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Preflashing & Latensification

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#1 Michal1

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Posted 11 July 2013 - 03:16 PM

Have you ever tried them? http://en.wikipedia....ersensitization What is the approprite source of light for astro imaging? How intensive and long the flash should be?

#2 Nebhunter

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Posted 11 July 2013 - 05:19 PM

Michal - I recommend the book by Wallis & Provin - A manual of advanced celestial photography. The best book to date out there and it covers everything basically including films and pre-flash - you name it. I got a used copy from Amazon or similar. Robert Provin used to visit here on occasion and it would be neat if he popped in again for this subject. Essentially they agreed that pre-flash was of benefit with E200 films with lenses over f/4 or longer telescopes. Lenses from f/2.8 to f/4 benefited from the sky background mostly giving it a neutral background. It lessens the contrast and brings up the dark details without affecting the highlights by very much. Light needs to be 5500 K colour temperature. Other temperatures will need Wratten correction filters.

The 3 pages dedicated to the subject are very intensive with information which I cannot condense here. I recall someone stopping down the lens to f/32 at 1,000 seconds or faster, focused at infinity with the light box close up.

I've been tempted to try this with my Tec, and point it at my large light table which is in that temperature range. Defocus the lens so that no details will be recorded which could affect the final outcome. The lens can also be removed and exposed that way but a large box is required.

Hope this helps. BTW - I'm still in the game at least for awhile.

Igor

#3 Hikari

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Posted 11 July 2013 - 05:50 PM

I doubt you are going to get much of a benefit with latensification and modern emulations. As the Wiki entry said it was done with the old Koadak film, which was never really that great.

You are looking for a very short exposure to simply add a bit of density above base plus fog. The only real why of knowing that would be to test. But as Igor states, a very short exposure. I would set the color temperature to daylight for color film with a neutral target. The target needs to be an evenly lit, or better, to expose the film out side the camera. A light source about 12 feet away should be very even.

I did pre-flashing for architectural photography to lower contrast by adding density to the shadow area.

#4 Michal1

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Posted 12 July 2013 - 04:40 PM

Thank you Nebhunter and Hikari. I'll have to get that famous book by Provin and Wallis.

So, if I understand it correctly, the goal of pre-flashing is to overcome the toe of the characteristic curve, i.e. the value of about 10^-2.5 lux-seconds for E200: http://www.kodak.com...ubs/e28/e28.pdf , right? This is my fantastic idea, please correct me, if I'm too far from the truth: I will use my guiding notebook as the light source. I have no stand-alone exposure meter, so I will use the exposure meter on Pentax 6x7. I will go with the camera into a dark room, where only the screen of the notebook shines. I will set the f-number, at which I intend to photograph. Then I will obtain some value of exposure time by the exposure meter. I expect this combination of f-ratio and exp. time corresponds to the value of illumination in the middle of the linear part of the characteristic curve, i.e. about 10^-1.7 lux-seconds for E200. So, to get the time needed to overcome the toe of the curve, I shoud use the time indicated by the exposure meter divided by (10^-1.7)/(10^-2.5) = 6.3, right? I believe I will manage to make a computer program, that will be able to give a screen blink of a specified length. Of course, some experimentation will be needed.

#5 Hikari

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Posted 12 July 2013 - 08:44 PM

Preflashing is great with sheet film as you can try some different preflash exposures on each sheet and develop. Tricker with roll film. Preflashing is trying to compensate for low-intensity reciprocity. Usually when we think of an exposure, we are really thinking of the "average" exposure--highlights and shadows fall above or below that. Shadows suffer from low-intensity reciprocity more than the rest of the exposure, the pre flash tries to compensate for the loss of shadow detail lost to reciprocity. If your exposure is mostly putting your detail in the mid tones of the curve and your sky glow is already above base plus fog, I am not sure what benefit a pre flash is--I am not saying not to try it, I really don't know if there will be a benefit or not.






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