Have you been following the digital rangefinder developments
closely enough to know the challenges of illuminating a full-frame
sensor with current rangefinder lenses?
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I know nothing of this but can you tell me how the challenges
differ from illuminating a full-frame sensor with existing SLR
lenses?
As both rangefinder and SLR lenses were designed to illuminate a
full 35mm frame won't they perform the same on a 35mm sized sensor?
Because there's no swinging mirror, rangefinder lenses can approach very close to the film (or sensor). Some almost touch the shutter, coming within 4-5mm of the sensor. This means wide angle lenses like a 35mm, 28, 24, or 19mm can be symmetrical, which makes correcting many kinds of aberrations easy, and makes the lenses relatively cheap and low cost.
Because of the mirror, the rear element of an SLR lens can't approach any closer than 38mm from the sensor. So those wide angles lenses are asymmetrical, using what are called "retrofocus" designs, like telephoto lenses, in reverse so the negative elements are to the front, the positive elements face the sensor.
Retrofocus lenses for SLRs have exit pupils at least 50mm from the sensor. The exit pupil is the "image" of the aperture, and is the place where light "comes from" before it hits the sensor. With a 50mm exit pupil, a lens won't have light that's more than arctan(43.3mm/2/50mm) = 23.4 degrees from perpendicular to the sensor. This is a bit much for many sensor designs to cope with (look at the corner problems with Canon 5D) and is just on the edge of acceptable.
Rangefinder lenses have exit pupils at the same distance as their focal length from the sensor. So a 35mm rangefinder lens has a 35mm exit pupil, and the angle on full frame is arctan(43.3mm/2/35mm) = 31.7 degrees from perpendicular. That lens would work worse than the worst of the "problem lenses" on 5D. It's even worse for wider lenses, a 28mm (very popular for rangefinders) is 37.7 deg from perpendicular, it's pretty much not going to work. A 19mm is 48.7 degrees, forget about it, and forget the 12mm Helier.
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