Double...exposure....

Discussion in 'The Darkroom' started by meesh, Mar 9, 2007.

  1. meesh

    meesh TPF Noob!

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    I was sitting in Barnes and Noble trying to read for free:greenpbl: and i came across this one B&W printing book. It said something about double exposure, or a technique in printing involving exposing the paper twice. Anyway, i forgot what the exact name was, but does this sound familiar to anyone? He mentioned how it gives more tonal range. Well does anyone print like this? Or similar to it? Can you describe the exact process? How would I do my test strips? Do I do one for each exposure? ....does any of this make sense?:lol:
     
  2. Steph

    Steph No longer a newbie, moving up!

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    Maybe google 'pre-flashing' to see if it is what you are looking for.
     
  3. EOS_JD

    EOS_JD TPF Noob!

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    Film or digital?

    Digital requires more than 1 image - see mi avatar
     
  4. Hertz van Rental

    Hertz van Rental TPF Noob!

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    If it is using just the one neg and is a means of 'extending' the tonal range of a print then it would probably be pre-flashing.
    I've explained this one before but I'm happy to do it again.

    Say you have a contrasty neg where you can get the shadow detail but the highlights block out, and if you print in the highlights you lose shadow detail.
    It is possible to burn in the highlights or dodge out the shadows but this isn't always practical.
    The highlights on the neg are too dense and not enough light can get through to expose the paper if the rest of the print is properly exposed. What is happening is that the neg has a slightly wider tonal range than the paper can handle.
    Sometimes pre-flashing can solve this.
    When you expose printing paper (or film) you are putting energy into it in the form of light. To get a minimum exposure (one that will form a latent image) you need to put a certain amount of energy in before anything happens.
    In the situation outlined above this energy threshold is not being reached.
    In Physics it is called 'hysteresis' (literally 'lagging behind').
    You come across it when you try to push a car. It takes quite a bit of effort to get the car moving, but once it is it takes far less effort to keep it moving.
    Pre-flashing is a way of getting things moving.
    You set up an enlarger with no neg and the lens stopped down. Do a test strip and process.
    You are aiming for about 50% of the strip to be white and the rest barely grey.
    Work out the exposure where you just get a discernable grey and then choose the exposure before it.
    What you are selecting is the maximum exposure you can give the paper before you get a recorded exposure.
    Expose a sheet of paper under this enlarger.
    Immediately put the paper under the enlarger with the neg in and expose for the shadow detail.
    What you have done is to 'charge up' the paper with energy (overcome hysteresis) so that very little more light is needed to produce a latent image.
    You should now find that there is considerably more detail in the highlights whilst retaining all the detail in the shadows.
    It can often be enough to pull back a print that would otherwise have not worked.

    It is possible to flash on the same enlarger as you are printing on but it is a fiddle. Flashed paper will start to lose the energy you have put in almost immediately so the quicker you use it the better. But it should stay fully charged for a while. I wouldn't leave it longer than 15 minutes, though.

    The same technique can be used on film to 'super-sensitise' it. Astronomers often do it for photographing stellar objects. It is tricky though.
     
  5. meesh

    meesh TPF Noob!

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    Thanks a lot for all the comments! And thanks for the in-depth explanation hertz! I wish I wouldve known that technique a few weeks ago instead of spending 2 whole days printing one image! I think that wouldve solved the problem right away! you should write a text book herts! lol... you really broke it down and made it easy to understand
     
  6. Hertz van Rental

    Hertz van Rental TPF Noob!

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    I should have added that the 'flashing' exposure has to be even.
    If you are using the same enlarger to flash and print then you can set it all up for the print (leaving the neg in) and just use a piece of tracing paper or ground glass* in front of the lens. This should produce an even illumination of reduced intensity suitable for the flash exposure.
    If your enlarger has the red swing filter under the lens it might be worth taking the red filter out and replacing it with a piece of ground glass/trace/opal perspex*. Then you just put it in place to flash the paper and swing it out of the way to print. It can save a lot of fiddling about.

    *The opaque material you use should have a neutral tint, no texture and soften the image enough so it just gives a soft even illumination.
    Ground glass (NOT frosted) is good.
    Plastic tracing paper works well too.
    Some Perspex (acryllic) works better than others so you will need to test.
    Often effective is a piece of clear Perspex with one side scoured by fine emery cloth to get a ground glass effect.
     
  7. JonnyVPA

    JonnyVPA TPF Noob!

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    haha its just like Antique LAYERS

    Just had to say it
     
  8. rabidzoomer

    rabidzoomer TPF Noob!

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    yeah we are doing this in class. This is my first year and first semester (almost up) and it is the hardest project we are ever going to do. We have to take i think 2 negitives and transfer them onto one paper and make it look right. My teacher showed up some examples and the best one was a baby face in a clock and was said to have taken a little over a week to get perfect. You need to take one negitive and transfer it to paper then take the other and transfer it and you will need to do it at a low time because you are exposing the paper 2 times. It is going to be very very tricky.
     
  9. Philip Weir

    Philip Weir TPF Noob!

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    I found this question rather interesting, as many years ago when we only had black & white television in this country [Australia] I specialised in making up what were known as TV packs, that is packs of various products were given to me in artwork form. As a mid tone blue and a mid tone red if photographed through a TV camera appeared on B&W TV the same tone, so I had to make up a pack reproducing each tone as a percentage grey. Anyway I used a 10 watt globe in the ceiling of the darkroom for reproducing the overall background tone of say 5-15% grey. I would place the photographic paper on the floor of the darkroom and expose for say one second, two second etc to get the right percentage grey. A similar excercise to what you are attempting, but much more accurate than an enlarger. You could give this method a try.

    www.philipweirphotography.com
     

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