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>...........................................What I've learned about VF
>screens is this: they can be optimized for only about a 3 stop range of
>light and a range of focal lengths. For example, a screen that would give
>you a good image with a 20mm 2.8 would probably be no good for a 1000mm
>f/16, and vice versa.
This is true for all-out brightening attempts (actually the range of
FL and aperture acceptability narrows as the brightness optimization is improved...), which is why general-purpose (perhaps somewhat dimmer,
but we can handle it, can't we? ;-) screens may be preferable to brightness-enhanced screens. I can use the basic Nikon B/E screens
easily for both focus and evenly-illuminated viewing with lenses
ranging from 8mm to 1000mm, and from f1.2 to f11 maximum aperture.
BTW, I did try F4 screens in my F3, and found no advantage, but
some disadvantage with unusual finders, like the big sports finder
(brightness enhancement may also limit the VF types that work well
with the screens).
Hope This Helps