mcc wrote in message <01bcfc18$13adc060$83e531cf@armadel>...

>This may be an obscure question, BUT --


No, it is a very relevant question, basic to getting sharp
images with different types of long FL lenses...

>When I look at a long lens on a camera it seems pretty clear why
>it would be more susceptible to camera shake and would require a
>faster shutter speed when hand held -- the length of the lens
>would amplify any unsteadiness in the camera. But is this a
>factor of the lens's physical length or focal length?

FL primarily (due to magnification of image movement), but the
lens-camera physical characteristics are also important...

>For example, if I took a 400mm lens (that actually was 400mmm long),
>a 200mm with a 2x teleconverter, and a 400mm mirror lens and shot
>an exposure with each lens on an equally unseady camera, would
>the images be equally unsharp or would the shorter physical lengths
>of the mirror lens and 200/2x TC show less unsharpness?

Assuming the lenses and combinations were of equal optical quality
at 400mm at the same effective stop used (a rather unlikely
occurance...;-), I would expect from the physical attributes for
the 400mm to give the sharpest results, the 200 + 2X to give the
next sharpest results, and the mirror alone to give the worst
(assuming that same order for physical length and weight...).
If you go back to basic physics, you will find that both increased
mass and increased length will help in steadying a system and
slowing its rate of vibration/oscillation.

>And how
>about "compact" lenses, like the Tokina 35-300 which I hear is only about
>105mm long (physical length) at 300mm focal length? The rule of thumb
>is to shoot at the inverse of the lenses focal length when hand holding
>shots -- but should that be physical length as opposed to focal length?

Generally it refers to FL, but you are right in questioning the
effects of physical length (and weight). An example: I have a
compact 200mm f4 lens, a noticeably larger and heavier 70-210mm
f4 zoom, and a VERY noticeably larger and heavier 80-200mm f2.8
- and at 200mm, I can get away with 1/60th (briefly, until I tire
holding it up in the air! ;-) with the big zoom, maybe 1/125th
with the smaller zoom, but no less than 1/250th with the compact
200mm... Or, I can hand-hold (briefly!) the big fast teles at
speeds that are impossible with their slower f4.5-5.6 counterparts.
The same effect occurs on tripods - mass and length help.
(BTW, you can find some VERY-long-lens images on my web page [up
to 1400mm] - if you find a pair of "-" marks under a photo,
highlighting the space between with your mouse will show what
lens was used to make the photo.)
Hope This Helps
David Ruether - http://www.fcinet.com/ruether