is preflashing the same as postflashing?

Discussion in 'Enlarging' started by BetterSense, Dec 12, 2010.

  1. BetterSense

    BetterSense Member

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    Sometimes I can bring the highlights of a print in by giving a quick hit of white light while the print is in the tray. Is this the same effect as paper preflashing or is there a difference?
     
  2. Ian Grant

    Ian Grant Subscriber

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    You can pre or post flash, both work in the same way, it's about being consistent though for repeatability. However it's better to do it with an enlarger or fixed light source and an accurate timer.

    Ian
     
  3. henpe

    henpe Subscriber

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    I have read about a post-flash technique in which you expose the paper less than normal. Develop until the first shadow areas begin to emerge. Then, start to flash the paper with very short bursts of light. Areas that have already taken tone are "self-masking", while undeveloped areas are perceptible to the flashes and get the exposure needed to take tone. The technique is described as a 'light-separation" technique usefull for bringing out delicate high-lights. Hard paper grades are recommended.
    I find the technique interesting, but have never tried it myself.
     
  4. Rick A

    Rick A Subscriber

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    I suggest you look up the "Emmerman process" . It's an easy way of using developer to self mask a print with blocked up shadows to gain some detail without loosing highlite detail.
     
  5. RalphLambrecht

    RalphLambrecht Subscriber

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    For all practical purposes, I agree with Ian. However, strictly speaking, there is a difference between applying flashing as a pre or post-exposure.

    First of all, let's define 'flashing' as an exposure too weak to create density by itself, opposed to 'fogging', which is an exposure strong enough to create density by itself. Furthermore, let's make a difference between a high-intensity (HI) and a low-intensity (LI) exposure to create the print, and let's assume that the LI exposure is your 'flashing' and the HI exposure is your main exposure or 'fogging'.

    If we agree on the above, the photographic effect of applying a HI followed by a LI exposure is greater than the reverse, because the HI exposure will minimize reciprocity failure. Also, latent image stability is a function of time, and consequently, the time between HI and LI exposure, cannot be entirely ignored (intermittency effect).

    Having said all of that, once you stick to one 'system' and don't jump back and forth, it makes no difference if you flash before or after the main exposure.
     
  6. Nicholas Lindan

    Nicholas Lindan Advertiser Advertiser

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    If you are flashing in the developer as the image comes up then you don't have much choice as it is always going to be post- flashing. Though I have heard of people soaking the paper in the developer, squeegeeing off the excess and then exposing the sheet so I suppose pre-flashing is a possibility.

    Taken to extreme flashing a partially developed image results in solarization; the 'self-masking' effect can cause the image to reverse.

    If one is flashing the paper when it is still undeveloped then the order of flashing and exposure will make no difference.

    I don't think you will see any 'strange effects' coming into play with paper flashing: intermittency effects only come into play with very short exposures such as those made with an electronic flash; reciprocity failure with photographic paper requires exposure times greater than 100 seconds or so; latent image stability can be measured in years, though you may see some highlight fogging if you wait a few days before developing the print.

    Testing for the above effects is pointless without methods for very, very precise control of exposure. And you need to use such exotica as interference ND filters, a quartz timed electronic shutter and a stabilized light source. Comparison exposures have to be cut from the same sheet of paper and the comparison made along the cut line so coating variation is minimized, the two samples have to be developed together. Strangely enough the more you control the variables involved the more the effects seem to vanish.

    The sheet-to-sheet variation in a box of paper and the effects of developer oxidation (even an hour in a tray makes a very measurable [though barely visible] difference) will overwhelm any other effects.
     
  7. Ian Grant

    Ian Grant Subscriber

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    It's assumed to be different but in practice as you say it's identical, I've done both side by side and the results back that up.

    I've never found even a slight difference between sheet to sheet in the same box/packet, or even different boxes/packets, even different sizes, abeit all of the same emulsion batch. In fact I did some 24"x20" prints last month and the test strips were done using paper from a 16"x12" pack.

    Ian
     
  8. RalphLambrecht

    RalphLambrecht Subscriber

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    I agree, but for the sake of discussing the theory of photographic processes:

    Given the fact that Abney did his first studies in 1893 and Webb confirmed and extended his findings in 1933, exposure times don't need to be very short to get the effect. Webb revealed that the significance of the intermittency effect depends on the frequency of interruption not the duration or the intensity.

    Reciprocity failure is always there. It does not magically appear with longer exposure times. Halving the light intensity and doubling the exposure time to compensate for it will always result in a lighter print. In fact, the ratio is almost constant for any given paper at exposure times ranging from 4 to 120 seconds. An exposure loss of 1/16 f/stop per doubling of exposure time is a good starting point.

    There is also a density change over time (hours and days) due to reciprocity effects. It's large enough to be measured with a typical densitometer. If I remember correctly, density first increases then decreases with the age of the latent image.