# Perspective (yet again, sorry!)

Started 2 months ago | Discussions thread
books are frequenty wrong
4

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.

This is correct, thanks

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. 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.

Unfortunately, this is wrong. The viewing point for a photo is forever fixed at the point from where the photo was taken. In the 3D world, moving closer and farther changes the relative sizes of objects that aren't the same distance from the lens. But once captured in a photo, the relative sizes are forever fixed.

Take the classic example of a portrait taken close up. The features are exaggerated. Try as hard as you can, no amount of moving the photo back and forth will alter this exaggeration. Printing the photo freezes the perspective, since there no "center of perspective" for a flat, perpendicular object

Take a photo. Now take a photo of the photo from far away and close up using a zoom the fill the frame. The two resulting photos will look identical.

Many classic works have serious  flaws. In this case, the flaw is somewhat laughable. Suppose you take a photo of a mountain from a mile away. Do you have to view the image from a mile away to have correct perspective? What about a macro? Do you have to glue the photo to your face?

Okay, technically, the edges of a photo might be smaller as you move the image closer, since they are now farther away. But most lenses and the human eye correct for this effect, which can't be categorized as "perspective" effect.

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