Re: Lens perspective distortion and DSLR crop factor

On May 14, 12:08 am, pgmacdon...@xxxxxxxxx wrote:
Hi all,

I attended the Rocky Mountain School of Photography last week and
their equipment guru made a claim that is inconsistent with my earlier
understanding of an issue related to Crop Sensor DSLRs. Maybe someone
out there could clarify for me.

I have always been under the understanding that if I took a Canon
24-70mm f/2.8 zoom and took photos with the following two bodies, the
resulting images would be exactly the same if we were to ignore pixel
density and quality differences of the zoom at different focal
lengths:

Canon 30D @ 31.25mm focal length, 5 feet from subject
Canon 5D @ 50mm focal length, 5 feet from subject

According to the individual at Rocky Mountain, the lens (perspective)
distortion normally associated with wide-angle shots (traditional 35mm
film) would be evident on the 30D example above because this
perspective distortion is inherent to the wide angle lens. My
understanding was that this was false and that the distortion is a
result of distance to subject. Could somebody please clarify this
issue for me?

Regards,
Paul

Ah, the problem may be that there are TWO definitions of distortion.
To an optical engineer and lens designer, distortion means that the
field angle to any point in the image is not exactly the same as the
field angle of the input point. This kind of distortion IS reduced by
reducing the format used by a lens, and hence by the cropping done by
digicams. That is, the distortion increases with distance from the
optical axis, which intersects the image plane at the center of the
image for most cameras. Thus cropping does not use the wide field
angles and does reduce this form of distortion.

Perspective distortion is another issue. This is the APPARENT
distortion of perspective caused by viewing an image from a different
effective vantage point than what it was taken at. This is especially
due to the fact that wide angle prints would require viewing from a
distance closer to the print than our eyes can focus at. This type of
distortion is ONLY a function of the taking and viewing distances, and
has no dependence on focal length of lens.

.

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