With the exception of a couple of very special cameras (which I will deal with
later), every SLR has a mirror which must be moved out of the optical path before
a photograph can be taken. When this mirror moves (and especially when it stops
moving!), it causes vibrations to be set up. These vibrations move the camera
(and attached lens) around and can lead to a loss of image sharpness.
This is especially the case when using long telephoto lenses or when doing
closeup (macro) photography.
The loss of image shapness is worst at intermediate shutter speeds, say
between 1/60 and 1/2 second. At very fast speeds the shutter is only open for a
small part of a vibrational cycle, while at long shutter speeds the vibrations
can die out, so only a small part of the total exposure is taken while the camera
is shaking. Note that an flimsy tripod can make vibrations worse, but even the
biggest, heaviest tripod can't eliminate vibrations completely.
In general, for lenses of 300mm and over (and macro work), users should try to
avoid using shutter speeds between 1/30 and 1/4 second for optimal sharpness. For
"normal" work with lenses of 100mm or less, sharpness loss due to mirror induced
vibrations does not seem to be an issue.
The solution to this problem is Mirror Lock Up (MLU), where the SLR mirror can
be locked in the "up" position, vibrations can be allowed to die out, then the
shutter fired. Not many current SLRs have this function (the Nikon F4 and F5 do,
as does the Canon EOS-1n), which is a pity. Some cameras do have what is called
"mirror pre-fire" though. This is a mode in which the SLR mirror moves normally,
but the camera delays the firing of the shutter by a couple of seconds, so the
mirror induced vibrations have a chance to die out before the shutter fires. Most
Canon EOS bodies have this function, for example.
Nature photographers planning on using long telephoto or macro lenses might
want to look for MLU (or mirror prefire) on any camera bodies they plan to
purchase. If a body doen't have MLU and you must use speeds around 1/15 with a
telephoto lens, tricks such as putting one tripod under the lens and a second
tripod under the camera can help, as can laying a sandbag, beanbag or even a hand
on the lens above the tripod support point.
A few cameras have a fixed, semi-transparent mirror, rather than a
conventional moving mirror. Obviously these cameras don't need MLU! The only
current camera of this type is the Canon EOS-1n RS, though the Canon EOS RT can
be readily found on the used market. The downside of these "fixed mirror"
(pellicle) designs is that they cost about 2/3 stop of light at the film, and 1
1/3 stops in the viewfinder. The upside is that you can see through the
viewfinder in real time. With a locked up mirror, the viewfinder is blacked
The figure below shows a typical vibration plot which illustrates most of the
points made above. I made the measurements based on the deflection of a laser
beam reflected from a camera, and it is part of an article I wrote for George
Lepp's "Natural Image" newsletter.