The Zone system - Part 4 Calibrating exposure and developing times, a way of achieving perfect tone control.

By Lars Kjellberg Film speed and developing times are not sacred cows. On the contrary, they often need to be adjusted for your pictures to be full in shadow detail and to control contrast. The speeds and times provided by the manufacturer should be regarded as starting points for you to make your own adjustments by. In this part of the Zone System course you will learn several different ways of calibrating for your personal needs. If you use one of these methods while going through your routines, your black and white negatives will be easier to print and the technical quality of your pictures will be better.

Why calibrate?
Exposure index (ISO, or film sensitivity setting) is calibrated because a camera's shutter and aperture aren't always correct, the exposure setting may not be correctly adjusted, and the film developed in your developing time may not hold the speed the manufacturer claims it does. If your equipment fails in a negative way (your shutter speed are too short, your aperture is too small and your meter shows values that are too high), your negatives will come out underexposed, which always results in a lack of detail in the dark parts of the picture. A simple calibration will reveal such defects and will provide a proper exposure index for your equipment, resulting in pictures with full shadow detail.
The contrast of the negative depends on several different factors. The most important ones are, of course, what film and developer you choose to work with. Every combination of film and developer demands it's own developing time. But contrast is also affected by developing time, agitation, concentration and temperature. Usually the contrast of the negative is regulated by varying the time of developing, while other variables are kept constant. The ideal contrast is not clear. It depends on the source of light in your enlarger, your choice of photographic paper and your personal taste. Besides that, it can be to your advantage to vary the time of developing with the contrast of the photography light in order to always keep the contrast of the negative unchanged. An enlarger with a diffuse light (cold light or a color head with light mixing box) demands more contrasty negatives than enlargers with condenser light. If you choose to print on a high contrast photographic paper your developing time has to be shortened so as to not make the picture's contrast too high. All this might seem difficult, with all the different variables to keep track of, but it really isn't that complicated.

The principle of calibration
The scale of tones start with black and end in white. Between black and white are a large number of grays. These gray shades relate to black and white in a predictable manner, they are always between black and white. If you calibrate black and white, you will also control all the gray shades. Black and white are at the far ends of the spectrum. Normally, the scale consists of seven stops of exposure and the point of metering is at the middle of the scale, 3 1/2 stops away from black and 3 1/2 stops away from white. When calibrating, you first check where the black and white points are located at standard exposure and standard developing. If you have to, you adjust these two points in order to control them perfectly. You will find out how to expose to hit the black point exactly, and how to develop to hit the white point exactly. When you know how to hit these two points, you can also figure out how the grays in between will fall.

Correct Exposure Index
Let's begin with a few methods of calibrating the exposure index (EI). Normally, you set the exposure meter on the camera (or a detached exposure meter) at the value recommended by the manufacturer. If you use a 35 mm camera you may even let the camera read the DX-code on the cassette. In most cases this is all that is needed, but you still recommend that you check if the shadow detail is good enough when using the standard setting.

1) The first method is the simplest and maybe the best. Place your negatives on a light table and look at them through a magnifying glass to see if there is a slight shade of gray in the lightest part of the negative (the part of the negative that turns almost black when printed). If large parts of the negative is almost completely or completely transparent, the EI should be adjusted somewhat lower. If, for example, you have used Tri-X set at 400, and the shadow part of the negative is too light, the EI should be adjusted to 250. This simple method of calibration is most accurate if you shoot an entire roll of film of a motive that has been placed in Zone 1 1/2, that is: a motive that has been exposed 3 1/2 stops below the meter reading. Expose the dark parts of the motive in Zone 1 1/2. Developed as usual and take a close look at the negatives on a light table. If the EI is correct, the dark parts of the negative should show a slight, but distinct, shade of gray.

2) Calibrating with a densitometer. Very few photographers have access to a densitometer. A densitometer is an instrument that measures blackness. All graphic art companies and photographic labs have one. Call them up and ask if you may use their densitometer to measure some negatives. Densitometers measures blackness in a logarithmic number that describes the proportions between the light that has been absorbed and the light that has been let through. If all light was let through the negative (which is an impossibility in reality) the densitometer would show the value of 0. If half the light was let through, the densitometer's value would be 0.30. A forth would be 0.60, and an eighth would be 0.90. All negatives consist of a film base with fog that absorbs some of the light. You call this base+fog (b+f). B+f absorbs light evenly across the entire negative and therefore does not affect the finished picture. To be able to reproduce on paper the darkest part of a negative, it has to be exposed so that the developed negative shows a densitometer value of 0.10 above b+f. This is what you have to measure with the densitometer.
Set the exposure meter to the films suggested speed. Photograph an evenly lit surface (a gray card for example) and expose it 3 1/2 stops below the meter reading. If the meter suggest you set the camera to 1/60 at aperture 8, you may choose to set it at 1/500 (3 stops below) and adjust the aperture to between 8 and 11 (another 1/2 stop below). Develop as you'd normally do and measure the negative with a densitometer. Let's start by measuring the b+f which is usually at about 0.30 for 35 mm, and 0.15 for 120 and sheet film. The value of the exposed negative should be at about 0.10 above the b+f. If the negative is too thin, less than 0.08 (not exposed enough) the EI should be lowered. Try 1/3 below for 0.07 and 2/3 below for 0.04. If the suggested speed of the film is 400, 1/3 below will result in an EI of 320 and 2/3 will result in an EI of 250. In a similar way the EI should be increased if the negative is too dense. At 0.14 above b+f the EI should be adjusted by approximately 1/3 and at 0.18 above the b+f by 2/3. Increased by 1/3, an ISO of 400 becomes 500, and increased by 2/3 it becomes 640.

