I went online to find what aperture the Diana's three settings correspond to and I even found them. They're f16, f6.3, and f4.5. The shutter speed is variable, of course, it's between around 1/30 and 1/200.
PS: On another site, it said the apertures are f16, f11, and f8 with a shutter speed of around 1/45. Well, that agrees for the "sunny" setting but for the other ones it disagrees.
PPS: I asked the experts on www.toycamera.com. Somebody said he used f16, f6.3, and f4.5. Of course, Dianas are no precision tools anyway so all this has to be taken with a thousand grains of salt. But it's useful to know. I think what I might do for photography under very low light (planned for an upcoming trip to Durham [UK]) is to use the "sunny" setting and "B" plus that light meter I will have to bring to use my Leica. I know I'm gonna be the freak of the week when I use a light meter for my Diana but hey! If the results come out the way I want them to come out I just don't care!
