How to calculate needed bellows draw?

[Ed Richards]

What is the equation for calculating the bellows draw for a lens at distances shorter than infinity? For example, a 600mm focused on a subject 50 feet away? (Assume the FFL is 600mm)

[Mark Woods]

Depends on the lens design.

[Ed Richards]

The FFL depends on the design, but I do not think the bellows extension beyond the FFL at various object distances does. At least for the relatively simple lenses used for LF.

[Heroique]

This might help.

From Ansel Adams’ The Camera:

“One of the fundamental formulas in optics,” he says.

1/u + 1/v = 1/f

u = distance to subject
v = bellows extension
f = lens focal length

* Be sure to use the same unit of measurement for your variables!

[Asher Kelman]

This might help.

From Ansel Adams’ The Camera:

“One of the fundamental formulas in optics,” he says.

1/u + 1/v = 1/f

u = distance to subject
v = bellows extension
f = lens focal length

* Be sure to use the same unit of measurement for your variables!

Yes, but from where are u, v and f measured in a complex lens?

Asher

[IanG]

Assume roughly where the aperture iris is.

Ian

[Struan Gray]

This might help.

From Ansel Adams’ The Camera:

“One of the fundamental formulas in optics,” he says.

1/u + 1/v = 1/f

u = distance to subject
v = bellows extension
f = lens focal length

* Be sure to use the same unit of measurement for your variables!

Yes, but from where are u, v and f measured in a complex lens?

u is the distance from the front principal plane to the subject (on the optic axis)

v is the distance from the rear principal plane to the film (on the optic axis)

The physical interpretation of where u and v are measured is more complicated in tele designs because the optics move the principal planes, sometimes well beyond the physical brass-and-glass of the actual lens. However, I think I'm right in saying that most photographic lens designs don't muck about too much with the position of the front principal plane - so u can be taken to be the distance from the lensboard to the object, certainly for LF lenses focussed on objects 50 feet away.

v is more complex, but only if you insist on finding the principal plane. Ed was right, in that the extension beyond infinity focus is the same for regular and tele lenses, so if you calculate that 'extra' extension, and have infinity stops or marks for your tele lenses, you can simply add the same amount and start shooting.

Alternately, a workable rule of thumb would be to note the difference between the flange focal length (the extension when focussed at infinity) and the marked focal length, and simply subtract that from all values of v calculated using the formula given above.

[Joanna Carter]

Hey Guys!!!

Why on earth would you go to all the trouble of calculating bellows extension at all???

Surely, the bellows length is determined by what you get when everything is sharp on the ground glass screen?

Or don't you have ground glass screens on your cameras?

[Struan Gray]

We're guys!

You'll be telling me I don't need that titanium spork next....

[Struan Gray]

PS: I use these calculations to see whether a particular lens I don't yet own and cannot try out anywhere is actually going to be usable on my camera, and if so, how close I can focus with a given bellows and rail length. From Ed's recent posting history, I'd guess he's doing something similar.