familiar with your equipment and techniques. Personalizing is up to you, and it's a lucky coincidence indeed if your optimum film speed and development time just happen to match the standard recommendations.

How should you go about personalizing your film speed? You could start shooting roll after roll at different speeds and soup them all in different developers, but aimless experimentation is probably worse than no experimentation at all. A more methodical approach will save you time and money, and you'll end up with better results. By following the directions in this article and Part 2 next month, you should be able to personalize your film speed after shooting and developing only two or three 36-exposure rolls of film.

Eliminating Variables

Because so many variables are involved -- camera, film, developing time, temperature, developer, dilution, agitation, and more -- let's start by eliminating a few if we can. Later, if you want, you can follow up your initial experiments with additional tests to fine-tune the process. But if you experiment with too many variables at the outset, you'll only wind up more confused than when you started.

First, let's limit the testing to one camera, one film, and one developer. Pick the camera, film, and developer you use most often for black-and-white work. If you normally use a separate hand-held light meter instead of the camera's built-in meter, then limit the test to that meter. If you rely on your camera's meter, you must use a camera which allows you to manually set the film speed.

Almost all modern 35mm cameras automatically set the manufacturer's recommended film speed by reading the DX codes (those black and silver squares) on the film cassette. Some cameras don't let you override the DX coding, so there's no practical way to personalize your film speed. If you're not sure about this, check your instruction manual to see if there's any way to manually change the ISO or ASA setting. (ISO is the modern term for ASA, but the numbers mean the same thing.)

Also, the camera you use should have a metered manual mode or some way of overriding the automatic exposure setting by plus or minus four f/stops. Again, refer to your manual.

It's important to realize that the results of your tests cannot be generalized to cover other cameras and meters, even when using the same film and developer. That's why I can't save you the trouble of this testing merely by telling you what works for me or anyone else. From one person to another, there are always variations between camera shutters, light meters, metering methods, and numerous other factors. The whole object of personalizing your film speed is just that -- to arrive at a personal  film speed that gives the best possible results with your own equipment and techniques. There's no shortcut.

Shooting a Test Roll

Let's start by shooting a 36-exposure roll of film. Ideally, you should test your film under the same conditions encountered in your everyday photography. If you shoot most of your pictures outdoors, then you should shoot your test roll in daylight. On the other hand, if you pursue indoor photography using studio floods or flash, you should expose your test roll under those types of lighting.

Begin by taking a series of shots at different film speeds around the manufacturer's standard ISO. (Important: Keep a written record of every shot you take during this test, including the ISO, shutter speed, and f/stop. Neglecting to do this can lead to confusion later.)

Assume you're shooting a typical outdoor scene. Try to find a scene that includes a broad range of highlights and shadows. Bright sunny days are best because the light is more contrasty, so it's a tougher test for your film. Also, the light is more dependable than on partly cloudy days, when barely noticeable variations in the fickle sunlight can throw off your testing.

If you're shooting an ISO 400 film such as Kodak Tri-X or Ilford HP5 Plus, take the first picture at the manufacturer's recommended rating of 400. Shoot the next three pictures at the next higher ratings above 400: 500, 640, and 800. Then shoot the next three pictures at the next lower ratings below 400: 320, 250, and 200. For reasons I'll explain in Part 2, I advise taking three more shots at even lower ratings: 160, 125, and 100.

Should you take these pictures with your camera in manual mode or automatic? I recommend using whatever mode you normally use in your everyday photography. That way, your personalized film speed will compensate for any metering bias built into your camera. But it's very important to make certain the camera is metering each shot identically. If you're shooting on automatic, don't switch modes during your test -- for example, don't change from center-weighted averaging to spot metering. Otherwise, the camera will measure the light differently and mess up the results.

For the same reason, make sure the camera is aimed exactly the same for each shot. Using a tripod wouldn't be a bad idea. Avoid spot metering modes, and don't change lenses or zoom settings. All of the pictures in this series should be identical, except for the ISO ratings.

You may find that certain ISO ratings won't work under the prevailing light conditions. On bright sunny days, a rating of 800 may not be possible unless your camera has shutter speeds faster than 1/1000 or a lens aperture smaller than f/16. In auto mode, your camera may signal an overexposure warning. If so, take your pictures in the shade instead of in bright sunlight. But find a shady place where the light doesn't change from shot to shot.

