Perspective (yet again, sorry!)

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Just Tim 4
Just Tim 4 Contributing Member • Posts: 591
Re: Perspective (yet again, sorry!)

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.


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.

And thirdly on our assumptions on the actual sizes of the objects depicted as governed by our experience. For instance your assumptions of the size of the half filled glass are not contained in the image but your assumptions of how big glasses are from your experience of them. They could be 5" tall or 8" tall and the point is that the assumption you make affects how you interpret the relative distance. This is the entire point of the illusion of the odd shaped room with the two people of the same size: We do not see reality in the images but make assumptions which conform to our experience of what is most likely to be correct, (it is more likely the room is square because that is our experience of rooms). The point is that there is no absolute reality in a 2D image, it is always interpreted by a human eye which makes assumptions modified by experience, (didn't I say that a while ago... ). The illusion also shows another important point, that a our assumption of distance between two objects is entirely dependant on our assumptions about the size of the two objects. It is not locked into an image by the relative sizes of objects but can change even though the relative size of object remain entirely fixed...

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.

I'm sorry to start yet another thread on perspective, but I thought these extracts from a very well respected book on photography from the era of film may be of some interest.

Yes, agreed with the extra addition in bold above.

Also if you read on, (I actually own the same edition of the book... spooky. ):

If we view a print at a distance other than the correct one the perspective achieved will be distorted, (didn't I say that as well?).

The end result is not really of consequence as all we do is the same as the extra point I added:

The point is that there is no absolute reality in a 2D image, it is always interpreted by a human eye which makes assumptions modified by experience...

In effect we change our assumptions of relative scale and distance to suit an understanding that is in line with our experience and our position as the viewer of the print. This is the sole reason why wide angle images *appear* to exaggerate distance and telephoto one *appear* to foreshorten, the incorrect assumptions we make when we try to interpret an image when viewing it from the incorrect distance. View them fro the correct distance (change only your distance to the print) and the *apparent* distortions disappear, viv-a-vis they must be relative to your position as the viewer.

This even works with the UWA distortions of the tennis ball, they are in fact correct projections of the shape onto a 2D plane, (remember the plane of focus or sensor is not parallel to the object but an an angle and so spheres will appear elongated) viewed from the incorrect position. All circles do it but your eye pretty much cancels the effect if it can form a different assumption of perspective when viewing. No circle is ever depicted as anything other than an oval unless it is central and square to the camera but you never notice because you make the assumption of the correct shape when you view and just don't question these assumptions until the distortion becomes severe.

This is why artists dispensed with linear perspective a long time ago, because it is so dependant on the position of the viewer. There is only one viewing position for *correct linear perspective*, which we almost never achieve when viewing images, and that is dependant on the assumptions we make about the actual objects contained within the image based on our experience, (we recognise the object and relate it to an object we know, and the absolute size of the object we know - not the one in the image).

And so you can't talk about images in terms of a mathematically correct perspective that you never actually see, because you always interpret perspective in terms of you position and experience and never from the point of the correct position and knowledge of the exact sizes of the objects depicted.

It therefore seems pointless to me to try and understand perspective in mathematical terms, but rather understand it in terms relative to how you actually see it with your human eye and assumptions.

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