Question for you paper preflashers

Steve Goldstein · Mar 12, 2021

Although I've never done it, I can understand how preflashing black-and-white paper can be a useful tool for some images. It can be done with an enlarger as the light source, but making multiple prints means a lot of negative handling as it has to be removed to preflash and then replaced to expose every print. I don't have the room for a second enlarger, even a small one, for preflashing and proofing. RH Designs sell the Paper Flasher II, which appears to use white light and has provision for filters, but I'm not aware of any similar unit.

Now that I've retired from a long career in analog and mixed-signal circuit design I've been thinking about building myself something similar to the RH Designs unit, although perhaps with less fancy timing capabilities. The basic circuit to current-drive an LED string is straightforward and is something I did 20 years ago when I built myself an LED safelight. What I'm wondering is whether I should include three sets of LEDs: white, green, and royal blue. It's not really any harder than doing a single color, just more LEDs and the ability to switch between them.

For those of you who preflash variable contrast paper, do you use green light, blue light, white light, or filtered light?

Brendan Quirk · Mar 12, 2021

I have flashed paper only a few times. Here is what I did, and it worked out well. Previously, I determined the parameters for proper flashing with my set up. When I decided I needed flashing, I flashed a few sheets before inserting the negative, and just worked with those. The flashing was done on the same enlarger, at minimum aperture, and with neutral density filters. For me, it was 5 seconds at f16 with a 50mm lens, ND4 and ND8 filters (total 5 stops density) on a Beseler 23CII at a height suitable for 8x10 from a 35 mm negative (so about 8x enlargement).

Pieter12 · Mar 12, 2021

I flash paper as needed. I use a small electroluminescent panel hooked to an enlarging timer that is on an arm that pivots out from the wall next to my enlarger. I place the paper on the table surface directly below so the light-to-paper distance is the same no matter the enlarger head position, and the flashing time is always the same. No need to move anything. I do not know the spectral output of the EL panel, but it works fine for me. If you use LEDs I would think you only need to expose for the low-contrast emulsion since that is what you're trying to boost.

Mainecoonmaniac · Mar 12, 2021

Steve Goldstein said:
Although I've never done it, I can understand how preflashing black-and-white paper can be a useful tool for some images. It can be done with an enlarger as the light source, but making multiple prints means a lot of negative handling as it has to be removed to preflash and then replaced to expose every print. I don't have the room for a second enlarger, even a small one, for preflashing and proofing. RH Designs sell the Paper Flasher II, which appears to use white light and has provision for filters, but I'm not aware of any similar unit.

Now that I've retired from a long career in analog and mixed-signal circuit design I've been thinking about building myself something similar to the RH Designs unit, although perhaps with less fancy timing capabilities. The basic circuit to current-drive an LED string is straightforward and is something I did 20 years ago when I built myself an LED safelight. What I'm wondering is whether I should include three sets of LEDs: white, green, and royal blue. It's not really any harder than doing a single color, just more LEDs and the ability to switch between them.

For those of you who preflash variable contrast paper, do you use green light, blue light, white light, or filtered light?

Before you try anything fancy, take a look at this video. Joe Van Cleave has a very inexpensive but effective solution to flashing.

albada · Mar 12, 2021

Steve Goldstein said:
What I'm wondering is whether I should include three sets of LEDs: white, green, and royal blue.

In a word, yes. Having separate green and blue lets you print all grades without using filters, and makes split-grade convenient. Separate colors is wonderful. I copied Mal Paso's final design in this thread:
https://www.photrio.com/forum/threads/led-split-printing-enlarger-lamphouse.173834/
The final design has no LEDs in the middle.

I suggest using red (not white), green, and royal blue in order to leave open the option of printing in color. Also, red allows you to position dodge/burn tools before the actual exposure starts. Turning on all LEDs provides white for composition and focusing.

Chips are available which provide the constant current needed for LED-strings. The Mean Well LDD-700L is an example, as well as the BuckBlock A009. Many more are available that contain only the closed-loop logic, and you provide the high-power components (MOSFET, inductor, sense-resistor); these closed-loop controller-chips cost only US$1.30 or so.

