On Thu, Apr 30, 2009 at 09:03:47AM -0400, Graydon wrote:
>
> Rather than include all of the *many* focal length and SR focal length
> combinations for the 55-300, I'll just note:
>
> Lens Focal Length SR Focal Length
> smc PENTAX-DA 55-300mm F4-5.8 ED 300.0 mm 252 mm
> smc PENTAX-DA 55-300mm F4-5.8 ED 300.0 mm 260 mm
> smc PENTAX-DA 55-300mm F4-5.8 ED 300.0 mm 268 mm
> smc PENTAX-DA 55-300mm F4-5.8 ED 300.0 mm 276 mm
> smc PENTAX-DA 55-300mm F4-5.8 ED 300.0 mm 284 mm
> smc PENTAX-DA 55-300mm F4-5.8 ED 300.0 mm 292 mm
>
> Shake reduction more or less has to be doing something based on a
> detected property of the image, presumably the actual distance to the
> subject (meaning the thing the camera has a focus lock on), rather than
> purely the set focal length of the lens.
Shake reduction has to be based solely on image magnification,
which in turn is directly related to actual focal length.
I can see two explanations:
The first number is the 'focal length' set on the lens,
and the second number is the true focal length in use
(calculated from set focal length, focussing distance,
and known properties of the lens).
The second number isn't really "SR Focal Length" - it's
something quite close to that, so the ExifTool author
called it that. As I'm pretty sure he doesn't have a
deal with Pentax to get the RAW specs, all the values
are reverse-engineered, and are best-guess descriptions.
(I know that was the case for the running exposure count,
because I'm the person who reverse-engineered that one).
There's a fairly easy way to check the first hypothesis;
mount the lens on the camera, set it to 300mm focal length,
and see if the image size (or field of view) in the finder
changes as you shift the focussing. With a change of over
15% it should be readily visible.
--
PDML Pentax-Discuss Mail List
[email protected]
http://pdml.net/mailman/listinfo/pdml_pdml.net
to UNSUBSCRIBE from the PDML, please visit the link directly above and follow
the directions.
