He just wants an EVF during mirror lockup. Macro photographers and
long telephot photographers use mirror lockup to minimize the
vibrations produced by the moving mirror. The problem is that, with
the mirror locked up, you can't use the SLR viewfinder, so you
can't see when that stalking Lion hunches for the leap, you can't
see when the butterfly's wings are spread nicely, etc.
These are really mechanical problems. Why do you NEED to lock the
mirror up? Bad mirror damping
Damping has nothing to do with it. Newton's third law of motion, for every action, there is an equal and opposite reaction. Mirrors move in an arc. To counter that requires counterweights (two of them, one on each side of the mirror, so you don't generate an axial torque) moving in an identical and opposite arc. No camera has such a system, and no camera is adequately damped for high macro ratios, telephotos with very high magnifications, or astrophotography.
or maybe bad shutter balancing
Again, without matching counterweights, it's not a question of "balancing", either.
(have
you EVER spoiled a shot due to mirror shock?).
Yes, literally hundreds.
OK, accepting that
shock IS an issue, why not use a pellicle mirror or prism like the
Olympus E-x0s?
I've used a pellicle mirror camera. They're fragile, they suffer mirror rot, they're so hard to clean that they make sensor cleaning look like a snap, and they cause vertical image smear, even with the pellicle is only a few microns thick.
There literally isn't room for a prism in current SLRs, unless you make the sensor even smaller (say a crop factor around 2x). The prism is a 45 degree solid beam splitter. It has to be the height of the sensor, and because of the index of refraction of the glass, it has an optical thickness about 1.5x the sensor height. There's not room between an SLR shutter and the back of the lens for it.
The next issue if that of the focal plane shutter -
if your sensor NEEDS a mechanical shutter, and you don't want to
build a stills camera with a continuous sector shutter (like a
movie camera) then EVFs aren't an option anyway. If you're working
with macro subjects at such high mag. that MLU is necessary then
are you not also likely to be using strobe lighting?
No, actually, you're not. Most high magnification macro photography is done with continuous light, typically a fiber optic cold light unit (although I'm doing more with white LEDs these days). These devices don't mix continuous and flash. For example, one of my macro lenses is a 25mm f2.5 Leitz Photar. At 10x magnification, that's an effective f25. it takes a lot of light to be able to compose and focus.
And astronomy?
You're likely to have several instruments on your eq mount anyway -
use a webcam on your pointer for timing if needed.
There's a little thing called focusing....
As for the
butterfly's wings - you can always view the subject direct! The
wildlife example makes no sense - you're gonna need a decently high
shutter speed to capture a dynamic subject like a leaping lion -
I didn't say "leaping", I said "hunching for the leap". Animals pose. They do an incredibly interesting variety of things at relatively slow speeds, before they go bounding or leaping. You need to see it the animal to catch an interesting pose.
If I were after an actual leap, I'd use a higher ISO that for a more "posed" shot. The action would go a long way towards gaining forgiveness for the noise.
why use MLU?
I see the desire - 'video' during MLU, but the NEED escapes me
(with one exception - Canon's implementation is surely to allow the
photographer to see beyond the visible spectrum).
Actually, it's specifically for focusing. You can zoom up to about 20x while it's engaged. The Canon really isn't built for use beyond the visible spectrum, just visible band of red, the H-Alpha wavelength at 656.3nm. (Canon didn't want another Sony "x-ray camera: scandal so the Da still blocks IR beyond 730nm or so)
Most DSLRs, in their quest to eliminate IR, eliminate over 2 stops of valuable H-alpha. This is partially because the IR blocker starts cutting out red, and partially because the blue "spectral tilt" filter is equalizing the sensors spectral curve (sensors are much more sensitive to red than a human eye. Some of the red needs to be knocked down).
Here's a pretty H-alpha picture. Not one of mine.
http://antwrp.gsfc.nasa.gov/apod/ap980828.html
--
Salvage troll posts! When you see a thread started by a troll, post something useful to it. It will drive the trolls up the wall.
Ciao!
Joe
http://www.swissarmyfork.com 