I recently built a LED controller that also contains a timer, which is about what you want to do. It's based on the Arduino Uno, which was a good fit for this task. The ATmega328p in the Arduino provides the PWM for the three LEDs. The software took a long time to write, and I plan to put it on github. If you decide to use an MCU as the brain, I'll send you a zip of this software if you wish.
Mark Overton

albada · Mar 12, 2021

Steve Goldstein said:
For those of you who preflash variable contrast paper, do you use green light, blue light, white light, or filtered light?

For scenes that are mainly shade with small or unimportant portions in direct sun, I preflash (or postflash; it makes no difference) with grade 00 (green only) to provide low contrast highlights instead of distracting paper-white.
Overall contrast drops, so I boost the grade by 1.5 or 2 to maintain good mid-tone contrast.

koraks · Mar 13, 2021

albada said:
The ATmega328p in the Arduino provides the PWM for the three LEDs.

If color is also on the horizon, the 8 bit pwm resolution of the 328 will not be sufficient.

RalphLambrecht · Mar 13, 2021

Steve Goldstein said:
Although I've never done it, I can understand how preflashing black-and-white paper can be a useful tool for some images. It can be done with an enlarger as the light source, but making multiple prints means a lot of negative handling as it has to be removed to preflash and then replaced to expose every print. I don't have the room for a second enlarger, even a small one, for preflashing and proofing. RH Designs sell the Paper Flasher II, which appears to use white light and has provision for filters, but I'm not aware of any similar unit.

Now that I've retired from a long career in analog and mixed-signal circuit design I've been thinking about building myself something similar to the RH Designs unit, although perhaps with less fancy timing capabilities. The basic circuit to current-drive an LED string is straightforward and is something I did 20 years ago when I built myself an LED safelight. What I'm wondering is whether I should include three sets of LEDs: white, green, and royal blue. It's not really any harder than doing a single color, just more LEDs and the ability to switch between them.

For those of you who preflash variable contrast paper, do you use green light, blue light, white light, or filtered light?

I never pre-flashed with anything but white light but colored light sounds interesting and may open up more opportunities.

radiant · Mar 13, 2021

I've been thinking of doing a "flasher" for many purposes; paper, exposing film test strips etc. Maybe even as a light table with constant light. Multipurpose thingy really?

Just a wooden box with opaque plastic sheet on top. Install WS2812 RGB leds, controlled by microcontroller. Et voila!

It should be possible to flash WS2812 for about 5 - 10ms pulses for film exposures. Greatest part is that you can control invidual colors and "brightness" too.

koraks · Mar 13, 2021

That should work fine.
I still have some big (90W) rgb modules lying around that I might use for this purpose. It's very low priority though. The thought came to me when i was doing some sabbatier ra4 prints.

David Allen · Mar 13, 2021

For pre-flashing paper there is absolutely no need to remove the negative or a second enlarger. The simplest way is to buy a thin piece of semi-opaque acrylic and hold this (or fix it with some BluTac / tape / etc) under the lens and do your pre-flash. No need to remove the negative, no need for an extra enlarger or light source, etc.

Also, if only the sky (or any other area of very high brightness compared to the rest of the image) is your problem (and despite the fact that you have intruding objects) then you can simple pre-flash the sky. The central idea to pre-flashing is that all papers have a threshold/inertia for exposure that has to be overcome (think along the lines of how hard it is to start pushing a car but, once moving, it becomes much easier). By pre-flashing you give the area that you have pre-flashed a head start over the un-flashed parts of the paper. Therefore, there is a GREAT difference between pre-flashing and post-flashing.

