Where is the optical center in a shutter mounted lens?

[koh303]

When calculating the focal plane distance to optical center (for knowing where the infinity focus is), is there a marker on shutter mounted lenses that denotes this position like the film plane marker on 35mm film cameras?
I am not really looking for an exact measurement, but more of a general understanding.

[Dan Fromm]

No such mark. It would be lens, not shutter, specific.

For most of the lenses used on LF cameras the nodes, front and rear, are near the diaphragm.

[Leigh]

Infinity focus is measured from the rear lens node (commonly designated H' on drawings) to the film plane.
In fact, that's the definition of infinity focus.

The lens datasheet should tell you the exact location of H and H' (front and rear nodes, respectively).
Those are usually measured from the front surface of the front element, along the optical axis.

- Leigh

[Dan Fromm]

Leigh, datasheets are available for relatively few of the lenses we love and use. Unfortunately the OP asked a question that wasn't specific enough to let us point him at a datasheet or tell him that none exists.

[Leigh]

Leigh, datasheets are available for relatively few of the lenses we love and use.
Sorry, Dan...

But the validity of that statement depends critically on how you define "we".

I have about 25 LF lenses, and datasheets on all but one of them. I don't have one for the 8 1/4" Gold Dot Dagor.

- Leigh

[BrianShaw]

From a practical standpoint, I use the lensboard as a rough approximation of optical center of a mounted LF lens.

[desertrat]

If you don't have a data sheet and really want to know where the nodal points are, you can make a fixture and do some measuring. I don't remember the details, but you can download "Photographic Facts and Formulas" by E. J. Wall from the internet archive. It has instructions on how to do this. I have the 1924 edition in hard copy, which can be downloaded. There were later editions but the latest ones aren't available for download.

[Leigh]

I use the lensboard as a rough approximation of optical center of a mounted LF lens.
That will be pretty accurate for lenses of "normal" focal length, perhaps 90mm to 300mm or so.
The error increases as the FL moves outside that range.

With short lenses, the flange focal distance (front of lensboard) is usually longer than the optical FL, while
with longer lenses, the flange focal distance is usually shorter than the optical FL.

- Leigh

[Leigh]

If you don't have a data sheet and really want to know where the nodal points are, you can make a fixture and do some measuring.
If you only need the rear nodal point (H'), there's no need for all that complication.

Set up your camera and focus on a distant object ("infinity").
Measure the distance from the film plane to the front of the lensboard (= the "flange focal distance" or FFD).

Subtract the FFD from the true optical focal length of the lens.
If the result is positive, the rear nodal point of the lens (H') is in front of the lensboard, otherwise it's behind.

- Leigh

[desertrat]

If you only need the rear nodal point (H'), there's no need for all that complication.

Set up your camera and focus on a distant object ("infinity").
Measure the distance from the film plane to the front of the lensboard (= the "flange focal distance" or FFD).

Subtract the FFD from the true optical focal length of the lens.
If the result is positive, the rear nodal point of the lens (H') is in front of the lensboard, otherwise it's behind.

- Leigh
That should work fine for modern lenses, but I have several old ones whose real focal length is somewhat different than marked. The measurements can be done accurately to within about 1/8 inch using methods described in the book mentioned above. My 12 inch T-R triple is actually 12-7/8 inches combined, and I have a three focus Seneca RR (probably made by Wollensak) having a combined focus of 13-3/8 inches and marked 12 inches.

[Leigh]

That should work fine for modern lenses, but I have several old ones whose real focal length is somewhat different than marked.
The instructions I presented earlier, being:
"Subtract the FFD from the true optical focal length of the lens."
are correct in all cases, by definition.

Perhaps I should have capitalized TRUE.

- Leigh