Bellows extension rule of thumb

[ndwgolf]

I’m trying to get my head around bellows extension compared to focal length of a lens (8x10)
What I mean is when I have tried in the past to take close up pictures with my 360mm lens on my Chamonix 8x10 it’s been very hit and miss trying to get focus..,,,., I keep hearing people on here quoting things like 1:1 and 1:3 followed but a certain length of bellows, can someone please try and explain this to me or point me in the right direction to learn about this?
Thanks in advance
Neil

[Louis Pacilla]

I suggest you read & study with the threads already posted on this forum regarding 8x10 close up photography. Read & study thoroughly and then ask any question your unclear on.

https://www.google.com/search?biw=16...71.Et7E-v8n3Hw

[Dan Fromm]

I think of it in terms of magnification. The magic formulas are:

rear node-to-film plane distance = focal length * (magnification + 1)

front node-to-subject distance = focal length * (magnification + 1)/magnification

For most lenses used for LF the nodes are near the diaphragm and the internodal distance is a small fraction of focal length.

If you think a little about the magic formulas, you'll see that for m = 1 the two distances are equal. For all other magnifications there's a pair of front node-to-subject and rear node-to-film plane distances for which film plane-to-subject distance is the same. Because of this, the best way to focus for close up work is to set extension to get the magnification desired and then move the subject or the camera-lens assembly to focus.

I use a tape measure to set the magnification. When the magnification or framing aren't exactly what I want I move the front standard (not much. If it has to move a lot, start over.) to fine tune.

The list has a list of books on closeup work that I recommend, with short reviews. Buy a copy of Lester Lefkowitz' book The Manual of Closeup Photography. Available at reasonable prices from vendors on abebooks.com, alibris.com, amazon.com, bn.com, ...

[G Benaim]

Keep it simple: for every extra length of bellows the size of the focal length at infinity add one stop. For instance, say you're using a 12" lens and because it's a close up you're racked out to 24". Compensate by adding one stop of exposure. I.e. If you would normally shoot at f64 and 1/10, you would now shoot at f64 and 1/5. Half stops for half the focal length, etc.

[Alan9940]

Keep it simple: for every extra length of bellows the size of the focal length at infinity add one stop. For instance, say you're using a 12" lens and because it's a close up you're racked out to 24". Compensate by adding one stop of exposure. I.e. If you would normally shoot at f64 and 1/10, you would now shoot at f64 and 1/5. Half stops for half the focal length, etc.

Not correct. A 12" lens extended to 24" would require 2 stops compensation.

[Jerry Bodine]

+1
Alan is correct. The general formula for LF bellows compensation is: Exposure Factor (EF) = [bellows extension / focal length]^2 . Bellows extension is the distance from film to lens. So, in the example presented, the factor would be 24/12 squared (2 squared = 4). A factor of 4 means two stops of correction. Of course, always keep the extension and focal length in the same units, either inches or millimeters. This general formula is valid for symmetrical lenses; it doesn't apply to lenses such as telephotos or significantly unsymmetrical lenses.

FWIW, my Leica Manual explains that, since many lenses for 35mm are UNsymmetrical designs, the formula is a bit more complex:
EF = (1 + m/mp)^2
where m is the ratio of image size to object size, and mp is the ratio of exit pupil diameter to entrance pupil diameter (mp = 1 for a symmetrical lens). Since mp is a number less than 1, the EF will be greater than that for a symmetrical lens. Fortunately, the built-in meters of many 35mm cameras provide the needed correction automatically.

[ndwgolf]

I think of it in terms of magnification. The magic formulas are:

rear node-to-film plane distance = focal length * (magnification + 1)

front node-to-subject distance = focal length * (magnification + 1)/magnification

For most lenses used for LF the nodes are near the diaphragm and the internodal distance is a small fraction of focal length.

If you think a little about the magic formulas, you'll see that for m = 1 the two distances are equal. For all other magnifications there's a pair of front node-to-subject and rear node-to-film plane distances for which film plane-to-subject distance is the same. Because of this, the best way to focus for close up work is to set extension to get the magnification desired and then move the subject or the camera-lens assembly to focus.

I use a tape measure to set the magnification. When the magnification or framing aren't exactly what I want I move the front standard (not much. If it has to move a lot, start over.) to fine tune.

The list has a list of books on closeup work that I recommend, with short reviews. Buy a copy of Lester Lefkowitz' book The Manual of Closeup Photography. Available at reasonable prices from vendors on abebooks.com, alibris.com, amazon.com, bn.com, ...

Hi Dan
So for my new lens that is a 210mm I would need to set my bellows to 420mm from the rear of the camera where the film holder sits to the front standard ~ middle of the lens and this will give me 2 times magnification. Then place the object getting photographed 210mm from the middle of the lens........is that correct?
Thanks
Neil

[Dan Fromm]

Try again, Neil.

If magnification = 2, magnification + 1 = 3. 3 * 210 = 630. To get 2:1 with a 210 mm lens, the diaphragm (your "bellows" is impossibly vague) should be approximately 630 mm from the film plane.

If magnification = 2, (magnification + 1)/magnification = 3/2 = 1.5. At 2:1 with a 210, the subject will be approximately 315 mm from the diaphragm.

Your arithmetic is also wrong for 1:1, where the distances are 420 mm film to diaphragm and 420 mm diaphragm to subject. What were you thinking?

[ic-racer]

Hi Dan
So for my new lens that is a 210mm I would need to set my bellows to 420mm from the rear of the camera where the film holder sits to the front standard ~ middle of the lens and this will give me 2 times magnification. Then place the object getting photographed 210mm from the middle of the lens........is that correct?
Thanks
Neil

Almost correct. That is the setting for 1x magnification, or 1:1. Try it. A flower 3cm tall should be 3cm on the ground glass. Add 2 stops to your exposure.

[David Schaller]

I use the formula and make a list of corrections for each focal length. Then round to the nearest third of a stop when taking the picture. When in doubt, on B&W negative film, give a little bit more exposure.