Telephoto compression is a myth - why is this fallacy so widespread?

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Tom Axford Veteran Member • Posts: 8,648
Telephoto compression is a myth - why is this fallacy so widespread?
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If you search online for “telephoto compression myth”, you find a large number of articles that tell you that telephoto compression doesn't really exist, it is just a myth.

The argument generally goes rather like this:

A telephoto image is exactly the same as a small crop from the centre of a normal focal-length image. Therefore the telephoto image contains no compression that is not present in the normal image, or even in a wide-angle image for that matter. Hence telephoto compression does not exist. Q.E.D.

This argument is seductively clear and simple, but misses the point.

Both telephoto compression and wide-angle distortion are perspective distortions that are caused by differences in the perspective between (i) viewing the image and (ii) viewing the original scene.

There seems to be a widespread lack of awareness among photographers that the position of the viewer's eye relative to the image affects how we see the image and that is what causes these common distortions. The image itself is just a step along the way. What really matters is what the viewer sees and that depends on two things: (i) the image itself, and (ii) the position of the viewer's eye relative to that image.

Of course, it is often implicitly assumed that an image is viewed in the “normal” way. This is typically defined to mean that the viewer's eye is (i) at a distance from the image that is equal to the length of the image diagonal and (ii) on a line through the centre of the image and perpendicular to the plane of the image

This definition of the normal way to view an image depends fundamentally on the size of the image. That is why viewing a small crop from an image and viewing the original image are normally very different. One may show perspective distortion while the other does not.

Any photographic image taken with a perfect rectilinear lens can be viewed in such a way that the image will appear to the viewer exactly as the original scene appeared. An object in the image will appear on the viewer's retina with exactly the same size and position as it would appear if the scene had been viewed directly when the photograph was taken. To achieve this, the viewer's eye needs to be placed in the same position relative to the image as the camera lens was relative to the sensor (allowing, of course, for scaling up the size of the image).

An image taken with a lens of “normal” focal length (defined as equal to the length of the diagonal of the image on the sensor) and viewed in the normal way will be seen with completely natural perspective.

An image taken with a lens of longer than normal focal length and viewed in the normal way will be seen with telephoto compression (not very noticeable until the focal length is several times greater than the normal focal length).

An image taken with a lens of shorter than normal focal length and viewed in the normal way will be seen with wide-angle distortion (most noticeable in the corners of the picture).

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