Just like with a teleconverter, brightness will fall off in
proportion to the magnification as a square function. A 2x
teleconverter costs two stops. Same will happen in a viewfinder ...
your f/1.4 lens will end up looking like an f/2.8 lens, and, more
critically, an f/4 lens will look like an f/8 lens through the
viewfinder. That makes it somewhat hard to focus accurately, unless
the lens is very long.
A 16x24mm focusing screen only has 40% the area of a 24x36mm focusing
screen. If the rest of the viewfinder optical system is of identical
efficiency, that cuts illumination by 1-2EV already. Magnify it to
look twice the size and it will be too dim to be an effective
focusing system.
Godfrey
On Mar 22, 2006, at 2:46 PM, Rob Smith wrote:
Is this actually the case? Is there any fundamental problem with
altering the magnification of the viewfinder to give a larger
image ? (other than that it would not be as bright). In principle I
would expect you to be able to have an MX style viewfinder with the
equivalent of a teleconverter in the viewfinder optic path to
achieve this and quite possibly there would be more elegant and
less bulky alternatives possible with just pentaprism design -
after all different viewfinders do have different magnifications
don't they? Most are not actually 100%.
In short, although it seems conventional for the viewfinder image
to be approximately the same size as the film/sensor size I see no
reason why this has to be the case.
Rob.
