On Thu, 11 Jan 2007 20:02:55 +0100, "Ståle Sannerud"
wrote:


I can't find the article but I remember reading about the comparison
between in-camera IS and in-lens IS. It stated that the in-camera IS
would prevent a full sized sensor from working with the in-camera IS. It
stated that the camera would be very large to allow for full sensor
movement and the light from the lens could fall off the edges of the
sensor, therefore even a larger then full sensor was needed, thus making
the camera even larger.

Am I blowing smoke or does anyone else remember this?


It's quite obvious if you think about it. The lens throws a circular,
fixed-size image into the camera. The size of this image circle is designed
to cover the film frame (or sensor, obviously) and not a smidgeon more than
necessary- or, if the image circle size IS significantly bigger than the
film frame you will have to carry around a lens that is correspondingly
larger, heavier and more expensive than would otherwise be the case.

Now, with in-camera IS the sensor has to move around to "chase" the image as
it wobbles around the sensor plane as your hands shake. The image circle is
of course constant and stays in place, so the sensor has a hard limit on how
far it can move in any direction before parts of it actually pokes out into
the darkness outside the image circle.

So, in-body IS is a good idea only as long as the image circle is
sufficiently larger than the sensor. A full-frame sensor on a full-frame
lens is a particularly bad candidate for in-body IS, an APS-sized sensor on
a "digital" APS-sized lens with reduced image circle size ain't too hot a
proposition either. Full-frame lens and APS-sized sensor gives you the most
leeway.

Compare this to an IS lens, which de facto lets the image circle chase the
wobbling image so that it stays in the same place relative to the sensor.

Camera shake shakes the image, not the image circle, which is fixed by
the optical axis (with IS is turned off). With IS now on, the
corrective element moves the image circle around so that the image is
placed correctly.

So yes, IS lets the image circle chase the wobbling image so that it
(the IMAGE) stays in the same place relative to the sensor. The
relationship between sensor and image circle is the same as for
in-camera IS. The difference is that with lens-based IS, the image
circle moves, whereas with in-camera, the sensor moves.

The image circle demands are the same for the two situations. However,
not having to have an IS group in the lens allows for a simpler
optical design (read: cheaper and maybe better performance).

KS