Re: viewfinder magnification

[Gonz]

Since the *istD is using only a portion of the full 35mm frame, I wonder 
 why they dont do something like engrave a tiny grove showing where 
100% is on the viewfinder, but cover something like 110% of the frame, 
so you can see where you are cutting if off in context.  I don't know 
that the issues around alignment would be, probably the same as you 
mentioned.  But maybe if they had a simple means of alignment with this 
technique when it was built using some type of fixture, it might not be 
so bad?  I'm just musing here...

rg
John Francis wrote:
I don't quite buy that. What determines the size of the viewfinder image is the
size of the frame the screen sits in (as long as we are talking +/- a
millimeter). Make that frame a little bit larger and you have a 100%
viewfinder. Of course, all elements that attach to the mirror box have to be
'accurate' but I don't see why that would be so difficult here.

Ah, but that's precisely what *is* difficult.  It's not just the size of the
viewfinder - it's position the boundaries accurately.
It's easy(-ish) to make a 95% viewfinder, because you only have to position the
viewfinder region to +/- one mm.  Make the frame a little bit larger, though,
and you don't have a 100% viewfinder; if you're off by that same 1mm you might
have a viewfinder that showed 97.5% of the image, cropping off 2.5% on the left,
and showing an extra 2.5% on the right that wasn't part of the true image area.
This would be bad. If you can see it through the viewfinder, people expect it to
show up on the image. The extra 5% allows for a certain amount of inaccuracy.
To get a true 100% viewfinder (no more, no less) would require at least an order
of magnitude more accuracy - everything would have to be aligned to a precision
of 0.1mm or better.  That would require a camera considerably more rigid, and
manufactured to much closer tolerances, than consumer-level prices can support.