The moon with 70-400 and a99 vs a77

Started Nov 26, 2012 | Discussions thread
Chimere Senior Member • Posts: 1,577
Entry pupil or front lens diameter ?

There might be a bit of confusion here: it appears to me that it is the diameter of the front lens / primary mirror and the primary focal length which is the trouble, not the f-stop (diaphragm diameter). The larger the diameter of the front lens / primary mirror & the shorter the f-length of it, the larger the diameter of the "tunnel" of light gets cutting through the layer of the air surrounding the globe, the more prone the image gets to be influenced by oscillations, gradients of warm and cold air.

The f-stop setting has nothing to do with it, as it is located in the optical centre of the lens, and it will not change the "light tunnel" diameter. In other words, as we all know, the angle of view with 2.8:1 is the same as 22:1.


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