What you and Moose have said now seems to make perfect sense but I'm
still curious about the reality. According to your arguments an f/4
lens on a 6x6 camera should have approximately the same screen focusing
brightness as an f/2 lens on 35mm. In fact, it should be brighter since
the eyepiece doesn't have to magnify the image as much.
Can someone with a 6x6 and 35mm verify this?
Chuck Norcutt
Dan Mitchell wrote:
>
> So there's 4 times as much light available on the mirror to be bounced
> into the eyepiece, and so if the image I'm seeing is the same size to my
> eye, it'll be 4 times as bright in the 120mm camera. (or it could be
> twice as large in each direction and the same brightness).
>
> It doesn't really matter how large the focussing screen is, or how
> much magnification happens between the focussing screen and my eye;
> there's Z amount of light available bouncing off the mirror in the 35mm
> camera, then there'll be 4Z that amount of light bouncing off the mirror
> in the 120mm camera.
>
> What happens between mirror and eye is then the usual tradeoff between
> image size and image brightness -- but the 120mm body has four times the
> light available when making that tradeoff.
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