# Perspective (yet again, sorry!)

Started 2 months ago | Discussions thread
Re: Perspective (yet again, sorry!)

WunWegWunDarWun wrote:

Tom Axford wrote:

I have an old copy of "The Manual of Photography", 1978 Edition (Focal Press) and with all the discussion recently about perspective, I thought I would look at what it had to say on the matter.

Here are two extracts from the section on perspective (in Chapter 4 "The Geometry of Image Formation"):

Perspective on taking a photograph

As we have already seen, the perspective obtained on taking a photograph - sometimes termed the true perspective - is governed solely by the viewpoint. If the viewpoint is fixed, there can be no change in perspective, even if we change to a lens of different focal length - although, as we have seen earlier, the image size will alter. If, however, the viewpoint is altered, so will be the perspective, and no change of lens will recreate the perspective obtained at the first viewpoint.

But does he explicitly address what happens to "true perspective" when changing the lens (to one with a different AoV) while maintaining the viewpoint ?

A later sentence says:

"Altering the focal length of the lens, without changing the viewpoint, alters image size only - leaving perspective unaltered."

and

Perspective on viewing a photograph

The perspective obtained on viewing a print - sometimes termed the apparent perspective - depends, firstly, on the relative sizes of objects in the print - and hence on the perspective obtained in the negative on taking the photograph - and, secondly, on the distance at which the print is viewed. Correct perspective is said to be obtained when a print is viewed in such a way that the apparent relation between objects as to their size, position, etc., is the same as in the original scene.

Unless by "etc." he means their print-relative size and position that will be the case regardless of viewing distance.

The apparent size of an object in the print depends on the viewing distance.

This is achieved when the print is viewed at such a distance that it subtends at the eye the same angle as was subtended by the original scene at the lens. The eye will then be at the centre of perspective of the print, just as, at the moment of taking, the lens was at the centre of perspective of the scene.

Unless this is accompanied by geometric diagrams with parametrized distances, angles etc. that relate a scene to a print and the print to a viewing setting making it clear what he means by "such a distance that it subtends at the eye the same angle as was subtended by the original scene at the lens" I'm afraid this is just as handwavy as the attempted explanations in the previous 3-900 threads.

I did not want to copy the whole chapter.  There is, indeed, much more explanation (including diagrams) that makes it clearer what he means.

A couple of later sentences extracted from this further explanation may help:

"For correct perspective, therefore, a contact print should be viewed at a distance equal to the focal length of the lens.  An enlargement should be viewed at a distance equal to the focal length multiplied by the degree of enlargement."

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