Bellows correction with tele-photo?

[Drew Bedo]

I have a Wollensak 15' telephoto that I have not really used much. Infinity focus is at ~12". How is the correction for bellows extension made for this type of lens?

In the field, I have been measuring lens to film plane distances from the diaphragm for my other lenses . . .works pretty well. Where should measurements be taken from when using this telephoto?

[ic-racer]

I have a Wollensak 15' telephoto that I have not really used much. Infinity focus is at ~12". How is the correction for bellows extension made for this type of lens?

In the field, I have been measuring lens to film plane distances from the diaphragm for my other lenses . . .works pretty well. Where should measurements be taken from when using this telephoto?

http://www.brucebarrett.com/bellows.ps

[Drew Bedo]

Link won't work.

[djdister]

Link won't work.

It was a postscript file - I converted it with Adobe Distiller, so here it is in PDF.

[Tin Can]

I have one also and have never used it. Mine is mounted for a Speed Graphic. I don't think my clumsy exposures would notice the tiny correction factor involved. I am interested in any comments on using this lens.

ymmv

I have a Wollensak 15' telephoto that I have not really used much. Infinity focus is at ~12". How is the correction for bellows extension made for this type of lens?

In the field, I have been measuring lens to film plane distances from the diaphragm for my other lenses . . .works pretty well. Where should measurements be taken from when using this telephoto?

[R.K]

15' telephoto, sounds a bit strange for me. probably it is 15" telephoto. If you infinity focus at 12" bellows extension, that mean you have 3” at the front of the lens. These 3" must be taken in account when you calculating bellow extension factor. Say you setup the camera, focused and measured length of the bellows draw =16”. Than you total bellows draw for calculation will be 16+3=18”.

[Leigh]

The 1:1 bellows extension always equals one focal length forward from the infinity focus position.

The infinity focus position is always the extension at which the rear node is exactly one focal length from the film.
That's the definition of the focal length of the lens in the first place. If that dimension does not correspond to the
location of the diaphragm, then the rear nodal point has moved, as with a telephoto lens.

While we commonly encounter a rear-node shift for telephotos, a similar situation may occur with very short FL lenses, where the rear node is actually behind the expected location, to move the lens farther forward at infinity focus.

The extension from infinity focus to 1:1 is always equal to the lens focal length.

- Leigh

[Drew Bedo]

Everyone: Thanks for the info.

So now, As I understand it, I will carefully focus at infinity, then measure the bellows extension and add the difference between this infinity focus distance and the nominal focal length of the lens.

If the infinity focused distance is 12" (it may be something else), 3" brings that back to 15". Now to focus closer; measuring the bellows extension at, say 14", I would add 3" and use 17 inches to calculate the correction factor . . .giving about 1/2 stop extra exposure.

Do I have this right?

[R.K]

Yes.

[blueribbontea]

This sounds a bit strange. A telephoto lens requires less bellows extension than a long lens. The bellows extension factor is concerned with the distance from infinity focus to a point of closer focus, and it is the greater distances that kick in the rule of fours, isn't it? The fact that this lens is a 15 inch, unless there is something I do not understand, is less significant than the distance from the infinity bellows position to the bellows position at a focus of 10 feet. I use this lens on my 4X5 RB Graflex and can focus that close, without extreme bellows draw precisely because it is a telephoto design. So why wouldn't I not simply measure the difference between the lens focused at infinity and the lens focused at 10 feet, r.e. bellows draw? What am I not understanding here?

Bill