At 22:19 1/4/02, Albert wrote:
I was just reading some specs on most p&s. Most start at 1/250s!!
WOW. Some show 1/300s, and some go to 1/500's. I thought that was
bad.. My friend commented that is the same shutter speed, as a
Hassy, so not a big deal. YES, but a Hassy 80mm starts at F2.8,
not F8!!
[snip]
John Lind wrote:
The basic problem with P&S leaf shutters is reliability at faster
than 1/400th second. The vast majority of leaf shutters go no
faster than 1/500th, whether they're in a P&S or in a professional
grade leaf-shutter lens for a medium format SLR. The limiting
factor is how fast the shutter blade(s) can move. As shutter speed
is increased, the length of time spent fully open begins to converge
to the time required for the blade(s) movement to the fully open
position. (Less of a problem at narrow apertures than at wide
ones.) If the shutter design allows too high a shutter speed
compared to how fast the blades move the result is uneven exposure.
Similarly, accurate exposure repeatability begins to suffer with
variation in how fast the blades actually move from one exposure to
the next. The practical design limit has been 1/500th second for
many years. Most P&S cameras top out at 1/500th, but some top out
at 1/400th or less. A very few claim speeds faster than 1/500th.
I've been *mildly* curious about their design and skeptical about
their exposure repeatability.
I have been reading about point and shoots lately and there is some
clever design going on. The Contax T3 claims a top shutter speed of
1/1200 second. What they have done is to use two sets of shutter
blades one open and one shut. When the shutter fires the closed one
opens and the open one closes eliminating the speed limitation of
reversing the direction of a single set of blades. As a result there
is very little light fall off at faster shutter speeds. However,
even they do not have the problem licked because shutter speeds above
1/500 are only available at F stops of F8 or smaller. The speculation
is that they can only achieve those speeds with the physically
smaller lens opening.
Winsor
--
Winsor Crosby
Long Beach, California
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