Frank Ernens <fgernens@xxxxxxxxxxxxxxx> wrote:
> > On the other hand all OMs have the disadvantage of light weight,
> > while sheer mass can help damp vibration
>
> Is there a scientific basis to this? Because, it seems to me, that
> some rather heavy objects - swimming pools of water, railway tracks,
> concrete paths - transmit sound (vibration) very well. The shape
> and material might matter more than the mass. Do we have any
> civil engineers subscribing? Surely we have a physicist or two.
>
Well, here's one physics major on the list (finished all of my
classes, only thesis left).
Consider a camera made by carving a little tunnel to a light-tight
box in the center of a mountain. Mount a very badly designed
shutter on this tunnel (one that really vibrates like all hell)
and take a picture. No matter how badly the shutter wants to
vibrate, its mass is so small compared to the mass of the mountain
that your picture (of a Nepalese yak) will come out vibration-free.
As the camera mass increases, its movement in response to shutter
vibrations decreases (conservation of momentum is the issue at
hand).
This is slightly different from vibration transmission through the
camera body, such as the transmission of sound. It would take a
damn loud shutter to cause the material to flex enough to affect
picture quality at all.
The best way to picture camera vibrations is like the little
vibrating device in a cell phone silent ringer (or a vibrator
for that matter). It's just a heavy mass on a stick, spinning
around. The center of mass must remain stationary in the absence
of external forces. Thus, when the rotating mass jumps from
one side to the other, the cell phone mass jumps to the opposite
side. In the camera, the rotating mass is the shutter, so when
it moves from one side to the other, the camera body tries to move
to keep the center of mass stationary. Camera motion will decrease
as the shutter mass becomes a smaller fraction of total mass.
> > and hand shake
>
> Again, I've seen no evidence. What little I have seen (to do with
> computer mice) suggests exactly the reverse - lighter cameras are
> easier to keep steady, due to the way human muscles work. This is
There are two competing effects here. As the mass increases, the
camera takes more force to move. Thus, once it's stationary, it
takes a larger force to induce hand shake, and it is therefore
more resistant to such movement.
On the other hand, humans are weak. When the mass gets to be very
large, just holding the camera still becomes tiring, so we begin to
produce larger and larger vibrations as our exhausted muscles give
out. This is a weakness of humans, not of the camera. :-) But,
since the camera is a tool, you have to take this into account and
not produce 30 pound (13.6 kg) cameras.
Anyway, it's all in tradeoffs. As the mass increases, it takes a
larger force (either from human motion, shutter motion, earthquakes,
or whatever) to induce movement. However, it also takes more effort
(in the form of human strength or a beefy tripod) to support which
can introduce other movement if support is barely sufficient.
Was any of that coherent? If you want equations, just let me know :-P
joey
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