Thanks, everyone.
It also explains well why cropped cameras have the AF points more spread out. The lens register distance is the same (we use the same lenses), so the mechanical design constrictions are similar, but the smaller sensor sees relatively less of the image, so it appears that the AF points are closer to the edges.
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You're quite right. Canon decreased the mirror size and shortened the back focus distance by a small amount for EF-S lenses, but nowhere near enough to provide the same ratio between back focus and format size that exists on 35mm. One beneficial side effect is the increased autofocus coverage. One of the downsides is that it makes fast wide angle lenses far more expensive (like a theoretical 15mm f/1.4).
Is an electronic viewfinder really a
step in the right direction?
It's a personal preference. The existing EVF technology is in the stone age compared to what's coming. But even what we have now is already superior in many ways to OVF. I use the EVF on my 5D2 (aka "LCD") as much or more than the OVF, even for handheld (actually, shoulder mount) work.
Advantages of an EVF are:
Zooming to any part of the image for critical focus
View the actual captured image, including white balance, black and white, etc;
Tilt/swivel.
Movie mode.
Live zebras, histograms, false color "raw" mode, guide lines, overlays, etc.
100% accuracy for framing.
Accurate DOF (typical viewfinder screens show f/1.2 as if it were f/3.5, making critical manual focus impossible).
Accurate rendition of the bokeh
Usable in much darker situations, including effective f/32 for macro, etc.
Contrast detect autofocus with movable focus points
Allows the entirety of the primary mirror to be used for phase
detect autofocus (instead of a small secondary mirror), so AF sensors
can cover the entire frame instead of just the middle third portion.
The possibility of reducing lag from 40ms to 0ms or even negative
lag (this would not apply if a mirror is still used for phase-detect
autofocus).
Disadvantages, of course:
Battery usage
Heat generation (in some circumstances can cause thermal noise to rise above the read noise floor).
Current technology may be relatively expensive, slow frame rate, low resolution, reliability, and/or bulky.
Dynamic range limited by the sensor instead of the eye/brain.
I'm asking because I can only imagine the
quality to be worse than a good optical viewfinder due to (a) the lower
spatial resolution, and (b) the compression/clamping of the dynamic
range into 18 or 24 bits ... but correct me if I'm wrong, I have only
ever seen cheap and/or old electronic viewfinders (that were quite
horrible, in particular on the resolution side).
The resolution and DR are definitely worse, right now. But it does have things like 10X zoom that result in far higher resolution (for that one part of the scene), which, for me, are enough to outweigh OVF advantages, even with today's technology.
How does this schematic change when the reflex mirror is changed to
a fixed pellicle mirror as found in the earlier EOS-1N RS?
Great question; I have no idea.
Given the geometry in the diagram, there does not seem to be any
reason why the AF mirror and sensor assembly cannot be made wider--that
is to say, it could conceivably be made to stretch lengthwise across
the frame. This would not be reflected in the cross-sectional
diagram. There must be other considerations as to why this is not done.
Agreed. I don't know the reason either.
I can see that one theoretical modification to this geometry
which would enable greater vertical coverage would be to use multiple
AF mirrors, the height of which would form a geometric progression.
That is to say, the bottommost mirror would cover the bottom half of
the frame, a second mirror would cover the quarter extending from the
horizontal mid-line upward, and so forth. Of course, this is not
practical, but in theory, multiple AF mirrors would enable greater
coverage. The problem of course is that this drastically increases the
complexity of the mirror assembly, and would create horizontal gaps
where the mirror attaches to the main reflex mirror, where no AF points
could be placed.
Neat idea!
I agree that an electronic viewfinder would be the future, given
that contrast-detect AF is not yet fast enough to yield satisfactory
performance. However, this raises questions about how to deal with the
behavior of the shutter, presuming that the main imaging sensor is used
as the VF.
If you go to EVF but *keep* the mirror, then you can dedicate the mirror entirely to AF, and you don't have to use the slow contrast-detect AF. (Or you have the choice of either, depending on whether you feel like waiting for the mirror to slap up and down or not.)