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.

That depends on whether you think it rains a lot in Manchester Glyn.

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 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.

AP and WDC are indeed from the same publisher. Publishers publish many titles. In this current issue (sent free for subscribers with AP) it even repeats Damien Demolder's day on the Thames with Pentax.

No Tony. And certainly no Algi, you haven't looked at their images properly.

Tony, if you want the same field of view and perspective to a 85mm lens as on 35mm film, then the nearest in the Pentax range is the 55mm f/1.4 lens.

The reason this is true is that you need to stand in the same place to get the same picture. If you had an 85mm lens on your Pentax DSLR that would be no good, you'd be cutting off parts of the subject. You'd have to move back to get it all in. So you change to a lens with a shorter focal length and get the same FOV as your 35,, camera.

Consider you are standing looking at a subject and you have several cameras with you with their standard lenses on. With your Minox sub-miniature the focal length is 15mm, with your Pentax Auto 110 it's 24mm, with your K-3 it's 35mm, with your LX it's 50mm, with your Hasselblad it's 80mm.....and so on. But they are all a standard lens and all give the same perspective because you haven't moved.

If you're having difficulty with this, and it can be tricky I'm sure you're not alone, then a book on photography in general should explain it very well. Try "The Lens Book" by Hicks and Schultz.

What I wrote is absolutely true Tony. If you think about it, it has to be.

What happens if you have a lens and you put a sheet of 5" x 4" film behind it? What happens if you put a 35mm frame behind it in the same position? Think about that and you'll see the lens itself doesn't change at all. What does change? Once you've got the answer you'll understand.

Perspective has nothing to do with focal length. It is entirely to do with how far away from the subject we are.

The only reason people thought FF had an advantage was that wide angle lenses remained wide angle. On APS-C they crop the image. However, since then we have had ultra short focal lengths that solve that problem.

The idea that "at least my 50mm is a real 50mm" is a bit of a misunderstanding, because a 50mm lens will always be a 50mm lens. Same DOF, same perspective from any given position. Different formats put behind it just dictate how much of that view we capture, a lot of it (wide angle) if it's a larger format, less of it (telephoto) if the format is smaller.

A lens does not change its properties because of what you place behind it.

The angle of view is just that regardless of format. There was a suggestion some years ago that we should start talking about, say, a 75 degree lens and that would give a better usea than focal lrngth. Never caught on.

It won't be a bargain (if it was a bargain) if you tell everyone about it...which is why in general we don't link to eBay auctions. It puts prices up as more people start to compete and it tends to sometimes upset anyone who has spotted something the world has missed....