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Pre and Post Flashing
Pre and Post Flashing
Published by Les McLean
17th December 2008
Default Pre and Post Flashing


Contrast control is the key to making the fine black and white print. Clearly, there are many other considerations, factors such as tonal range, image size print colour to name just three, all play a very important role in the search for the ultimate black-and-white print which best portrays the subject in the way it was visualised. To achieve that aim we need to be aware of, and sometimes use, every method in the book to express the thoughts and feelings experienced at the time we make the exposure. One very old dodge that I frequently employ is the use of white light to pre or post flash the paper to control contrast.


Flashing is a method of controlling contrast by pre-sensitising the paper with an exposure to white light that overcomes the paper’s inertia. The initial exposure to white light will produce no tone it simply eliminates the inertia to light. Therefore, when exposing the paper with a negative in the enlarger, known as image forming light, a certain amount of the exposure is not producing tone, and more importantly, detail. However, when flashing is employed, all units of image forming exposure passed through the negative will produce tone, therefore less exposure is required. Consequently, because of the reduced exposure the lower values are less likely to block up, resulting in better separation in the shadows. At the other end of the tonal scale the highlight detail is improved because, in effect, the paper is getting more exposure to the image forming light that produces tone and detail. Because the inertia is overcome by the white light flashing exposure all light transmitted through the negative is “working” to ensure that the detail on the negative is recorded on the paper, hence the lowering of overall contrast and the improvement of detail throughout. In describing how I make the prints illustrating this method I use two different terms to describe the light, “white light”, is any source of light that can be used for flashing, for example an enlarger with no negative in the carrier, or the Paper Flasher that I use. However, it is important that you can accurately control the time of the exposure and that the light source will provide consistent illumination. “Image forming light” is the light projected on to the baseboard by the enlarger with the negative in place.

You need a second light source, attached to an accurate timer to make the flashing exposure, either a second enlarger or the Paper Flasher manufactured by RH Designs ( Flashing can be carried out with only one enlarger in your darkroom but it involves removing the negative but that is not really practical. A second method with only one enlarger is to leave the negative in the carrier and place a piece of diffusing material, such as Opal Perspex under the lens, similar to the procedure used when employing a colour analyser in colour printing. This method will make the flashing exposures quite lengthy. I use the Paper Flasher that can easily be attached to the enlarger and find it very quick, convenient and accurate.


Using a black card cover a section along the whole length of the test strip. With a pencil put indicator marks at about 1” intervals along the opposite edge of the test strip, 8 to 10 should be sufficient. Expose the test strip to white light in the same way as you would when making a normal test strip, I use 1 or 2 second intervals. Move the card in sequence through each of the remaining indicator marks exposing for the chosen time at each one. Process the test strip in your normal way and after a short wash, dry, in either a microwave oven or with a hair dryer. The maximum pre-flash exposure is the one before the step that shows tone. In the example here (fig1) I used increments of 2 seconds and the first tone can just be seen at 4 seconds so I used 3 seconds as the correct pre-flash exposure. To make a print that requires a pre-flash exposure all tests, as well as the prints, should be pre-flashed prior to the image forming light exposure to ensure that you maintain consistency.


The image of the tree and branches was made on a bright morning in early spring and I visualised a final image that conveyed the aged texture of the branches contrasting with the softness of the leaves and grasses. I also wanted to retain the impression of the sunlight spreading across the background grass. To retain the delicacy in the leaves required a softer paper grade but I needed to use a harder grade to retain the texture in the branches. I decided to use Oriental Seagull VC with grade 3.5 dialled into my enlarger to give me the contrast required to show texture, and I knew that I could control the delicacy and softness of the leaves and grass by using the pre-flashing technique.

I made a normal test strip that was pre-flashed as described above to determine the correct exposure for the final print. Having determined the correct image forming light exposure, I covered part of the sheet of paper before giving the pre-flash exposure to show the effect of flashing (fig 2). The final print received 3 seconds pre-flash followed by an image forming exposure of 8 seconds. No further manipulation was required. (fig 3)

I could have compromised and made the final print on a hard grade to show the required texture in the branches but the highlights would have been too bright and without detail. Burning in was never an option for the delicate mid tones in the fine twigs would have been unacceptable and too dark. Making a print on a softer grade to render the correct tonality in the grasses and leaves would not have produced the grittiness required in the textured branches. Pre-flashing the paper when faced with these problems is simple and provides the exact result that I visualised.


