Pre-flashing is a very useful technique that doesn't really fit anywhere else. It doesn't take long to describe so we hadn't the heart to charge for it. It is a means of getting more highlight detail at a given exposure.
Everyone is familiar with the problem. You make a print that is perfect except that there is insufficient texture and detail in the highlights. The picture on the right makes it clear. You can see a diagonal line across the front of this Greek chapel, with much more texture on the dark side than the light side. This is because part of the image (the darker part) has been pre-flashed and part has not.
The effect here is somewhat crude, which is an inevitable consequence of web reproduction with reasonable download times, but if you try it yourself you will find that it can be very subtle and can bring in highlight detail in an almost magical way.
Pre-flashing relies on overcoming the inertia of the paper. You can re-read the free module on density, etc., or for a brief recapitulation, read on.
Any photographic emulsion requires a certain minimum exposure before it will record any image at all. This minimum exposure is called the inertia. If the highlights do not receive enough exposure to overcome the inertia of the paper, they will lack detail, even if you can see that detail on the negative (the darkest part of the negative, of course).
Of course you can simply give more exposure, but this will inevitably darken the rest of the picture as well -- the mid-tones and the shadows. Pre-flashing enables you to extend the effective dynamic range of the paper. Dynamic range is the range across which you can see both shadow detail and highlight detail: there is more about it in both Subject Brightness Range (free module) and the planned chargeable module on Paper Grades which we haven't finished yet.
As its name suggests, pre-flashing is a technique of giving the whole paper a weak, brief, uniform exposure. This overcomes the inertia and ensures that you will get some detail in the highlights. The word 'flash' is slightly misleading, as it may well be a few seconds' exposure at a small aperture.
The usual technique is to raise the enlarger head as high on the column as it will go (for ease of reproducibility); set the aperture fairly small; defocus the lens; and give an exposure of a few seconds. Anything too short may be hard to time reliably; anything longer than about 10 seconds is rather tedious. Another possibility is to use a weak, bare bulb at a set distance, instead of an enlarger. Because we have two enlargers in our darkroom, we normally use one for the pre-flash exposure and another for the actual printing exposure: it saves winding the printing enlarger up and down. Although we have two identical enlargers (Meopta Magnifaxes with Meograde heads) there is no need for the pre-flash enlarger to be anything fancy: any old rattletrap, with any old lens, will do. You can probably find someone at your local camera club who would give you one.
The aim is to give the longest exposure that results in no density at all. This may sound odd but as soon as you try it you will see how it works.
Take a piece of paper -- a test strip an inch wide and a few inches long (2-3cm x 10-15cm) is perfectly adequate -- and make a series of exposures from 0 seconds upwards (the 0 is to give you a pure white for comparison) with no negative in the carrier. Let's say you try 0, 2, 2.8, 4, 5.6 and 8 seconds at f/16. These correspond, of course, to half-stop intervals. Develop and dry the test strip.
With any luck, you will find that part of the test strip is blank; then there is a very faint grey; then denser greys. The correct exposure for the pre-flash is the last one before there is any perceptible density. Once you have determined the correct exposure for a given paper/developer combination (at a given enlarger height and lens aperture, of course) it remains constant: you do not have to test it again, though of course, it does not take long to do so.
If even the shortest exposure gives a perceptible grey, try again with a smaller aperture or the enlarger further up the column. If none of them gives a perceptible grey (unlikely), try again at a wider aperture.
Pre-flash a whole sheet of paper at the appropriate exposure, then print normally. That's all there is to it.
Unfortunately, you cannot pre-flash large batches of paper. After a few hours, or at most a few days, the pre-flash wears off thanks to latent image regression: the displaced electrons fall back into their previous orbits. This is why the manufacturers don't do it for you!
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© 2005 Roger W. Hicks