Bellows Extension/Focusing Distance Formula

[Vlad Soare]

Hi guys,

If D is the distance between the lens and the subject (focused), F is the focal length of the lens, and E is the distance between the lens and the ground glass, is there a mathematical formula that calculates D as a function of F and E?

Thank you.

[aduncanson]

The lens equation:

1/F = 1/D + 1/E

Works for "thin lenses". For real lenses D is measured from subject to Front Nodal Point and E is measured from ground glass to Rear Nodal Point. The difficulty comes in knowing the locations of these Nodal Points. For telephoto and retrofocus lenses they can be quite far away from the center of the lens.

[memorris]

I was taught a simple way to calculate exposure compensation for bellows extension.

Convert the focal length of the lens to inches and think of that as an aperture setting... For example, a 210mm lens id about f/8 1/3. So then look at the distance between the standards in inches as another aperture setting. The difference in stops is the compensation.

If you have a 210mm lens and have the bellows extended to 16 inches you should adjust the exposure by 1 2/3 stops.

[Jerry Bodine]

Vlad,

Using your nomenclature, then according to my Leica Manual:

D = (E x F) / (E - F), where D and E are measured to the lens diaphragm

[Vlad Soare]

Thanks, guys.
I understand that it's difficult to know where the nodal point is, but I don't need extreme precision. A rough approximation will do.

I'm trying to figure out what the longest lens usable on my camera is, and under what circumstances (at what distance from the subject, that is). The camera is a Chamonix 045n-1. The maximum extension of the camera is 395mm.
Let's say I wanted to use a 360mm lens. With the bellows extended out to 395mm, the lens would be focused at 360*395/(395-360) = 4063mm.
So, can I assume that a 360mm lens can be used on my camera with subjects from about 4 meters to infinity?

[Dan Fromm]

Vlad, I just checked y'r arithmetic. You're right.

[aduncanson]

Yes, but with a 360/5.5 Tele-Xenar with back focal length of 184mm you could focus to just about 1 meter.

Take maximum image distance = focal length + max extension - back focal length.

[Jerry Bodine]

I've not done much close-up work, but it seems to me that a WA lens (e.g., 90mm Super Angulon) would not require very much bellows draw. Any drawbacks to this?

[cowanw]

Here's another one from Deardorff's (Kellsey) Corrective Photography.
if r=(times of reduction;i.e. linear object size divided by linear image size)
D=(r+1) x F
e=D/r
F=D/r+1
Regards
Bill

[Dan Fromm]

I've not done much close-up work, but it seems to me that a WA lens (e.g., 90mm Super Angulon) would not require very much bellows draw. Any drawbacks to this?

Short working distance. Possibly, great stress possibly, so-so image quality.