I am pretty sure Marianne's experiment is flawed

Started Jan 22, 2013 | Discussions thread
Leonard Shepherd
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Re: I do not agree it is "flawed" but neither does it take into account all possible factors
In reply to Leo360, Jan 22, 2013

Leo360 w

Marianne's experiment is not perfect but it captures the essential part: the light that enters the lens near the periphery of the front element does not reach the AF sensors.

Perhaps being overlooked is AF uses only the central portion of the image covered by the sub mirror. Changing from an FX body to a DX body without changing the focus distance reads from a smaller part of the scene. The exposure does not change switching from FX to DX.

A central disk is not new - all mirror lenses from last century have one.

Blocking off part of the front element does reduce it's physical dimensions. Provided 42% of the surface area is blocked out the light transmission is reduced by one stop.

With a lens like the 200 f2 anything on the front element is very much out of focus and has little as distinct from no effect on autofocus, unless disk quite a bit larger than the one used in the experiment is used.

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Leonard Shepherd
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