sentences:  In SLR camera systems, typical mount diameters are in the range of 44-54 mm, with flange distances around 45 mm.This limits the maximum possible f number to 1.0 to 1.2, with rather strong vignetting towards the edges of the image.Flange distances are significantly smaller for rangefinder and mirrorless cameras (even below 20 mm), theoretically enabling designs down to something like f/0.7 or even faster.