Focal Length Confusionism

[Ari]

I may have blacked out and not fully regained my senses, but something is puzzling me.
When I put an old Paragon 12" lens on the camera and focus at infinity, the distance from lens board to film plane is 10".
Similarly, on a newly-acquired Wollensak Vesta 14", the bellows are at 10" when I'm focused at infinity.
Only a 12" Cooke lens lines up with 12" bellows, my other lenses use shorter bellows than the FL indicates.
Am I missing something?
Thanks

[Dan Fromm]

Probably not.

Flange-to-film distance given focal length depends on the lens design. For example, my 101/4.5 Ektar, a tessar type, has a larger f-to-f than my 105/3.7 Ektar, a Heliar type. To add to the fun, lenses in barrel have f-to-f distances that are only loosely related to focal length. For example, I have a 150/6.3 CZJ Tessar in a deep mount whose mounting surface is nearly at the front of the barrel.

For lenses of normal construction -- non-tele, non-retrofocus -- diaphragm-to-film distance is usually approximately focal length, but not always. 6/4 double Gauss types are often somewhat telephoto, i.e., sit a bit closer to the film than the focal length would suggest. This is why many 1950s and early '60s "normal" lenses for 35 mm SLRs have 58 mm focal lengths. Designing a 50 mm 6/4 double Gauss type with adequate performance and enough back focus to work in an SLR took more time than was available ...

[Mark Sawyer]

...the distance from lens board to film plane is 10".

That might be the problem. Measure from the aperture's location, not the lens board's.

[Drew Bedo]

Ah yes . . .The Nodes.

[ic-racer]

Focal length is from rear nodal point to image at infinity. f' in the image below.
The issue you are having is that the rear nodal point is not usually marked on large format lens barrels, shutters or lens boards.

[Alan Gales]

Ah yes . . .The Nodes.

Isn't there a node thread in the Image Sharing and Discussion section?

[Ari]

That might be the problem. Measure from the aperture's location, not the lens board's.

Yes, of course you're right. Thanks, Mark.

[Mark Sawyer]

Focal length is from rear nodal point to image at infinity. f' in the image below.
The issue you are having is that the rear nodal point is not usually marked on large format lens barrels, shutters or lens boards.

Focal length is from the rear node to the focal plane at ANY distance. Infinity is just a standard distance to measure at, hence the bellows extension factor for close ups; you've changed the focal length of the lens by focusing closer. Quite right that the rear nodal point is seldom marked, but it's generally pretty close to where the aperture is (exactly where the aperture goes is one of the "design freedoms"), so close enough for all practical purposes.

[Dan Fromm]

Son-of-a-gun! The OP asked why flange-to-film distances of lenses with similar focal lengths aren't much the same and was distracted into accepting explanations based on invisible points.

[Ari]

I wasn't distracted, I was just...hey, what's that shiny thing there?!