Quote Originally Posted by MattKing View Post
For some telephoto lenses, the nodal point is a long way forward of the front element and way outside the body of the lens.

And retro-focus construction is most often used for wide angle lenses. In that case it results in the nodal point also being outside the lens itself, except in this case it is behind the rear element. - some times a considerable distance behind.

Unless you know where those nodal points are, you cannot measure the distance between them and the film plane, and therefore cannot use single measurements from a fixed plane to determine an exposure calculation.

That is when the various tools that involve measuring magnification (e.g. using a ruler in the field of view) are most useful.

And that is why you can't build a scale on the camera for this purpose.

EDIT: for certainty and clarity, I mention that a "telephoto" lens is not the same as a long lens - it is a lens that is shorter than its focal length.

A bit of random googling yields an example of the Nikkor 1200mm f/18 lens which is focused at infinity when the bellows it is mounted on is extended to just 755 mm - about 450 mm or 18 inches less than the focal length of the lens.

The lenses you are using may not be strongly telephoto, or strongly retro-focus, in which case the nodal points may all be near the middle of the lenses.

But in any event, a scale on the camera wouldn't work.
Gotcha, okay that makes sense, but at least the ruler idea make sense still, however my question now is how the heck do I know what I have on my lenses that I own now, is there a marking point that I'm supposed to look for, I don't remember seeing any of those zeros with lines through them...