clintonr@xxxxxxxxxxxxxxxx wrote:
Technically speaking, shutters at or near the optical center of a lens are more
"efficient" than any other shutters, meaning they
are able to control the total exposure throughout the image more evenly. But
some leaf shutters are placed behind the optics,
usually on the less expensive models.
Most critically, though, I suspect, is that leaf shutters are cheaper!
----------------------------
Leaving aside cheaper, lighter and smaller, there are 2 significant reasons
that leaf shutters contribute less to vibration compared to focal plane
shutters. First, the mass of moving parts is much less. Second, much of that
moving mass moves out and in radially in a full circle around the lens, which
doesn't tend to displace it, as does a horizontal or vertical motion at the end
of a lever arm with a focal plane shutter.
Jamie Costello wrote:
Matt, have you ever had it tested? I am not being flippant or sarcastic; I
only ask because I have read that although leaf shutters may be rated at
1/500th, it is almost impossible to get then to physically move that fast.
It leads to approx. 1/2 stop overexposure error - barely noticeable, but
still there and probably why you don't see a 1/1000th speed on a leaf
shutter.
It's slightly more complex than that. The way a conventional leaf
shutter works, the center is open longer than the outside. At lower
speeds, the portion of the total time which has the blades part way open
is small, and there is no problem. At high speeds, the speed with which
the blades are able to open and close relative to total exposure time
becomes important. Strange things happen like the effective shutter
speed varying with aperture and/or something that looks like vignetting.
How strong this effect is depends on shutter design - harder with big
opening - and shutter location within the lens. This is one reason why
leaf shutters require regular cleaning. Oil, dust, etc. on the blades
may not affect overall timing while slowing the travel of the blades and
giving anomalous results that may be blamed on other things.
Moose
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