If you're measuring your exposures with a hand-held light meter and shooting in manual mode, you'll run into another problem. Film speeds are spaced one-third of an f/stop apart, but aperture rings on lenses work only in increments of half a stop. Some modern electronic cameras let you adjust exposure in one-third stop increments, but otherwise you won't be able to reach the intermediate settings without guessing. Don't worry about it. Shoot your pictures at half-stop increments and forget about the one-third stop settings. The error between one-third and one-half is only about one-sixth of an f/stop, which isn't terribly significant.

The Blank-Card Test

After you've completed this series of ten exposures, it's time for the second part of the test (on the same roll). For this you need to find a fairly large surface that's evenly lit with no shadows or bright spots. The ideal target is a smooth, neutral-colored sheet of cardboard large enough to completely fill your viewfinder. It's not necessary to use a photographic gray card (a special card which is a calibrated shade of medium gray). However, the surface should be of uniform tone and texture. Avoid strong side-lighting, and be sure you aren't casting a shadow on the surface when you aim your camera at it. Most importantly, point the camera straight at the surface and make sure you can't see anything else in the viewfinder. It's not necessary to bring the image into focus. (See photo.)

Set your camera or meter for the manufacturer's standard ISO. Shoot your first picture at whatever aperture and shutter speed the camera or meter chooses. If you're using a hand-held meter, take a reflected (not an incident) reading and point the meter straight at the target so it's not reading anything else.

Now increase the exposure by three f/stops above the normal setting and shoot a second picture. It doesn't matter if you increase the exposure by changing the lens aperture or the shutter speed or some combination of both. However, maintaining the same shutter speed is preferable if your camera has a mechanical shutter. Mechanical shutters are less accurate than electronic shutters, so they introduce an unwanted variable into the test. (It's easy to tell whether your camera has a mechanical or an electronic shutter -- if it'll fire at all shutter speeds without batteries, it's mechanical.)

Return the aperture and shutter speed to the same settings used for the first picture in this series (the normal exposure). Then decrease the exposure by four f/stops and shoot a third picture. Again, you can decrease the exposure by changing the lens aperture, the shutter speed, or a combination.

So far you've taken three pictures in this series at the following settings: 1. The correct exposure as chosen by your camera or meter. 2. Three stops over the correct exposure. 3. Four stops under the correct exposure.

Now change the ISO setting and repeat the series. Keep repeating the series at different ISO settings both above and below the manufacturer's standard ISO until you run out of film. For example, with an ISO 400 film, you should take a series of three shots at each of the following ISO settings: 400, 320, 250, 200, 160, 125, 500, 640, and 800.

That should finish up your 36-exposure roll. Develop the film according to your normal routine. The manufacturer's standard developing time and temperature is a good place to start.

Incidentally, it's a good idea to mix and dilute the film developer with distilled water instead of tap water. This eliminates one more pesky variable, because the quality of tap water can vary from place to place and even from season to season in the same town.

Next month, in Part 2 of this series, I'll explain how you can determine your personal film speed by making a few test strips and prints from your finished negatives. The results may surprise you!

Table 1: Example Test Roll for an ISO 400 Film

Frame #ISOSubjectExposure
1400typical scenenormal
2500typical scenenormal
3640typical scenenormal
4800typical scenenormal
5320typical scenenormal
6250typical scenenormal
7200typical scenenormal
8160typical scenenormal
9125typical scenenormal
10100typical scenenormal
11400neutral surfacenormal
12400neutral surface +3 stops
13400neutral surface -4 stops
14320neutral surfacenormal
15320neutral surface +3 stops
16320neutral surface -4 stops
17250neutral surfacenormal
18250neutral surface +3 stops
19250neutral surface -4 stops
20200neutral surfacenormal
21200neutral surface +3 stops
22200neutral surface -4 stops
23160neutral surfacenormal
24160neutral surface +3 stops
25160neutral surface -4 stops
26125neutral surfacenormal
27125neutral surface +3 stops
28125neutral surface -4 stops
29500neutral surfacenormal
30500neutral surface +3 stops
31500neutral surface -4 stops
32640neutral surfacenormal
33640neutral surface +3 stops
34640neutral surface -4 stops
35800neutral surfacenormal
36800neutral surface +3 stops
37800neutral surface -4 stops

[end]


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