When faced with a particularly difficult high contrast negative, where the highlights appear to be completely burned out, I use controlled fogging to help print in the detail. Fogging actually puts tone on the paper whereas flashing simply eliminates the inertia. A correctly pre or post flashed and developed sheet of photographic paper will show no trace of tone, but a pre-fogged sheet will clearly have an even grey tone. How then can we fog paper and retain delicate highlight detail and not end up with a degraded flat image?


When I made the exposure for this image the excessive contrast dictated that I should over expose and under develop the film, but I knew that I would still have some problems when I made the final print. I exposed the film to record detail in the dark metal stove and marked the film for minus two stops development. However, I knew that I would have to post fog the paper to have any chance of making a balanced print. Print 1 is a straight print on grade 4 Ilford Warmtone VC paper (fig 5).

The tonality and detail in the walls and stove are as I visualised but the highlights are totally burnt out. I made a second print that was pre-flashed as described above, and also burned in the highlights with image forming light that did produce a little more detail in the highlights but it is still unacceptable.

Print 3 (fig 7) was made on Ilford Warmtone VC grade 4 and received the same pre-flash and image forming light exposure and burning in as print 2 (fig 6). The window and open door were given an additional 10 seconds exposure with image forming light and with grade 4 dialled into my enlarger. Finally, I gave the print 9 seconds post fogging exposure and selectively burned in the highlights with white light for a further 20 seconds. Compare the final print with print 2 to see the effect of the deliberate fogging, particularly in the highlights. Note the difference in the detail in the window and building seen through the open door.


After I had determined the image forming light exposure for the stove and walls I made a post fog test strip. First I pre-flashed the tests strip, this was followed by the image forming light exposure and finally burned in the table and open door for the additional time that I determined from the original test strip. I left the test strip on the baseboard with one side covered with a black card to provide a reference point when I assessed the result. Using my Paper Flasher I then exposed the test strip to white light for increments of 3 seconds, (fig 4). You can see that a 9 second exposure puts tone in the very bright highlight on the ground outside and the extra burning in with white light to the windows and open door has successfully shown detail but retained the impression of the bright light outside the building.

The final print has received a base image forming light exposure and burning in selected areas with image forming light. In addition it has received a 3 second pre-flash exposure to white light, a 9 second post flash exposure followed by a further 20 seconds of selective burning in with white light. The brightest highlights have received 32 seconds exposure to white light but the print has not lost the luminosity conveyed by the very bright outside light. When making the test strips to calculate the various exposures required to make this quite complicated print it is essential to be consistent, before I begin printing I spend a little time planning the steps required to produce the final print. In making this print the sequence used to make the various “white light” and “image forming light” exposures is critical, and is as follows: 1) pre-flash exposure: 2) base image forming light exposure: 3) burn in with image forming light where required: 4) post fog whole print: 5) post fog burn in: The test strips were made in this order with each strip given the previous strip’s exposures, for example, test strip 3 was given 1 and 2 exposures first, I call this incremental printing. Having worked out the required exposures using test strips, it is essential that the final print is made in exactly the same sequence.


Some printers now consider that techniques such as flashing are redundant because of improvements in variable contrast paper. Certainly, the facility to use more than one grade on the same sheet of paper, or use my own method of split grade printing, has given us more flexibility when dealing with problem negatives. However, I said at the beginning of this article I believe that we should use all methods available to us to produce The Fine Print and I hope that this description of the old method of flashing will convince those who perhaps doubted that statement. I believe that this method of post flashing can be useful in many applications when printing high contrast negatives. Precise control can be applied when using post flashing together with hard filtration. Burning in difficult highlight areas, using white light, which is simply fogging paper, is also an application that I have been using for several years. The pursuit of The Fine Print that “sings” requires us to use all available methods, flashing is but one. Put them all together and with enough practice every print made in your little darkened room could prove to be a little gem.

