Re: Lens perspective distortion and DSLR crop factor

pgmacdonald@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

If both - or any - cameras are at the same distance from the subject, the perspective drawing is the same for all, regardless of focal length, wide angle or not, and regardless of image or film/sensor size. Any part of the scene common to both images will have the same drawing, which is determined solely and only by the camera position, and nothing else.

It is commonly stated that certain lenses produce optimum perspective for portrait shots, typically 85mm on a 35mm camera. This is fallacial; the reality is the desired perspective is obtained from the shooting distance, and that distance needs an 85mm lens to fill the frame. If you shot with a shorter lens from the same viewpoint, the perspective will be identical - but the image will be smaller. If you go closer to fill the frame, then the perspective will change, and this gives rise to the commonly but wrongly held belief that the lens controls perspective. It does not; distance from subject controls perspective.

Minor distortions with particular lenses are not relevant to your question.

Colin D.

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