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11-25-2020, 01:07 PM   #16
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QuoteOriginally posted by stevebrot Quote
The first point is true, though the combination of mount diameter and short register can put a crimp on things. The second point is often false. Angle of incidence is important.

But that is a mattor of lens design. Just because you put the lens closer to the sensor does not necessarily mean angle of incidence change.
A DSLR lens with a small rear lens element may give worse angle of incidence than a mirrorless design.

It is just that on old lens design for film, SLR lenses usually have an advantage in this regard.
But that does not mean that lenses designed for digital sensors has the same advantage. Then the limitating factor of moving the lens further away from the sensor can be a bigger disadvantage, as it may require a more complex optical formula.

11-25-2020, 03:24 PM   #17
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QuoteOriginally posted by house Quote
but I though you used the retro focus design to make the rays more parallel.
Ummmm...not quite. That is a happy side-effect of using retrofocus with a long flange focal distance.

Retrofocus was originally the name for a series of Angénieux wide angle lenses dating from the early 1950s. The proper name for the design is inverted telephoto or sometimes reverse telephoto. A full explanation would involve a lot of text and diagrams, but I think I can distill it down to a few points...
  • The focal length of a simple lens is the distance from the lens center to the focus plane. The same is basically true with symmetrical-design complex lenses.
  • The above works well when the focal length is long enough to accommodate the rearward elements and whatever hardware might be between the lens and the film/sensor. It does not work well as focal lengths decrease and stuff like focal plane shutters and mirrors get in the way of the rear element when attempting infinity focus.
  • While this is a huge problem for SLRs with lenses shorter than about 50mm, it can also be an issue for mirrorless cameras if the focal length places the rear element in the way of the shutter curtains or frame guides when focused to infinity.
  • Retrofocus design addresses this problem by moving the lens center rearward so that it is well behind those rear elements.
Consider a 10mm lens on Sony E mount where the mount flange is only 18mm forward of the focal plane with non-negotiable thickness of the shutter and sensor stack. If not retrofocus, the 8mm lens in the video would be placing glass within a few millimeters of the shutter. That does not make for very easy lens changes. Below is a diagram of a retrofocus lens...

Notice that f' (the focal length) is measured from well behind the rear elements.

Telephoto lens - Wikipedia


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