Post-flash vs. Pre-flash

[NedL]

Whether or not there is some technical or subtle difference between pre-flashing and post-flashing a paper negative, I don't know. I cannot see a difference: both reduce contrast by about the same amount and increase the effective speed by about the same amount. I tend to pre-flash when I have time to plan ahead, and post-flash when I'm in a hurry and want to get the camera loaded and head out.

Today I found a reason why post-flashing might be better. I took a photograph of a big oak tree, with dark green leaves.... the paper in the camera was not pre-flashed. I was debating whether to make the exposure for post-flashing or not... even VC paper is pretty blind to dark green... but since this is heading for a salt print I decided to make the exposure for no flashing.

After I made the exposure, I realized I'd made a mistake and underexposed by a full stop. Now I should still be able to salvage the negative by post-flashing it.

If the paper had been pre-flashed and underexposed for that, I'd be out of luck. If I'd been planning to post-flash and I'd overexposed for that, the negative might still have been salvageable by skipping the flash and using weak developer.

Waiting to flash the paper leaves more options open.

 

[TheoULF]

Hi Ned,
What are your exact experiences with flashing the paper negatives?
What is the most effective way to do it? And all the other things like distance to paper, long (manually) or shorter flash intensity ? How many f-stops are the difference ? Whatever.

I am interested in all factors related to this issue. It would be nice of you to share all details!

Best regards
Theo

 

[NedL]

Hi Theo, Welcome to APUG!

(there was a url link here which no longer exists) about pre-flashing paper negatives.

• Most effective way is probably whatever the easiest and most convenient way is for you. I use my enlarger because it is easy, but before that I used a flashlight bulb in a soup can suspended about 2 meters above the paper, with a simple pushbutton switch to turn it on and off. I like to flash VC paper through a green filter.
• Distance to paper: most important thing is that it is the same every time so that the times are repeatable. I put my enlarger up almost as high as it can go.
• I think longer flashes are better because it is easier to repeat them precisely. I don't use an enlarger timer, but a dodging card and metronome, so I like the times to be >10s.
• In my experience, most paper gains about 1 stop, or just a little more, for photographs made outdoors. The effect is most noticeable in the shadows.

When I make test strips to determine the flash time for paper, I let them dry completely, hold them up in good light, and choose the longest exposure that shows no tone at all. You need to test each kind of paper you use separately, but once you know the time it is quite stable.

Happy to answer anything else if I can.

Joe Vancleave had a really good post about this over at f295.org, but I don't have a link to that one.

Ned

 

[TheoULF]

Hi Ned,
Thanks a lot for your answers and link.I 'll run into my darkroom as soon as possible to test my first paper sheets.

This issue is very important to me because I am working with my DIY made ultra large camera (12x16") and paper negatives. Some still life /tabletop pictures need up to 1 hour to expose (often with f-stop 45 or 65).

The developed negativ is often my final step, that means the negativ is the final picture. And this can be very tricky to find out the right exposure time + right contrast of paper.

If you are on Facebook you can see the first posts / pictures by joining the group "The Fascination of Black and Wnite Negatives " which was founded by me about three weeks ago. ( Wnite was a mistake in the earliest morning
I remember , it should be ...White ..)

I am sure that l 'll come back with more questions to you soon.

Best regards from the country with the longest nights and days
Theo

 

[NedL]

Unfortunately, I'm not on facebook, but paper negatives can be very beautiful in their own right. I think pre-flashing might help you gain some control over the contrast. A yellow filter can also be used with VC paper. My understanding from APUG member Ray Heath is that the yellow filter needs 1 stop more light when used indoors ( my experience is that it needs an extra 2 stops outdoors ). Also you can use a dilute paper developer and develop the paper longer. I don't use dektol any more, but when I did I liked to use it diluted 1+8, and I'd make that up from normal strength dektol that had been already used to make a few darkroom prints, and also let it sit ( in a bottle ) for a few days. Also if you have any trouble with development streaks, it can help to presoak the paper negative for at least 2 minutes before developing it ( try to lay the paper onto the water in one quick fluid motion, even water droplets can cause spots ).

Good luck and have fun!

 

[Ian Grant]

It makes little difference whether you pre flash of post flash as the effect is the same, it's about being consistent. I use a timer and a second enlarger for the flashing exposure, with no filtration.

Ian