This technique is quite different to split-contrast exposure which (with the #00 + #5 method) puts a low contrast veil over the whole images that, to my eyes at least, reduces micro-contrast throughout. Pre-flashing is a very easy technique to learn and a powerful one for certain negatives:

Mark your test strip with a pen (so that later you can accurately judge the change in exposures).
Expose the test strip.
Develop, fix and rinse the test strip.
View the test strip at a level of light similar to where you show your prints (for fibre-based Baryta paper it is important that the print is fully dry before assessing the test strip - I have an old microwave in my darkroom to dry test strips).
Identify which exposure gives a slight tone darker than the paper base.
The exposure to select is not the one with a slight tone but the previous section of the test strip (i.e if your exposures are increasing from left to right and the third exposure has a slight tone then use the time from the adjacent - to the left - section of the text strip).
Use this pre-flash on your paper and then do a normal test strip to assess your desired exposure time for the image including the pre-flash exposure.
One thing to keep in mid is that you may have to work at a smaller aperture when making enlargements with pre-flash because it makes the paper more responsive to light (because the pre-flash has overcome the paper’s threshold/inertia for exposure and is more responsive to light)

Bests,

David.

Temporary website address: http://dsallen.carpentier-galerie.de

albada · Mar 13, 2021

koraks said:
If color is also on the horizon, the 8 bit pwm resolution of the 328 will not be sufficient.

I've found that 8-bit is not adequate for B&W either. The 328p has one 16-bit timer that can generate two PWM outputs, so I configured it to provide 10-bit PWM for green and blue, which gave me a maximum error of .01 or .02 stops, which is insignificant.
But that doesn't help color.
Does color need finer control over color levels than B&W? I notice that Kodak's CC filters have a resolution of 5 counts. With 45 counts/stop, the resolution is 5/45 = 1/9 stop. 10 bits when 5 stops down is a PWM of 1023/32=32, yielding a worst resolution of 1/32 stop, so I think 10 bits would be fine for color. Or did I make a mistake in that analysis?

@David Allen wrote, "Therefore, there is a GREAT difference between pre-flashing and post-flashing."
The paper only remembers how many photons struck each spot, and it makes no difference when those photons arrived, so pre- or post-flash will produce identical prints, which you can verify in the darkroom.
This thread discusses this topic: https://www.photrio.com/forum/threads/pre-flash-post-flash-and-chemical-fogging-question.165843/
Posting #6 explains why pre/post-flashing has the effect it does.

grain elevator · Mar 13, 2021

I so far have used an even lower tech solution than any presented here: low power flashlight bounced off the ceiling. I might eventually move to a lamp hooked to an enlarger timer - I don't see what a microcontroller could add.
Curious about how diffusion under the lens with negative inserted works - don't you still get a soft image? Maybe not with an opaque enough diffuser. Need to try that, too.
IME, using the pre-flashing exposure that just so doesn't give a grey veil isn't always right. For one thing, after flashing the safe lights and enlarger light spills will continue to fog the paper, but more importantly, even the brightest highlights will receive additional exposure when printing, and not the same amount for every negative.
@michael_r, what do you use for localized flashing? I imagine one could even do it in the developer try and get almost real-time feedback?

radiant · Mar 13, 2021

David Allen said:
For pre-flashing paper there is absolutely no need to remove the negative or a second enlarger. T

You don't but it makes things much more easier and repeatable. Negative average density varies, enlarging height and aperture all sum up and you need to calibrate each flashing.

Don_ih · Mar 13, 2021

I put the paper on the floor and turn on the overhead light for a second or so. But I hardly ever preflash - I usually just use my hands to burn in a stop of exposure to places that need it.

David Allen · Mar 13, 2021

"The paper only remembers how many photons struck each spot, and it makes no difference when those photons arrived, so pre- or post-flash will produce identical prints, which you can verify in the darkroom".

Pre-flashing pre-sensitises the paper by exposure to white light so that the paper’s inertia is overcome. This can be useful because all photographic paper has to absorb a certain amount of white light before it starts to reveal tone or show detail. This is particularly important for dense highlights that, by normal exposure, may not transmit enough light to overcome the paper’s inertia. This exposure to light will mean that the paper is more sensitive to light coming through the negative. Therefore, the highlight detail is improved because, in effect, the paper is getting more exposure to the highlight area that has a range of tone and detail.

There IS a great deal of difference between pre-flashing and post-flashing a paper. The purpose of pre-flashing is to give the minimum amount of light necessary to overcome the inertia that may occur with dense areas of the negative. The paper then responds to all of the varying tones in the negative. What post-flashing does is to place a single veil of light across the whole image. It is in effect a process of fogging the paper to a controlled amount. If you have exposed the paper to the negative and there was insufficient light passing through the highlights to overcome the inertia of the paper this area will have no detail and the post-flashing (fogging) of the paper will simply lay down a grey tone with none of the variations of tone and detail that is in the highlights of the negative.

