In a message dated 11/10/2007 1:39:10 P.M. Pacific Standard Time, [EMAIL PROTECTED] writes: Perspective (and thus perspective distortion) is the result of the distance and angular relationship of the camera and the subject. It is independent of focal length and optics.
This web page shows a sequence of exposures taken from a single position with focal lengths from 16mm to 135mm using the two DA* zooms, the K10D, manual focus and exposure. The final image in the sequence is a crop of the 16mm exposure to show the same portion of the scene as the 135mm exposure. Aside from the slight misfocus of the 16mm exposure and its increased DoF (as well as a tiny vertical shift from changing lenses) you can see that perspective is invariant with focal length. http://homepage.mac.com/godders/perspective/ Perspective distortion on vertical and horizontal scene elements (convergence, divergence, keystoning, etc) happens when the plane of the sensor (or film) is not parallel to the plane of the subject. Another form of perspective distortion is seen when using a very wide angle lens and a small aperture to permit a very deep zone of sharp focus, and you have subject matter in the near field as well as the far field. The near field items will look disproportionately larger than the far field items ... this is called "foreshortening". Again, it is not variable by focal length but is simply made apparent by a wider field of view. --- Rectilinear distortions (aka barrel, pincushion or curvilinear distortions) are optical effects the come from lens design. A fish- eye lens is a lens corrected to produce a spherical projection, a rectilinear lens is a lens corrected to produce a flat-field projection. Pentax Fish-Eye-Takumar 17mm f/4 lens on 4/3 format, natural curvilinear projection: <http://www.gdgphoto.com/FoV-43_11-50/content/bin/images/large/ P1030616.jpg> The same lens, image corrected for rectilinear projection: <http://www.gdgphoto.com/FoV-43_11-50/content/bin/images/large/ Pe1030616.jpg> Godfrey ============== Thanks, Godfrey. Very complete explanation (although a tad confusing. I found Bob's cardboard tube example about my level. :-)) But I am getting it. So I will print this out and read it over again and see if it falls into place. I probably, just for my own interest, should do some of my own test shots. Because I found all of your shots very illustrative. So I might try using the 16-45 on a building to explore this more. Thanks again, Godfrey, thanks mucho mucho for taking the time. Marnie aka Doe :-) Hopefully this thread has been of help to someone else as well. --------------------------------------------- Warning: I am now filtering my email, so you may be censored. ************************************** See what's new at http://www.aol.com -- 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.
