Today I was looking for a lens to put on my MPP 5×4 to shoot a still life in the studio, and I picked out a 150mm f2.8 lens which originally came from a photocopier. The reason I know it’s origins is because about 15 years ago, a slightly eccentric neighbour was breaking up an old photocopier outside my house, so I asked him if he would give me a lens if he found one inside. I got it and I’ve had it sitting in a box of odd lenses and unusual bits of glass since then. Putting it on the camera, I really liked the shallow focus and beautiful soft background, but with a fixed aperture of 2.8, I had to find some way to control the exposure. I decided that some very slow X-Ray film and a diffuser over my tungsten light would give me a manageable exposure which I could time in seconds. I cut some strips of X-Ray film and put them in a dark slide, then did a couple of test shots, one at the exposure I expected and one with more exposure. The second one gave me the kind of negative I was after, so I cut a piece of film to the full 5×4 size and exposed it. I took it into the darkroom and processed it in a tray of paper developer for a minute and a half, gave it a quick stop and fix, then washed it. The negative looked much softer in the background than it looked on the focusing screen (this is something I’ve noticed a lot) and I thought it would make a nice print. Then I remembered that somewhere in my studio I had a 150mm f2.8 projector lens which gave really nice out of focus softness, probably better than this one I’d just shot, so I thought I’d expose a second shot through it and compare them.
I had a faint idea that I’d read somewhere that two lenses with the same focal length and aperture should produce the same depth of field, so I was interested to see if it was so. After shooting and processing the second sheet of film through the projector lens, I could see immediately that they were quite different. The projector lens had a much shallower focus and was far softer in the out of focus areas, so I must have misremembered the thing about comparable focal lengths.
Anyway, I present the two images here for comparison.