On Thu, Apr 30, 2009 at 01:52:36PM -0400, John Francis scripsit: [snip -- what's going on with the EXIF "SR focal length" value?] > 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).
That would be my expectation. [snip] > 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. Finding some overlapping-on-this-angle-of-view tree branches at various distances gives very visible field-of-view changes in the finder as the focus is moved from the near branch to the further branches. Going from infinity down to the minimum focusing distance with the manual focus ring with the lens set at 300mm, I get, stepping the focus ring down through the feet markings (∞, 30, 15, 10, 7, 5.5, 4.6): smc PENTAX-DA 55-300mm F4-5.8 ED 300.0 mm 292 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 268 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 260 mm 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 252 mm Note that the 292 is with manual focus and the focus ring turned as far as it would go toward infinity; the DA 55-300 doesn't quite get to 300. Going UP from the minimum to infinity on the metric focusing ring (1.4, 1.6, 2, 3, 5, 10, ∞) with the manual focus ring with the lens set at 55mm, I get: smc PENTAX-DA 55-300mm F4-5.8 ED 55.0 mm 64 mm smc PENTAX-DA 55-300mm F4-5.8 ED 55.0 mm 64 mm smc PENTAX-DA 55-300mm F4-5.8 ED 55.0 mm 63 mm smc PENTAX-DA 55-300mm F4-5.8 ED 55.0 mm 62 mm smc PENTAX-DA 55-300mm F4-5.8 ED 55.0 mm 60 mm smc PENTAX-DA 55-300mm F4-5.8 ED 55.0 mm 58 mm smc PENTAX-DA 55-300mm F4-5.8 ED 55.0 mm 56 mm So it looks like depending on distance to the subject the actual focal length varies by 40mm in the DA 55-300 at 300, and by 8mm at 55. Margus' DA* 60-250 pic showed 196mm as the actual distance to that implausibly pretty random tourist with the lens set at 250mm. I'd be very curious what the closest-focus-to-infinity change was; 250-196 = 54mm, which is considerably larger than the 40mm difference for the DA 55-300 at 300mm. It makes me wonder how close the 60-250 gets to 250. -- Graydon -- 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.
