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FAQ: Stopping down a lens combo
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rjlittlefield
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Joined: 01 Aug 2006
Posts: 18868
Location: Richland, Washington State, USA

PostPosted: Wed Jun 06, 2018 6:22 pm    Post subject: Reply with quote

manu de hanoi wrote:
when you stop behind the front lens, you reduce the NA (I undesrtand the NA is defined by the angle of the most outer rays), that would seem, theoretically, to be more damaging to the resolution than stopping behind the back lens.

Your application of the theory overlooks a critical fact. In fact, stopping the back lens reduces the NA also, by blocking rays that go through the periphery of the front lens from getting through to the sensor. For resolution, it makes no difference whether the rays get blocked sooner or later; the only issue is that they do get blocked.

At image center, there is theoretically no difference between stopping the front lens and stopping the back lens, assuming that you end up at the same NA and effective f-number. The same rays get selected either way. In practice, there may be a little more veiling glare one way or the other due to stray reflections.

Away from image center there are differences that can be very important, as discussed above. This is because changing the stop position changes which parts of the glass are used.

Quote:
Perhaps the blur you get by stopping behind the back lens could be removed by using monochromatic light?

Certainly the colored banding would go away. But I suspect that a good deal of the blur is due to other aberrations such as coma that would remain.

--Rik
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