# How many meters gained with 300mm focal length ?

Started Nov 17, 2012 | Discussions thread
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 Re: How many meters gained with 300mm focal length ? In reply to Barrie Davis, Nov 18, 2012

Barrie Davis wrote:

On a standard film or "full frame" camera a 50mm lens gives similar perspective to the human eye, but at a much reduced field of view (narrower field of view).

Unfortunately that second comment is not really correct.

Lenses do not impart a perspective at all, whether they are mounted in a camera, or in a human eye.

Perspective depends only on where things are viewed from, that is, the position in space. If you view the photograph from the same relative distance as the camera looked at the subject, then EVERY perspective looks natural regardless of the focal length used to shoot it with.

Lenses themselves do not impart perspective, true, but the combination of lens and imaging sensor does. Hence why I mentioned "full frame" (ie 135 format). Most compact cameras give lenses in "full frame" equivalent, so it is a useful metric.

A normal lens is defined based on the fact that we view naturally images at a distance approximately equal to the length of the diagonal. On that basis, a normal lens is one that has a focal length equal to the length of the diagonal of the imaging sensor (be that film or digital sensor). The diagonal of 135 format is 43.2mm which is commonly, for some strage reason, rounded to 50mm by lens manufacturers.

Thus an image taken with a 43.2mm lens on a "full frame" camera viewed "normally" will appear to have normal perspective, while an image taken with a 300mm lens on the same camera will appear to have compressed perspective.

In the OP example, if he is looking at it at 100m with the 300mm lens then it will "appear" to be 100/6=16.6m away, and if at 50m it will "appear" to be 8.3m away.

The important thing is that it is a ratio, not a fixed amount of distance gained.

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