Where is Lens Plane for a large format lens?

[jford]

Can anyone tell me from where on a large format lens (or any 'thick lens' for that matter) the lens plane is measured from? If I have a 90mm Super Angulon mounted on a lens board, where is the lens plane? ie, from where will it be 90mm (nominally) to the film plane when focussed at infinity?

The possible answers I can think of are:
-The closest bit of glass to the film
-The front of the lens board
-The rear of the lens board
-Some other point nominated by the lens manufacturer.

Note: If I had a 90mm Super Angulon I may be able to work this out. Needless to say, I don't.

All help appreciated.

John Ford.

 

[Ole]

The correct answer is - none of the above.

It's from the front nodal point of the lens, which is usually (but not always) somewhere inside the shutter. But Schneider are noce, they provide all this information and exact specifications for us! Check out http://www.schneideroptics.com - under "info" and "Vintage lens data" you find the most common older lenses.

 

[jford]

Thanks, Ole. This document- http://www.schneideroptics.com/info/ads_&_brochures/pdf/large_format_lenses.pdf from Schneider explains all. The flange focal distance is what I was after.

Now, to see if I can find that information for other lenses...

 

[Ole]

There's a list of older lenses where I pointed you first...

Most older lenses are there, at least until you get to the really weird ones.

 

[Claire Senft]

If the lens is symetrical then I would expect the diaphram to be in the optical center.

 

[MichaelBriggs]

from where will it be 90mm (nominally) to the film plane when focussed at infinity?

The correct answer is the rear nodal point or principal plane. For the equation 1/Dsubject + 1/Dimage = 1/f to work, the distance to the subject is measured to the front nodal point and the distance to the image is measured to the rear nodal point. On quasi-symmetrical lenses the nodal points tend to be close together and near the aperture. The positions can be far from the aperture for telephotos and wide-angle lenses for SLRs.

As jford realized, for most photographers the flange focal distance (FFD) is the most useful number -- this the distance from the image to back of the shutter when the lens is focused on infinity. Most LF manufacturers provide this value. Some provide the positions of the nodal points. If the data isn't available, there are ways of determining these values.

 

[Ole]

If the lens is symetrical then I would expect the diaphram to be in the optical center.

If the lens is symmetrical, the two optical nodes will be symmetrical about the aperture.

In real lenses the rear and front nodal points will never coincide - that can only happen with theoretical lenses of zero thickness. But they can get close...

The reason why there are two nodal points is the necessary thickness of the lenses, wich is difficult to get rid of in anything more complicated than a pinhole. A Fresnel gets pretty close to zero thickness, but with a lot of other "side effects".