The primary cost savings in the design of an SLR viewfinder has to do
with the size of the prism and the accuracy which which it has to be
assembled. A 100% coverage viewfinder prism for 35mm format is large
and expensive, and has to be assembled extremely accurately or it
reflects an image which is not what will be captured. The usual cost
savings technique is to use a smaller prism and only cover 85-96% of
the format total area ... this reduces immediately the size and cost
of the prism, and reduces manufacturing cost further as it doesn't
have to be assembled quite as accurately.
Format coverage has little to do with viewfinder optical
magnification and eyepoint. Those are optical considerations based
upon both marketing and engineering requirements.
There is a small effect on total brightness from reducing coverage a
few percentage points, but it is mostly insignificant. Efficiency of
the prism itself, and of the mirror and focusing screen, influence
brightness significantly. Mirrors with semi-silvered components to
support focusing and metering cost somewhere in the 0.2-0.3 EV range
on light transmission. The focusing screen design and supporting
optics count for a tremendous amount ... generally more than the
semisilvered mirror. Generally speaking, EV deltas of less than
0.6-1.0 EV are difficult to recognize by eye. Screen design can
account for 1 to 1.5 EV changes in apparent brightness.
Magnification again has only a little to do with it...
So I tend to side with John Francis' thinking on this: it was the
inclusion into the viewfinder of ancillary information (aperture,
metering indicators, shutter speed, etc) that reduced viewfinder
magnification in more recent cameras and not a direct result of AF.
The manufacturers added this information starting in the mid-1970s as
a result of customer demand for more informative displays.
Godfrey
On Jul 22, 2005, at 10:34 AM, P. J. Alling wrote:
I'll admit optics I'm not so good with. I'd hardly try to grind my
own lenses but manufacturing I
know quite a bit about and optics isn't even half the problem.
John Francis wrote:
I could equally well say you don't quite understand the optics of a
viewfinder if you think the physical size of the pentaprism/mirror
makes any significant contribution to eventual image size.
On Fri, Jul 22, 2005 at 12:55:38PM -0400, P. J. Alling wrote:
I don't think you quiet understand precision manufacturing if you
don't understand the cost savings in
making a smaller viewfinder.
John Francis wrote:
On Fri, Jul 22, 2005 at 11:18:07AM -0400, P. J. Alling wrote:
Kostas Kavoussanakis wrote:
Not true, the viewfinder is irrelevant to auto focus. The
camera doesn't use the viewfinder for focusing, you do.
Viewfinders are only for aiming and composition in AF, that's
why it's brighter, (though the LX with a modern screen is also
brighter and is very good for focusing). Their importance in
focusing is minimized. There is no technical reason why a
viewfinder needs to be smaller, the reason for that is cost.
(Well sensor size has something to do with it as well but still
cost rules).
There's no cost saving in making the viefinder image smaller
(unless
you go really cheap, and substitute a pentamirror, in which case
the
smaller image size compensates for the additional light losses).
The reason viewfinder images got smaller is because there was a lot
of additional information being displayed as well as the image. To
get all that stuff inside a comfortable viewing angle something had
to give. Increasing the viewing angle wasn't an option; having to
move your eye around to see all the viewfinder information is a
pain.
--
When you're worried or in doubt, Run in circles, (scream and
shout).
--
When you're worried or in doubt, Run in circles, (scream and
shout).
