Post flashing

Discussion in 'B&W: Film, Paper, Chemistry' started by cliveh, Oct 14, 2013.

  1. cliveh

    cliveh Subscriber

    Messages:
    4,538
    Joined:
    Oct 9, 2010
    Shooter:
    35mm RF
    On a different forum, I read about pre and post flashing. I have never heard of post flashing. Can someone tell me when this technique should be used and the advantages it gives?
     
  2. mike c

    mike c Subscriber

    Messages:
    2,200
    Joined:
    Jan 5, 2009
    Location:
    Los Angeles
    Shooter:
    Multi Format
    Would not not think there would be much difference either way. With prints post flashing works better for me for the simple reason that the neg has to be removed from the enlarger. I use the enlarger for the flashing light source . Do not think it would make a difference with negs ether.

    Mike
     
  3. jovo

    jovo Membership Council

    Messages:
    4,124
    Joined:
    Feb 8, 2004
    Location:
    Jacksonville
    Shooter:
    Multi Format
    I don't really know if there's a difference in the result, but the idea is to raise the print's exposure threshold so that selected areas register immediately when there is an overall exposure. It just seems logical to me to do that first.
     
  4. ROL

    ROL Member

    Messages:
    792
    Joined:
    Oct 27, 2005
    Location:
    California
    Shooter:
    Multi Format
    I thought post flashing was face saving vernacular for the unplanned result of accidentally turning on the lights before the print has been fixed, almost always in the presence of students. :laugh:
     
  5. Michael R 1974

    Michael R 1974 Subscriber

    Messages:
    6,200
    Joined:
    Feb 2, 2010
    Location:
    Montreal, Ca
    Shooter:
    Multi Format
    Are you referring to film exposure or when printing?
     
  6. cliveh

    cliveh Subscriber

    Messages:
    4,538
    Joined:
    Oct 9, 2010
    Shooter:
    35mm RF
    Printing.
     
  7. Michael R 1974

    Michael R 1974 Subscriber

    Messages:
    6,200
    Joined:
    Feb 2, 2010
    Location:
    Montreal, Ca
    Shooter:
    Multi Format
    Usually when you flash paper, you give it just enough pre-exposure to non-image light so that the paper has been brought just to the threshold at which any further exposure will produce tone. The effect is to lower the contrast of the paper. It is therefore a technique of much more limited use with VC papers than with graded papers. With graded papers it could be used to get intermediate grades. And some people also used it so that they could use higher grades - which sometimes gave higher density blacks than lower grades (Richard Henry showed this effect with his meticulous tests of flashing with Ilfobrom).

    With VC papers, there isn't much point in flashing an entire sheet. However it can still be a useful technique for bringing in stubborn highlights if applied locally (and there are various methods of doing that). When done in this way the effect is essentially the same as burning in with a very low contrast filter.

    Actually I'm working on an article to show completely (including data) what flashing does (using MGIV), the differences between pre/post, and the effects of flashing at different grades. If there is interest on APUG I will post the article.

    There are some threads here on flashing in the enlarging forum.
     
  8. Rudolf Karachun

    Rudolf Karachun Subscriber

    Messages:
    47
    Joined:
    Jul 29, 2009
    Shooter:
    Large Format
    Good explanation of advantage of post flashing and technical advise how to do it given in the Ghislain Lootens book " Lootens on Photographic Enlarging and Print Quality"
     
  9. Bill Burk

    Bill Burk Subscriber

    Messages:
    4,621
    Joined:
    Feb 9, 2010
    Shooter:
    4x5 Format
    Where Lootens used the term Flashing, he meant something quite different. He isn't talking about raising the exposure to a threshold to hold a delicate highlight. He wrote about a pictorial effect which he used to darken corners dramatically. He does mention all four times when you can flash paper; before, during or after enlarger exposure and also after print is in the developer - he cautions you about the risk of Solarizing (Sabbatier effect). He recommends flashing after enlarger exposure but doesn't explain.

    If I had to guess his reason to recommend flashing after exposing in the enlarger... I imagine you can have a clear short-term memory mental image of "where" the exposure has already happened, so a better chance of flashing the correct corners.

    I don't use flashing as a general rule. So, without experience, I can't add knowledge of whether "before, during or after" makes any difference. I am certain there is a sensitometrically-measurable difference. But I am guessing there is little difference visible on the print, and that the distinction exists to explain the way of working.
     
  10. Michael R 1974

    Michael R 1974 Subscriber

    Messages:
    6,200
    Joined:
    Feb 2, 2010
    Location:
    Montreal, Ca
    Shooter:
    Multi Format
    Bill, one relatively simple illustration of pre vs post non image exposure (note not necessarilly limited to below-threshold exposure but also fogging) is in Kodak's safelight test procedure. The procedure is designed to test the effect of safelight (ie non-image) exposure both before and after the enlarger exposure.
     
  11. Bill Burk

    Bill Burk Subscriber

    Messages:
    4,621
    Joined:
    Feb 9, 2010
    Shooter:
    4x5 Format
    I was trying to find that safelight test illustration because, yes, it clearly shows the degradation that an unsafe "safelight" can cause.

