 John Swanson |
Characteristics of Lenses for Digital SLRs I need a reality check. I have a 17-85mm zoom lens, and it is a 28-135 equivalent in 35mm terms, since my camera has a 1.6 crop factor. If I set this lens to 17mm, do I have a DOF equivalent to a 17mm lens or a 28mjm lens? Also, will I get the perspective shift (emphasis on foreground objects, shrinking of distant objects) of a 17mm or 28mm lens? It seems that there is less perspective shift than I expected, perhaps because the shift occurs mostly at the outer edges of the lens, and the cropping factor keeps some of this shift out of sight in the viewfinder and final image.
|
|
|
|
||
|
Jon Close |
The "crop factor" confuses people because the same lenses are interchangable on 2 different formats - 35mm film (24mm x 36mm) and APS-C sized digital (~15mm x 22.5mm). This rarely comes up in comparing 35mm film (where 50mm is "normal") and medium format (where 90mm is "normal") since the lenses are designed to be exclusively for one format or the other. Using the 17-85 on the APS-C digital gives the same angle of view as 28-135 on a 35mm film camera. But the 17-85 remains 17-85, there is no change in the actual focal length. Perspective is based on distance from the subject, not focal length. Using 17mm focal length on the DSLR and 28mm on the 35mm film SLR, framing the scene the same way, you will have the same subject distance and perspective with both cameras. However, there will be a difference in depth of field. Depth of field is a function of subject distance, aperture and focal length. Composing the same scene with the 2 different formats, 2 of those variables are the same - subject distance and aperture - but the third, focal length, is not. The shorter focal length of the APS-C will result in greater depth of field than the 35mm film shot. The difference is roughly approximated by the 1.6x crop factor. To get the same depth of field in the 35mm film image at f/8, the APS-C DSLR would need f/5.
|
|
|
|
||
|
Harry Lichtman |
Hi Jon- Thanks for that answer/description. To take the dof question a bit further, are you aware of any dof scale charts on the web for the 1.6 x crop sensors? I have only been able to find dof calculators, and I'm looking for a quick reference for the field for use of hyperfocal distances. Granted, with the added dof available with this sensor and 17-50mm lens, it will not be as critical selecting the focussing distance. Any help wold be appreciated. Thanks, Harry
|
|
|
|
||
|
This old forum is now archived. Use improved Forum here
Report this Thread |
||