I'll try to be more clear.

Say you frame a square poster on a wall somewhere, such that it fits exactly in the viewfinder.

What you get on film differs from what you see by the amounts in the diagram: the poster has been cropped by 5 cm on both left and right sides, by 12.5 cm on top, and you'll see a strip of the wall below the poster measuring 3 cm.

Now if that poster is 1 m, you'll only get 1 m - 5 cm - 5 cm = 0.9 m of it on film, left to right.

You'll be missing 12.5 cm of it on top - only 87.5 cm of it on film, top to bottom - but record another extra 3 cm of the wall below it on film.

If that poster measures 100 m (rather big for a poster, i know

), you'll get 100 m - 5 cm - 5 cm = 99.9 m of it on film, left to right.

You'll again be missing 12.5 cm of it on top - so only 99.875 m of it on film, top to bottom - while again recording another extra 3 cm of the wall below it on film.

Or (as said before): through the viewfinder you'll see 5 cm extra on either side, 12.5 cm extra on top, but miss 3 cm on the bottom.

At any distance from the lens.

You could look at it this way: the field of view of the lens is a four sided pyramid, going out from the center of the lens, getting wider the greater the distance to the lens.

That pyramid is inside a larger pyramid - the field of view of the viewfinder - going in the same direction, with the sides of both pyramids parallel to each other.

The distance between the sides of these two pyramids is 5 cm. And it is and remains just that - 5 cm - no matter how far away you are from the lens and viewfinder, i.e. how wide the base of the pyramid.

The centers of lens and viewfinder are offset vertically, thus so also are the pyramids: hence the distance between the sides is 12.5 cm on top, and the viewfinder's pyramid's lower side is not outside the lens' pyramid as it is on the left, right and on top, but has crossed the plane of the lens' pyramid's lower side and is inside, above the lens pyramid's lower side. The distance between the lower sides is 3 cm, and that too no matter how far from the lens and viewfinder.

Hm... Perhaps a failed attempt to put it across in a more clear and easier to grasp fashion...