3) Calibration using a 0.10 ND-filter (a gray filter with a blackness measured to 0.10). Kodak sells ND-filters of different blackness values. This filter can be used as a reference when you evaluate the correct EI.
The ideal blackness for a negative that has been exposed 3 1/2 stops below normal, is 0.10 above the b+f. Therefore you need to use an unexposed and developed negative (filmbase+fog) with an ND-filter placed on top of it as a reference when the correct EI is evaluated. Start by photographing a detail of a motive that has an even tone and is evenly lit, for example a gray card. Expose it at 3 1/2 stops below the meter reading. Develop as you normally would. Compare this on a light table to the gray shade of the b+f+ND 0.10. If the developed gray is lighter than the reference, the EI should be decreased. Your equipment is underexposing, which can be compensated by a decrease of EI. Do another shoot with a decreased EI and compare it again to the reference. If you find it difficult to compare them just by looking at the negatives, they can be printed on a high contrast photographic paper. A high contrast paper provides the most visible result and is easier to evaluate. Print the b+f+ND 0.10 to a middle gray shade on the hard paper. Then print the exposed and developed negative at the same exposure time and compare the developed prints. If the b+f+ND 0.10 turns a lighter shade of gray on the print, your EI is too high and your negative is too thin (underexposed).

Calibrating the normal developing time
To balance the contrast of the negative, the developing time needs to be calibrated. Here are some different methods for calibration.

1) The visual method. It's rather hard to judge whether the developing time is correct or not just by looking at the negatives. It's essential to first make prints and then evaluate if the contrast in the prints is correct. Print on a paper of average grade. Print more than one negative and compare the dry prints in a normal light (not too bright). Do you experience the contrast as being correct? Or are the prints too low or high in contrast? If the prints are too contrasty, the developing time should be shortened. If the prints are too soft and gray, the developing time needs to be longer. You may use a more sophisticated way of deciding your developing time, but this visual method should be the definitive check. If you think that the prints are generally too harsh, the developing time should be shortened no matter what the densitometer says!

2) Contrast can easily be measured with a densitometer. Expose two negatives, one black-point negative and one white point-negative. One should be exposed 3 1/2 stops below the meter reading and the other 3 1/2 stops above the meter reading. The white-point negative is, after developing, the dark negative, and the black-point negative is the light one. Measure the blackness in both of them and calculate the difference. If, for example, the white point negative has a blackness of 1.42 and the black point negative has a blackness of 0.32, the difference in blackness is 1.10. The contrast is 1.10. As mentioned earlier, enlargers have different sources of lights and they demand negatives of different contrast. The approximate negative contrast desired when printing in an enlarger with diffuse lights is 1.20. A print made in an enlarger with a condenser light should measure between 1.00 and 1.05 in contrast. These values are approximates and should be regarded as starting points for your own printing. Personal taste may require negatives of deviating contrast.

3) Print the white point and the black point negatives simultaneously in the enlarger. Place them in the negative carrier in a way that ensures that half of each negative is printed. Between the negatives is a small slice consisting of the b+f alone. In the finished print, this slice should come out completely black, the black point should come out almost black and the whitepoint almost white. If the white point is too dark, then the contrast is too low. You need to increase your developing time. If the white point is too bright, the contrast is too high, and this can be adjusted by shortening the developing time.

4) Use a fact sheet. The contrast of a negative can be described by another number: contrast index (CI). The contrast index describes the inclination of the film curve. An ideal CI for a negative that is to be printed in a condenser enlarger is 0.50. The corresponding number for a print that is to be printed in a diffuser enlarger is 0.57. Some manufacturers provide fact sheets with charts showing the developing time appropriate for a certain CI. Along the y-axis of the chart are different developing times and along the x-axis are the different CI values. The chart has a number of lines and every line represents a developer. Suppose you are going to shoot T-MAX 100 and develop it using D-76. You want the negative contrast most suitable for a diffuser enlarger, this means a CI of 0.57. Draw a line parallel to the x-axis at 0.57 until that line crosses the D-76 line. Draw another line down to the x-axis and read the developing time, which according to the Kodak fact sheet is 8 minutes. This method is very simple and doesn't demand any darkroom work. The drawback is that not all manufacturers provide fact sheets like Kodak do.

Calibrating plus- and minus times
For calibrating plus- and minus, you can use a similar method as when calibrating the developing time. Normal developing assumes that the contrast of the motive spans seven stops. That is why you expose at 3 1/2 stops below and 3 1/2 stops above. You get a difference of seven stops. At minus 2 developing the contrast is nine stops. The two frames exposed are minus 3 1/2 and plus 5 1/2. At minus one, developing it is minus 3 1/2 and plus 4 1/2. The corresponding frames for plus 1 developing is minus 3 1/2 and plus 2 1/2. The contrast should be the same in all examples, and this is done by varying the developing time. Start from the following approximate times: For minus 2 developing time should be shortened by 30 %. For minus 1 by 15 %. For plus 1 time should be increased by 20 % and for plus 2 by 40 %.

 

Recommended Books and Supporting Reading Material

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