If you try both pre-flashing and post-flashing in the darkroom you will immediately see the difference. Pre-flashing is a technique that I employed very often when using fixed grade papers but rarely use now on multigrade papers. It remains a very useful tool for particular negatives.

"You don't but it makes things much more easier and repeatable. Negative average density varies, enlarging height and aperture all sum up and you need to calibrate each flashing".

The need to pre-flash a paper for a particular negative is usually only identified after you have done a test print and realise that it would be the solution to realising a fine print from a certain negative. Therefore, you already have the negative in the enlarger, focussed, cropped and ready to go. Therefore, the use of a diffuser under the enlarger is convenient. Yes a separate source of light can make things easier but many people do not have the luxury of extra enlargers, a large darkroom, etc. The key point is that the level of pre-flashing required is unique to each negative. If one finds that pre-flashing is a technique that you need to employ very often this implies that there is something seriously wrong with your negative’s exposure and development technique.

Bests,

David.

Temporary website address: http://dsallen.carpentier-galerie.de

koraks · Mar 13, 2021

albada said:
Does color need finer control over color levels than B&W?

Yes, much more so.
In your analysis you're forgetting 2 things:
1. The non-linear relationship between pwm duty cycle and paper response
2. The ratio between red:green:blue
No. 2 could also be solved in hardware making the resolution issue less significant, but 1 is trickier to solve with hardware.
However, 12 bit pwm controllers are dirt cheap, so why bother...

Steve Goldstein · Mar 13, 2021

David Allen said:
Pre-flashing pre-sensitises the paper by exposure to white light so that the paper’s inertia is overcome. This can be useful because all photographic paper has to absorb a certain amount of white light before it starts to reveal tone or show detail.

David, Thank you for your very thoughtful posts in reply. This is how I understood it, that flashing was most useful prior to image exposure. You mention it being done with white light - this makes sense for graded papers but variable contrast with its two or more emulsions might benefit from the ability to flash green or blue because those emulsions have different sensitivities, hence my original question. But perhaps there's little difference between preflashing and localized burning with the green or 00 filter. @Peiter12 raised a good point about this, I think.

David Allen said:
The need to pre-flash a paper for a particular negative is usually only identified after you have done a test print and realise that it would be the solution to realising a fine print from a certain negative. Therefore, you already have the negative in the enlarger, focussed, cropped and ready to go. Therefore, the use of a diffuser under the enlarger is convenient.

I wonder about this part. Isn't the paper's "inertia" (really, the region well below the toe) a property of the paper? In that case, a standardized exposure should be able to raise the sensitivity to just below the threshold, right?

Finally, I'm only looking to build a simple pre-flasher for black-and-white printing, not a full-up enlarger head for black-and-white or color. I think PWM, no matter how many bits, is probably overkill since an on-off switch controlled by a timer will do. I'm a long-time analog circuits guy (board level, chip-and-wire hybrids, and monolithic) and have spare parts left over from my LED safelight project and pretty much only need buy the LEDs. Yes, it would be more elegant to include the timer function, and I mentioned this in my original post, but now I'm thinking I just might get lazy and use the old Gra-Lab 300 sitting unused in my basement.

radiant · Mar 13, 2021

David Allen said:
The key point is that the level of pre-flashing required is unique to each negative.

I've always assumed that pre-flashing is done to the point where paper starts to show any signs of tones. I'm guessing the papers inertia is "moving" on any amount of light? I mean you can pre-flash a just a bit (far from the time required for tone) and still get the effect?

Figuring correct partial pre-flash seems to bring another variable to the (already) quite complex formula to get good looking print.

I've used pre-flash on direct positive papers and on those papers I think the pre-flash is done to the "almost tone" level always.

David Allen · Mar 13, 2021

Hi Steve,

‘White light’ is something of a misnomer and a carryover from colour enlargers where one could flick a switch to remove the filters from the light path. No enlarger uses a true ‘white light’ because the vast majority of light sources are some form of tungsten lightbulb which emit a colour temperature in the yellow region. In practice an enlarger lightbulb is fine for both pre-flashing paper and making prints from negatives.