Good printing and enjoy the flashing.
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By B&W Neil on 17th December 2008, 11:38 AM

Les, many thanks for posting this - great creative stuff RH Designs Paperflasher is one of my favourite tools in the darkroom - thank goodness you and Richard designed this as I used to stuggle with flashing with the enlarger before it arrived.

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By Trevor Crone on 17th December 2008, 11:46 AM

Thanks Les, great article. I really must get one of Richard's paper flashers. Removing the negative carrier+negative each time can be a bit of a pain.
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By B&W Neil on 17th December 2008, 12:00 PM

Originally Posted by Trevor Crone View Post
Thanks Les, great article. I really must get one of Richard's paper flashers. Removing the negative carrier+negative each time can be a bit of a pain.
Trevor, they are great little gadgets and they can do much more than just flashing the whole of the paper. Mine is hardly ever attached to the enlarger and I mostly use it hand held (keeping the distance constant) and flash small sections of the print which I deem need flashing. A good example would be just flashing a part of the sky. I believe this is covered in the manual but it is along time since I have looked. Flashing a whole sheet is so very easy with Flasher

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By Trevor Crone on 17th December 2008, 12:49 PM

Cheers Neil.
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By lee l on 17th December 2008, 03:01 PM

Originally Posted by Trevor Crone View Post
Thanks Les, great article. I really must get one of Richard's paper flashers. Removing the negative carrier+negative each time can be a bit of a pain.

Not to take anything away from the RH Designs product (I love my Stop Clock Pro), but until you get one of their flashers you can do flashing without removing the negative and carrier. I used to do about 500 custom B&W prints weekly in a small lab, and found it necessary to use flashing to get my best work out. I used a piece of 1/16 inch thick translucent milky/opal perspex over the enlarging lens at a percentage of the full exposure. In my case that worked out to about 20% of the overall exposure on a very consistent basis. I had to work fast, so had no time for a lot of fine tuning, but this approach allowed me to get the work out at higher quality and in good time. No doubt an enlarging meter would have helped for fine tuning the flash, but I had none to hand.

The RH Designs Flasher will no doubt be easier to calibrate.

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By Trevor Crone on 17th December 2008, 04:04 PM

Thanks for that Lee. Although I don't do a lot of flashing I've been meaning to get myself a piece of opaque perspex/glass. It would be useful to give it a go.
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By Argentum on 17th December 2008, 04:38 PM

My durst modular 70, like a lot of enlargers, has a red filter which you can swing under the lens for use whilst composing on the baseboard. It also has a second filter on that stalk which is a diffusion filter which is for flashing. i.e. these diffusion filters can be obtained to fit at least durst enlargers.
My Durst L1200 only has the red filter so I bought some diffusion material and made a little square filter holder for it which I just hold under the lens for flashing on that enlarger. If I ever come across a red filter holder for sale of the correct size for my L1200, then I'll snaffle that and fit it with diffusion material.

The diffusion material I use is "Leelux 400" and I use two layers of it in my home made flashing filter. That gives around 10% print time for a paper flash. Leelux 400 can be obtained from any supplier of Lee filters in sheets.

As mentioned White Opal perspex works too.
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By Trevor Crone on 18th December 2008, 10:58 AM

Just ordered a sheet of white opal perspex, A4 x 3mm thick on Ebay, just under £3 including postage. Will either hold it under the lens with the negative in place or attach a piece to the enlargers swing filter post. Thanks guys for all the suggestions.
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By Dave miller on 18th December 2008, 12:04 PM

Originally Posted by Trevor Crone View Post
Just ordered a sheet of white opal perspex, A4 x 3mm thick on Ebay, just under £3 including postage. Will either hold it under the lens with the negative in place or attach a piece to the enlargers swing filter post. Thanks guys for all the suggestions.
I have found that flashing with the negative in place is not always as successful as flashing with white light. I put this down to the fact that no matter how thick the diffuser I am still trying to flash with a defused image which varies the light intensity in proportion to the negative density.
The effect can best be seen when exposing pen lines. If they are exposed though a diffuser with the negative in place, one gets a graduation of tone (which may range to black) instead of a pure even black that is obtained using a pure white light.
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