Today I printed some 35mm HP5 negs for a friend that had been shot recently, the photos were made in a swimming pool. The prints needed a lot of magenta to print, Grade 4 or 4 1/2, I'm guessing this is normal due to the diffraction? encountered underwater. I'm wondering if there would be a filter that could be used underwater to increase contrast. I ask because I'm thinking what works outdoors to increase contrast might be very different to what is needed underwater. I will be suggesting to the photographer that she develops the negs from underwater for a higher contrast.
Underwater photography considerations
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It is more likely to be a reduction in contrast from being under water Diffraction would not as far as I can see make any difference to the negative contrast. Underwater light is also quite soft anyway so will need a bit of a boost to help the contrast.
Underwater diffraction will alter the apparent focal length of the lens. There used to be a table of what lens you should use to get the apparent same field of view. Even that is not a definitive science, Salt water and fresh water will differ so what chlorinated swimming pool water is I have no idea. From memory, To give you an idea in sea water the diffraction was such that to get the same field of view as a 50mm lens you would have to use something around a 35mm
Underwater diffraction will alter the apparent focal length of the lens. There used to be a table of what lens you should use to get the apparent same field of view. Even that is not a definitive science, Salt water and fresh water will differ so what chlorinated swimming pool water is I have no idea. From memory, To give you an idea in sea water the diffraction was such that to get the same field of view as a 50mm lens you would have to use something around a 35mm
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Diffraction between water and glass results in a 1/3 reduction. So a 35mm lens above water acts like a 50mm underwater as bike rider mentioned. You need to shoot with strobes to bring back the color (if she shoots color at some point) and get good contrast. Especially the deeper you go the water sucks out the colors leaving blues.
I've not never shot BW film, only color Ektachrome. I think 200 ISO if I recall correctly. But have converted a few after scanning to BW. DIgital processing leaves a lot of flexibility with adding contrast if she goes that route. You can see three of them here. Good luck.
https://www.flickr.com/photos/alanklein2000/albums/72157627032961729
Can you post an example?
This is only an iPhone pic (flatbed has been packed away). It was printed on grade 4.5 filter on new Ilford RC paper.
Very nice. Artistic. I didn;t know you could do this underwater with a iPhone. Which one? Was it in a case?
No, it is the different refraction of rays in water and air that does so.
What the OP seemingly refers to by diffraction is wild scattering of light due to turbitiy in the water.
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