    So I went to Google... and it led me back to myself on APUG... "The Kodak test was a still-life Siamese cat sculpture with polished silver pots".

    Now WHERE did I see those cats?
     
  12. Bill Burk

    Bill Burk Subscriber

    Messages:
    4,621
    Joined:
    Feb 9, 2010
    Shooter:
    4x5 Format
    Found 'em. Kodak Filters for Scientific and Technical Uses, Kodak Publication No. B-3 - Page 51.

    Doesn't show whether exposure before or after makes a difference. But the side which had "prolonged" exposure to safelight (the proper safelight filter with the proper bulb at the proper distance... but for too long)... was clearly degraded.
     
  13. NedL

    NedL Subscriber

    Messages:
    1,627
    Joined:
    Aug 23, 2012
    Location:
    Sonoma County, California
    Shooter:
    Multi Format
    Funny this topic coming up now. I use pre-flashing for my paper negatives to help control contrast a little and increase speed. I learned about it by reading Joe VanCleave's notes over at f295.org but I know he's posted his method here at APUG as well.

    Like here:
    http://www.apug.org/forums/forum62/65770-preflashing.html

    I make a test strip for each paper type, and pick the exposure that is 1 second shorter than the faintest visible tone in good light after dry-down. Joe uses graded paper but I use VC paper and only preflash through a green filter, so I'm differentially pre-flashing the low contrast emulsion.

    The reason this post was timely is that recently I've made a number of paper negatives using post flashing instead. This was due to being in a hurry or wanting to reload a one-shot camera while a paper was developing and not wanting to turn on my enlarger during that time. Using exactly the same enlarger height/aperture and filters, for exactly the same amount of time that I use for pre-flashing, I am unable to see any difference. So in my context, post-flashing can be more convenient... pull the paper out of the camera, stick it under the enlarger for 17 seconds ( or whatever for the paper type ) start my pre-soak and load the camera with a fresh sheet of paper. I'm almost sure I remember Joe saying that post-flashing was almost the same as pre-flashing, but not exactly the same. But in practical terms, my experience says I can use them interchangeably. Whatever the difference is, it's too small to matter for my mostly pinhole photos. I also use paper in lensed cameras, and have post-flashed a few of those in the past few months too, with equally acceptable results.

    I have never used pre or post-flashing during printing, so I can't comment on that at all.
     
  14. Sponsored Ad
  15. Michael R 1974

    Michael R 1974 Subscriber

    Messages:
    6,200
    Joined:
    Feb 2, 2010
    Location:
    Montreal, Ca
    Shooter:
    Multi Format
    Bill, when all else fails you can always go back to my "article" with the Kodak and Ilford resource links (shameless plug). Here it is under publication K-4:

    http://www.kodak.com/global/en/consumer/products/techInfo/k4/k4TestSafelite.shtml

    The point I was making is that the test is intended to show the effects of the same amounts of non-image exposure both before and after the image exposure (analogous to pre and post flashing).
     
  16. Bill Burk

    Bill Burk Subscriber

    Messages:
    4,621
    Joined:
    Feb 9, 2010
    Shooter:
    4x5 Format
    Ah, you mean this Article?

    http://www.apug.org/forums/showthread.php?t=114562

    You are right, it illustrates the difference between pre- and post- flashing. In the diagrams shown, Pre-flashing appears to affect the entire scale, while Post-flashing appears to affect the toe (highlights). The diagram is not an actual photograph, as it is in Publication B-3, but the kitties and kitchenware photo doesn't show pre-flashing.
     
  17. pentaxuser

    pentaxuser Subscriber

    Messages:
    7,928
    Joined:
    May 9, 2005
    Location:
    Daventry, No
    Shooter:
    35mm
    Les McLean, formerly of this parish, describes both pre and post flashing with illustrations in articles on his website and in his book Creative Black and White Photography - worth purchasing if you don't have a copy.

    pentaxuser
     
  18. cliveh

    cliveh Subscriber

    Messages:
    4,538
    Joined:
    Oct 9, 2010
    Shooter:
    35mm RF
    I may have got this wrong, but surely pre-flashing softens the paper contrast and raises the inertia of exposure level. However, is not post flashing a form of uniform fog of the paper already exposed to the negative? As at this point the exposure has gone beyond threshold.
     
  19. NedL

    NedL Subscriber

    Messages:
    1,627
    Joined:
    Aug 23, 2012
    Location:
    Sonoma County, California
    Shooter:
    Multi Format
    Clive, with my paper negatives, I'm exposing them as if they had been pre-flashed. So the total exposure to all light is the same either way. I don't think this is analogous to post-flashing while making an enlarged print, unless the print is intentionally underexposed before flashing...

    I'm curious about this too... I'll check out Les McLean's site.
     
  20. Athiril

    Athiril Subscriber

    Messages:
    2,887
    Joined:
    Feb 6, 2009
    Location:
    Melbourne, V
    Shooter:
    Medium Format
    I haven't tried post-flashing, but pre-flashing makes a HUGE difference when pushing the crap out of films.

