Topic: Diana+ lenses and apertures question

[imagesfrugales]

Hi there,

I will get a 38mm lens for my Diana+ in a couple of days and can't find any infos about the the effective apertures. They are said to be 11, 16, 22 and 150 for the regular 75 mm lens. Would the apertures for the wide lens calculate as 5.6, 8, 11 and 75 since 38 mm is about 1/2 of 75 mm? It surely would if the aperture would be in the focal distance plane, but it is much nearer to the body. Any experiances or thoughts?

[Francois]

Usually single lenses have a focal length that is the same as the lens to film distance...
I doubt Lomo would have taken the time to make a complex multi-element lens for the Diana+.
I would just use the same old formula for calculating the aperture... (focal length / diameter=fstop)

[imagesfrugales]

The 38 mm lens must be a more than 1 element retrofocus design because of the flange to film distance of about 55 - 60 mm. The aperture is part of the body and about 15 - 20 mm behind the lens element of the 75 mm lens. The physical aperture wide open is 75 : 6 = 12.5. Makes no sense imo, should be much bigger for an effective aperture of 11, but that's only raw guessing without real knowledge.

[imagesfrugales]

OK, i got it. The effective aperture is calculated by the focal length devided by the entrance pupil of a lens. It depends on the location of the aperture hole and is a projection of it. See last chart on the bottom of this site:
http://laurenz-pelz.de.tl/Website--g-Puchner-g--Die-Kamera--k1-Aufbau-k2-.htm
Sorry that I haven't found an english site with a similar extremely simple chart.

So, if the aperture is located closer to the film plane, the aperture hole is smaller than the entrance pupil displayed in blue color. Obviously, the image circle becomes too small if the aperture hole is too close to the film plane.

Now deviding the aperture numbers by 2 for the 38 mm lens should be a sense-making first approach.

[Francois]

Since the apertures are in the camera, values are fixed...