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F-stop printing

F-stop printing - Richard Ross shows how to apply those familiar camera exposure techniques in the darkroom

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Category : Darkroom Printing
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Apply those familiar camera exposure techniques in the darkroom
Richard Ross LRPS

flashPhotographic emulsions, on film and paper, are sensitive to the amount of light falling on them, and to achieve the correct exposure that amount must be carefully controlled. Emulsions respond to the volume of light that they are exposed to and, ignoring reciprocity effects, the same result can be obtained with either a bright light for a short time or a dimmer light for a longer time. In a camera, the brightness of light is controlled by the aperture and the time by the shutter speed.

On most cameras both adjustments are made in terms of a 'stop'. One stop, usually called one f-stop, is a halving or doubling of exposure and is the basic control of light volume in all forms of photography. On a typical camera shutter speeds are 1/125sec, 1/250sec, 1/500sec etc. Each of these represents a doubling of shutter speed. In the case of lens aperture, the area of the opening controls the volume of light so to double the area, the diameter must be increased by a factor of the square root of 2, which is 1.4. The reason for the rather odd looking number sequence on a lens aperture ring now becomes clear; each is 1.4x the previous hence f/2, f/2.8, f/4, f/5.6 etc.

Photographers habitually think of f-stop exposures when using a camera without turning a hair, yet the majority abandon the principle in the darkroom and expose enlarging paper in times measured simply in seconds. As a result, they often run into inconsistencies and get unexpected results. Taking the f-stop principle into the darkroom can reap huge benefits in time and paper savings, help you to understand and predict the behaviour of materials with confidence and give you better prints into the bargain! And it's not difficult - f-stop sequences abound in photography. We've already looked at two, and a third can be found in the ISO film speed settings - for example 100, 125, 160, 200, 250, 320, 400. If you examine this sequence you can see that every third figure represents a doubling, or one full f-stop. The sequence is therefore in thirds of an f-stop. You can use any or all of these sequences when determining the exposure time for a print.

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Consider these test strips. The first uses a simple time sequence of 5, 10, 15, 20, 25 seconds. You can see that the difference in density between 5 and 10 seconds is much greater than that between 20 and 25 seconds, which makes it difficult to estimate a base exposure. Now look at the second, exposed for 5.6, 8, 11, 16, 22 seconds - a familiar sequence! Immediately you can see that the density increases across the strip are consistent. In terms of exposure timing, this is a half stop sequence. It's a full stop sequence in lens aperture terms because, don't forget, there's a square root involved. Don't be confused by the maths, it's not necessary to understand the maths to use the system, although if you are mathematically minded, the whole f-stop principle is based on the fact that a photographic emulsion's response to light is logarithmic.

Gene Nocon first popularised the principle of using f-stops for determining print exposure times in his book Photographic Printing, first published in 1987. He based his technique on intervals of a quarter of a stop and published a table of exposure times in quarter stops. He took this a stage further and designed and produced an enlarger timer that worked on the same principle. A typical quarter stop sequence is 10, 11.9, 14.2, 16.9, 20, 23.9, 28.4 etc. Gene made all his test strips in these quarter stop sequences and determined any burning in requirements in terms of f-stops, not seconds. Tables of f-stop exposure times can be found in a number of books on photographic printing and these usually also incorporate times for dodging and burning.

Dodge & Burn
It's in dodging & burning that one of the huge advantages of the f-stop method of printing starts to become apparent. Suppose you're printing a landscape and the sky needs some burning in. From your test strip, you decide that a half stop extra is required, and you make a small test print for proofing purposes. When you come to make the final big enlargement, the base exposure will need to be longer because of the larger print size, so make a new test strip to determine the new base exposure. Now, you already know that a half stop burn in is required for the sky so there is no need to establish this time from another test, just look it up from the table. Expressing burning and dodging times in terms of f-stops enables you to reproduce the print at any size, or on a different type of paper, without the need to repeat any test strips apart from the one required to establish the base exposure.

After a little practice with the f-stop method of printing you will find that the amount of testing you need to do is greatly reduced because you will start to understand how the paper responds to light - you will be able to estimate the burning in time required to increase the density by a given amount because density is exactly proportional to f-stop changes. A half stop increase in exposure produces the same result whether the base exposure is 10 seconds or 60. The same applies to dodging. Some printers express dodging and burning times in percentages of the base exposure, which means that they too can scale a print up or down easily. The problem with percentages however is that if you increase exposure by 50% and then decrease it by 50%, you don't end up back where you started, 10 sec +50% is 15 sec. 15sec -50% is 7.5sec, whereas a half stop increase followed by a half stop decrease results in the original time.

You don't need any specialist equipment to use f-stop printing methods. Starting with the lens aperture sequence f/5.6, f/8, f/11, f/16 etc, gives you half stops, and a simple average of two adjacent entries will give you quarters with only a small error - for example half way between 5.6 and 8 is 6.8, whereas the correct quarter stop point is 6.7. Similarly, the ISO sequence 80, 100, 125, 160, 200 can be divided by ten to give a third stop sequence of 8, 10, 12.5, 16, 20 seconds, with sixths of a stop determined by averaging as before. These sequences are easy to remember as they're so often encountered. Sixths of a stop are fine enough intervals for everyday printing on medium paper grades. However for the finest quality work, especially on harder papers, smaller intervals are necessary and there are advantages to be gained by using a specialist f-stop enlarger timer. Using such a timer avoids the need to memorise f-stop intervals, and usually f-stop timers incorporate exposure sequences so you can store dodging & burning times, making it much easier to produce a number of copies of a print. In addition, if you change the base exposure time the timer automatically recalculates all the burn in times for you. One source of f-stop timers is RH Designs http://www.rhdesigns.co.uk.

Once you make the change to f-stop printing, you will find that other aspects of enlarging respond to the method also - you can for example calibrate your enlarger magnification in f-stops, so if you change the print size you can read off the new exposure time. If you use a dichroic colour head with variable contrast paper, you can calibrate the filter controls so if you change the paper grade, you can read off the new exposure time. And keeping print records is easier too. Once you have made the change to f-stop printing, you will never go back!

About the author
Richard Ross has been printing monochrome photographs for 13 years. Quickly converted to the f-stop method and unable to obtain a Nocon Timer (by then out of production), he designed and produced his own f-stop timer in 1994. His business, RH Designs, now manufactures a range of enlarging timers and exposure meters.

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