Want separation of principal points for Rodenstock, Fuji, and Nikon lenses

**Leonard Evens** · 22-Feb-2013, 15:15

At one point I had sources for information about lens specifications for lenses of various manufacturers. But I can't now find anything which tells me the separation of the principal points for different lenses that I own. Some websites that I used previously no longer seem to exist.

These are
Rodenstock 150 mm Sironar-S, 90 mm and 75 mm Grandagon-M
Fuji 300 mm compact
Nikon 210 mm.

I think I do need the separation of the principal points to determine where to measure subject distance from if I know the rear flange focal length which tells me the position of the rear principal point. In principle, I think I could measure the distance from the flange to the front focal point by reversing the lens and focusing at infinity, but I would prefer to look it up somewhere if I can

**ic-racer** · 22-Feb-2013, 20:55

4mm for the Fuji, if you mean the f9 300 AS as "compact."
11.5mm if you mean the f5.6 300 LS as "compact."
Why do you think the flange location is related to H2? For example, on the f5.6 300 LS the distance from the flange to H2 is 30mm

**Leonard Evens** · 22-Feb-2013, 21:54

Originally Posted by ic-racer

4mm for the Fuji, if you mean the f9 300 AS as "compact."
11.5mm if you mean the f5.6 300 LS as "compact."
Why do you think the flange location is related to H2? For example, on the f5.6 300 LS the distance from the flange to H2 is 30mm

The rear flange focal length is the distance from the front of the lensboard to the film plane when the lens if focused at infinity. If it is less than the focal length, the difference between the two, tells you how far in front of the front of the lensboard the rear principal plane is. If it is more than the focal length, the difference between the two tells you how far behind the front of the lensboard the rear principal plane is.

**Dan Fromm** · 23-Feb-2013, 06:38

Leonard, you've told us what you want to know, also what you want to do. What do you want to accomplish? Are you trying to focus by tape measure or are you trying to find out what's possible?

**ic-racer** · 23-Feb-2013, 10:44

Originally Posted by Leonard Evens

The rear flange focal length is the distance from the front of the lensboard to the film plane when the lens if focused at infinity. If it is less than the focal length, the difference between the two, tells you how far in front of the front of the lensboard the rear principal plane is. If it is more than the focal length, the difference between the two tells you how far behind the front of the lensboard the rear principal plane is.

Of course you are correct. I had mistaken what you were asking. As you may have observed the f9 300 AS the rear point just happens to be at the flange, whereas the f5.6 300 LS they are not at the same place.

**Leonard Evens** · 23-Feb-2013, 11:48

Originally Posted by Dan Fromm

Leonard, you've told us what you want to know, also what you want to do. What do you want to accomplish? Are you trying to focus by tape measure or are you trying to find out what's possible?

Except for closeups, the error made in measuring subject distance from the front of the lensboard would be too small too notice. Even for closeups, unless the lens is of telephoto or inverse telephoto design, the separation between the principal planes would probably be too small to matter. But sometimes for closeups with such lenses, more accurate focusing might be achieved measuring the subject distance precisely since the depth of focus might be quite large. So it might be nice to know the positions of the principal points.

Even so, I would just like to know where I can find such information, even if I might only rarely need to know it for some esoteric purpose.

**Leigh** · 23-Feb-2013, 12:28

Just Google the lens and add "filetype:pdf", as in
apo-sironar-s filetype:pdf

I assume you mean the apo-sironar-s.
There is no sironar-s, although you could use that as a search term.

- Leigh