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Camera
This is noisy. With a medium-format SLR, the mirror is four times the size of a Nikon's and very noisy.
A final problem with an SLR is exposure latency. If you wait for the decisive moment and press the
shutter, the camera doesn't take a picture until it has stopped down the lens and flipped up the mirror.
This takes between 50 and 100 milliseconds for the average 35mm SLR.
Note that a few 35mm SLRs have been built with fixed semi-transparent mirrors called "pellicle
mirrors". The Canon EOS line includes a cheap discontinued EOS RT model and an expensive current
EOS-1 RS model. The RS's mirror sends one-third of the light to the viewfinder and two-thirds to the
film. Thus the viewfinder is more than 1 f-stop dimmer than a standard camera and the film gets 2/3 fstop
less light than with a standard camera. Advantages are that the picture gets taken 6 ms after you
press the shutter release, you retain your view of the subject at the exact moment of exposure, the motor
drive can operate at a blistering 10 frames per second, and there is less vibration.
Twin lens reflex (TLR)
A twin-lens reflex has two lenses (the twin lens) and a
mirror to bounce the light from one of them onto a
ground-glass focusing screen (the reflex). Lacking a
prism, a TLR tends to be lightweight. Since the mirror
remains fixed at all times, a TLR tends to be quiet and
exposure lag is minimal. TLRs are mechanically very
simple. Consider that in an SLR the lens must have an
automatic diaphragm that remains open until the instant
before exposure, then stops down quickly to taking
aperture. With a TLR, there are separate taking and
viewing lenses and therefore the aperture knob can
directly open and close the diaphragm blades.
TLRs suffer from potential misalignment, e.g., when the image is focussed on the ground glass by the
viewing lens, it might not be focussed on the film plane by the taking lens.
TLRs suffer from parallax. The viewing lens is higher than the taking lens and captures a different
image. If the image is a mountain 20 miles away, the three inches of separation won't be significant.
However, you can forget about doing macro work and you might get interesting framing errors if you're
close to the subject.
The classic collectible TLR is the Rolleiflex, which takes 120 and 220 roll film. Though there is no
technical reason why TLRs couldn't be built for other film sizes, virtually all make 6x6 cm images on
120 film. The TLR that poor photography students use is the Yashica 124 (see photo at right; I took it
during my junior year at MIT). The TLR that wedding photographers use is the Mamiya because you
can change the lenses. The only TLRs currently in production are the Rolleiflex, which is priced from
http://www.photo.net/making-photographs/camera (5 of 9)7/3/2005 2:22:21 AM