Something to ponder!

OK, am I thinking of sensor size then? I.e. dof for any given lens on APS-C is greater at a specific aperture than the full-frame?

That's correct.... One of the few advantages of FF is that the
same shots have a shallower DOF. If you want more DOF FF
becomes a disadvantage.

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Well, if I could bag this thread, it would do my roses a power of good.
First off, there has been a lot of talk about perspective without saying explicitly which perspective you are talking about. Yes, I can see that it is linear perspective, but there are several other varieties as well.

Yes, magnification does change LINEAR perspective. However, you need to get out and take a couple of shots to demonstrate this. You need about 200 yards of quiet street with houses on either side. Stand in the middle (when it's really quiet) and take two shots (max wide, and max tele) with the same lens and without moving your feet. The centre of each shot should be at the centre of the far end of the street. Now print the two shots off at (say) 10 x 8 and draw lines along the converging parallels (kerbs, top of ground floor windows, rooflines, etc) and look at the angles between them. Do note that the results will be different if you have used a narrow street of terraced houses in (say) Salford or Leeds, and a wide street in leafy Surrey. Of course, the most obvious difference between the two shots will be at the end nearer to you, but that is a function of the acceptance angle of the lens at the two magnifications.

Now think of the way that linear perspective was taught before cameras with zoom lenses - Leonardo's Window. This waas a simple frame (or maybe a small sheet of glass held at arm's length by an artist - who drew what could be seen within the frame. Try this for yourself, and then try the difference by moving the frame a bit nearer. This is exactly what a zoom lens does - it alters the size of the image without altering the proportions of objects within the image, but only the way in which they are depicted.
You can get a good explanation of Leonardo's Window at:
http://legacy.mos.org/sln/Leonardo/LeonardosWindow.html

And to answer the question of which focal length equates to human vision, it is about 37-38mm (for 35mm cameras) provided you make certain assumptions about the human. There is no straight answer because we are all different, our eyes are spaced a different distance apart, they automatically focus to infinity unless something nearer is required, they all have slightly different fields of view to right and left, up and down; while humans have a general field of view of around 180 degrees, the periphery is not generally procesed for detail (only for movement).

But, in the end, does it all really matter? We zoom in and out (or change lenses) compose, take the shot, go home and process the image and are either pleased with the result or not.

Geoff

Not normally but the effect is often used in films so it does have its uses.

[quote="wikipedia"]Forced perspective can be used to deliberately misrepresent an object's size, making something appear larger or smaller than it really is. This is common in film, where a distant castle in the background may in fact only be a cardboard model a few feet high (and much closer to the camera). These are forms of optical illusions.[/quote]John K

It matters if we want to understand what we are doing. My main concern is that we shouldn't mislead anyone who is genuinely trying to get to grips with something.

Indeed and thanks. For a few years now I've watched the occasional arguments (50mm is the same as 75mm, no it isn't, yes it is, ad nauseum) and never really understood exactly what the answer was. I can see much clearer now (pun intended ) and understand better why I didn't get the same result from my Pentax 50mm 1.7 as I did from my Olympus OM10 50mm 1.8.

Regards

Tony

I aggree with John, understanding how changing focal lengths/position affects the picture is important in getting the picture you are after. Changing the focal length does not have the same affect as stepping forward back when framing an image.

Perhaps a quick rough sketch will demonstrate that the Photozone
article is correct..... at least for the brighter Pentax Users

The sketch shows a WA shot and a Tele shot looking down a road.
Note the kerbline angles stay the same all the way to the vanishing
point (which isn't shown here). Therefore each lens produces it's
own individual perspective view and you can't obtain one picture
from the other by cropping. The format (FF or APS-C)
wouldn't change the kerb angles either.

The WA lens kerblines always converge faster than the
Tele kerblines which become closer to parallel with
longer Teles.

Moving the camera providing you stay on the centerline
won't make any difference.




If you can't see it from the sketch.... try it yourself with a
camera and two lenses (or a zoom).

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No, no no Algi, whatever are you thinking?

If you stay where you are and blow up the centre of a wide angle lens shot to be the same as the telephoto shot, the perspective of the two will be identical.

The only thing that affects perspective is camera position and this is so well known and established that I can't imagine where you're coming from with this.

It's quite clear from the sketch..... you can take the kerbline
all the way back to the vanishing point and the angles will not
alter and they will always be different for a WA and Tele.
In the sketch the WA kerbline is about 40deg off vertical and
the Tele is about 7deg.

One normal use of a WA is distorted faces and bodies such as this
link
You can't get that shot with a Tele and you can't recover normal
perspective from it either there will always be distortion.

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I'll just refer you to the explanation in the lovely A4 Pentax book "The Eyes" that accompanied the M series lenses. It sums it up very well and if I have time later I might scan the two relevant pages.

Any book on photography will do, but this one is very clear indeed.

Algi, stop drawing sketches and

Do the bloomin' experiment.

All you need to do is what Tony did.

Stick the camera on a tripod, take two images at different focal lengths, crop a chunk out of the middle of the WA one to match the narrower angle of view of the tele one, and then compare the two.

And then come back and eat humble pie. It tastes better than you imagine.

This is a good example of telephoto compression.....
http://www.flickr.com/photos/unclejefejefe/4594201017/

The cars look on top of one another and the pedestrians look
very similar in height. Notice also the steep angle of the
kerblines........ Quite obvious that this couldn't be part
of a WA shot blown up.

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I think we all know this Algi, but you haven't responded to ChrisA's request in the previous post. You are barking up the wrong tree with this one!

Cheers
David

How do you know it wasn't taken with a wide angle lens and cropped?

John K