    My example is with colour film, Superia 800 exposed at 12800.

    FTR, my pre-flash method was double exposure, I held a tissue over the lens and metered through that with camera meter, and determined the pre-flash amount. It was with a Canon AE-1, I used that 'trick' of winding tight the left hand spool and holding it, while pressing the rewind button then gently advancing the frame advance lever.

    C-41 @ 6 minutes.

    12800 no pre-flash
    [​IMG]
    Superia 800 @ 12800 no preflash by athiril, on Flickr

    12800 with 'zone 3' pre-flash (Zones 1 and 2 did nothing essentially, all of a sudden at this point density suddenly jumped up, contrast improved, shadow detail increased).

    [​IMG]
    Superia 800 @ 12800 Zone 3 preflash by athiril, on Flickr
     
    Last edited by a moderator: Oct 15, 2013
  21. Michael R 1974

    Michael R 1974 Subscriber

    Messages:
    6,200
    Joined:
    Feb 2, 2010
    Location:
    Montreal, Ca
    Shooter:
    Multi Format
    Yes pre-flashing (or post-flashing) lowers paper contrast. Both pre and post-flashing can put uniform fog on the paper if done incorrectly (or if that is the intent - which it occasionally is). Perhaps what you're missing here is the procedure. Assume you want to pre-flash. You first do a test strip using non-image light and find the longest exposure time that does not result in any print density/fog. That is your pre-flash exposure. Then you incorporate the pre-flash into all further test/work prints. Since you're determining your image exposure with paper that has been pre-flashed, the pre-flash exposure is taken into account when you find your print exposure. So there is no unwanted fog.

    Post-flashing is harder because you have to work backwards, first determining the maximum amount of image exposure that yields no highlight density, then finding the amount of post-flash required to just bring in the highlight detail from the image exposure. If done correctly the end result would be the same as the pre-flash (although the flash exposure time might not be the same). Since the end result is the same as a pre-flash, and the pre-flash procedure is easier/more intuitive, I can't really envision a situation wherein one would post-flash, unless it is part of a broader sequence of masked exposures.

    Bottom line is that wherever you flash, you lower paper contrast. So in general it is a much less useful technique with VC papers than with graded papers.
     
    Last edited by a moderator: Oct 15, 2013
  22. Mr Bill

    Mr Bill Member

    Messages:
    359
    Joined:
    Aug 22, 2006
    Shooter:
    Multi Format
    Hi, I'm just the opposite - I think post-flash is more intuitive. The way I do it is to first make a print (actually, test strips). When I get to where I'm happy, except for lacking a bit of highlight detail, I'll often try flashing. Typically I'll make another test strip, doing several flash exposure on it. I use a piece of cardboard over the print, same as I would for a print-exposure test, moving it in steps. So one test print might also get flashing tests of 1, 2, and 4 seconds. If the highlights are doing what I want, I can select a flash exposure or test further.
     
  23. Michael R 1974

    Michael R 1974 Subscriber

    Messages:
    6,200
    Joined:
    Feb 2, 2010
    Location:
    Montreal, Ca
    Shooter:
    Multi Format
    No argument from me if it works for you. I probably should have said pre-flashing is more intuitive to me. The results in the end should be the same.
     
  24. JamesMorris

    JamesMorris Member

    Messages:
    53
    Joined:
    Sep 29, 2009
    Location:
    Sydney, Aust
    Shooter:
    Large Format
    It'd be great to see this article here.
     
  25. Bill Burk

    Bill Burk Subscriber

    Messages:
    4,621
    Joined:
    Feb 9, 2010
    Shooter:
    4x5 Format
    It'd be great to see the article.

    I hope you answer my burning questions: Is pre- vs post- just a way to distinguish the sub-threshold exposure from deliberate graying exposure? Does it matter if the total flash exposure occurs at any combination of before, during or after enlarger exposure? My gut tells me it doesn't matter and conventionally we do this because it is easier to figure out what amount of flash is desired by keeping the purposes separated in our heads.
     
  26. Michael R 1974

    Michael R 1974 Subscriber

    Messages:
    6,200
    Joined:
    Feb 2, 2010
    Location:
    Montreal, Ca
    Shooter:
    Multi Format
    I have similar questions, Bill. Still working on these tests and it will be a few weeks yet. Lots of data to go through. I've used localized and/or masked pre-flashing fairly often with extremely high contrast subjects, and always did it the way I was taught and the way it made most sense to me, but I've always wanted to do a rigorous "study" on it to hopefully answer some of the nagging questions I've had. Pre vs post is one of them, but the one I've had the most trouble getting my head around abstractly is what effect there might be (if any) if the flash exposure on VC paper is done with different filters. The conventional approach is to simply flash with white light, but I wanted to investigate this further. Another one of those rat holes I guess. The procedure works fine for me as-is, but who knows, maybe there are additional controls, or maybe a myth or two can be busted. There is little written about it, and no real evidence for any assertions. The exception is Henry's book - but that was with graded papers. So I kind of see this experiment as a continuation of his work but with VC papers.