Trying to consider pre-flashing only the blue or green sensitive parts of the emulsion has no practical use. This is because Multigrade papers are actually coated with three emulsions all sensitive to blue light but with varying amounts of green dye. Therefore, all emulsions react equally when exposed to blue light with very little difference when using ‘white light’ (i.e Tungsten lightbulbs).

"Isn't the paper's "inertia" (really, the region well below the toe) a property of the paper? In that case, a standardized exposure should be able to raise the sensitivity to just below the threshold, right?"

Yes every paper has a different inertia. Indeed back in the ‘old days’ all commercial photographers and laboratories were careful to buy multiple boxes of a paper from a single batch run because the sensitivity could vary significantly between each emulsion coating run. I have not seen such variations in decades now.

The key thing is that, with the vast majority of negatives, making enlargements from negatives requires no need to even consider the inertia of the paper because we do not ‘see’ its effect and, in the majority of instances, achieving a fine print can be achieved without pre-flashing the paper.

I have seen people suggest that pre-flashing every paper is a means to ‘speed up’ the paper and reduce exposure times but this would come at the cost of changing the base contrast of the paper. In practice I can not see where this would be useful.

Bests,

David.

Temporary website address: http://dsallen.carpentier-galerie.de

Doremus Scudder · Mar 13, 2021

michael_r said:
... the net effect of localized flashing is not much different (perhaps not different at all) than burning at grade 00/minimum contrast ...
... flashing the entire sheet adds no value with VC papers...

This is your best answer. Unless you're using graded paper (which, unfortunately, is rare these days), you can just burn the necessary areas with the #00 filter or 170Y or a #58 green filter or whatever and get the same result. I hardly ever pre-flash anymore.

Doremus

albada · Mar 13, 2021

Steve Goldstein said:
Finally, I'm only looking to build a simple pre-flasher for black-and-white printing, not a full-up enlarger head for black-and-white or color. I think PWM, no matter how many bits, is probably overkill since an on-off switch controlled by a timer will do. I'm a long-time analog circuits guy (board level, chip-and-wire hybrids, and monolithic) and have spare parts left over from my LED safelight project and pretty much only need buy the LEDs. Yes, it would be more elegant to include the timer function, and I mentioned this in my original post, but now I'm thinking I just might get lazy and use the old Gra-Lab 300 sitting unused in my basement.

Perhaps you could use a low-power light or nightlight, attach a diffuser under it, hold it over the table as high as you can with one arm, and start the Gra-Lab with the other arm. You would have repeatable brightness and time, and therefore, consistent flashing.

Pieter12 · Mar 13, 2021

Sometimes the areas needing detail are difficult to burn without making custom masks or tools and pre-flashing is quite effective. And it certainly works with VC papers. That's all I use and I can easily see the results.

Pieter12 · Mar 13, 2021

albada said:
Perhaps you could use a low-power light or nightlight, attach a diffuser under it, hold it over the table as high as you can with one arm, and start the Gra-Lab with the other arm. You would have repeatable brightness and time, and therefore, consistent flashing.

That's kind of what my set up is, just the arm isn't mine, it is a 24" long piece of PVC pipe attached to the back wall with an electroluminescent panel on the end connected to a Graylab timer. 1 second is what I have determined works for preflashing.

Steve Goldstein · Mar 14, 2021

Thanks for the pointer, I watched Joe Van Cleave's video on YouTube. The next one up was from The Naked Photographer, who has a different method (diffuser under the lens with negative in place and no filtration). While watching the second one I remembered that I have an old 52mm Expo-Disc that I can attach to my enlarger lens with my 40.5mm-52mm step-up ring, so I don't need to build or buy anything to experiment with this.

I'll have to find a candidate negative to compare pre-flashing with green burns, and will need to come up with another project to start learning that software. Life could be worse.

Thanks to everyone who contributed to this discussion, it was educational.

Question for you paper preflashers

Steve Goldstein

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Pieter12

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Steve Goldstein

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