> I got the f/stops is f/stops part. That has to do with the
> surface brightness on any given area of the film and screen.
> The mystery was why the eyepiece brightness is not higher for
> the larger format.
> Presumably the image magnficication through the eyepiece is
> less for the larger format and should appear brighter?
Because the viewfinder eyepiece is a lens with an F/stop too.
Some cameras have bigger eyepieces (larger apertures) than
others. All have different magnifications. If you increase the
magnification AND the aperture, you can maintain the same
apparant brightness while increasing the magification.
Who knows, you might be able to have a lens with better
characteristics in other areas too. The eyepiece on the Pentax
645n is a classic example--your eye has to be perfectly centered
otherwise the image blacks out. Just like trying to look
through binocs at night that have too small of an exit pupil.
When you are working in a studio environment, the
exposure(brightness) of the subject is determined by the
flash-to-subject distance (and power output). Provided you are
using the exact same F-stop, the exposure to the camera is
exactly the same regardless of how far or near the camera is to
the subject AND what focal length the camera lens is. It's like
the sunny-16 rule--it works regardless of the lens or
subject-camera distance.
A viewfinder's optics are exactly like the camera lens in the
above example and the focus screen is a "backlit object" which
the lens is focused on. Our eyes are essentially the "film" in
this example. Changing magnification (apparant optical size of
the focus screen) is a matter of changing the optics of the
viewfinder's eyepiece. Remember, the actual surface area of the
focus screen does not change in any way--therefore it's actual
brightness is fixed.
A television or computer screen is just as bright if you are
inches away from it or sitting across the room from it. If you
sit across the room and use binocs to view the screen it's the
same as if you were sitting right in front of the screen.
Now, there are tradeoffs, which have absolutely nothing to do
with brightness.
AG
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