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Ed Richards
24-May-2011, 18:40
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
24-May-2011, 19:12
Depends on the lens design.

Ed Richards
24-May-2011, 20:33
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
24-May-2011, 20:34
This might help.

“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
25-May-2011, 00:22
This might help.

“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
25-May-2011, 00:59
Assume roughly where the aperture iris is.

Ian

Struan Gray
25-May-2011, 01:12
This might help.

“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
25-May-2011, 02:26
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? :rolleyes:

Struan Gray
25-May-2011, 02:31
We're guys!

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

Struan Gray
25-May-2011, 02:33
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.

Joanna Carter
25-May-2011, 02:33
We're guys!

You'll be telling me I don't need that titanium spork next....
Well, if you're a guy, then it's an essential :p

Joanna Carter
25-May-2011, 02:47
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.
Maybe it's my simplistic approach but, if I were buying a lens, I would look at the fact that, at 1:1, the bellows draw on a standard lens, would be double the focal length; but, since Ed is looking at focusing at 50ft, I doubt he would be using that much more than the infinity length.

Emmanuel BIGLER
25-May-2011, 02:51
The following formula is universal and valid whichever the lens design might be, wherever the principal planes could be located, e.g. in a telephoto design (I have a 360 Schneider tele-arton where the principal planes are located at a real exotic place ;))

Additional Bellows Extension beyond the focal point = (magnification) X (focal length)

Landscape photographers, unlike macro aficionados, are not familiar with the (image / object) magnification ratio, but a rough estimate of the magnification is very easy to know when you can approach your subject to measure it; for example a person 1m80 tall ( approx 6 feet) framed for a "portrait" image including feet and head with a 4x5" camera, image height is about 120 mm for the 4x5 in portrait orientation, hence the magification is 120/1800 = (1/15) = 0.06667

This magification ratio is what you need in oder to frame the subjet completely, this is independant from the focal length in use
It the f.l. is 600 mm, the additional bellows extension beyond the focal point will be 600/15 = 40 mm.
The (infinity->focus) minimum bellows extension actually depends strongly on the lens design.
If the 600 mm is an apo-repro lens of symmetrical design, the minimum bellows draw at infinity will be about 600 mm.
But usually people know it if they have been successful focusing a landscape image, hence they know their minimum infinity-focus bellows draw. They just need the additional bellows draw as indicated by the simple & universal formula above.

There is a very simple tool ( 100% free, a D-I-Y cardboard device !) designed for calculating the exposure correction in close-up, named the quickdisc, something that gives you a direct reading of the magnification on a the ground glass.
More on Mr. Salzgebers's web page, the designer of this clever and useful device (in English)
http://www.salzgeber.at/disc/

Le last objection will be: eh, I cannot approach my subject, hence the previous technique fails since I have no means to estimate the magnification ! The answer, then is : probably your subjet is, say, not at infinity, but not very far from infinity and your bellows draw will be very close to the infinity-focus setting ;)

eddie
25-May-2011, 03:44
Easiest way for me is if you have a 100mm lens and u focus and the bellows measure 150. U add one stop. If it is 200mm then add two stops.

Easy as pie.

GPS
25-May-2011, 04:04
The following formula is universal and valid whichever the lens design might be, wherever the principal planes could be located, e.g. in a telephoto design (I have a 360 Schneider tele-arton where the principal planes are located at a real exotic place ;))

Additional Bellows Extension beyond the focal point = (magnification) X (focal length)

Landscape photographers, unlike macro aficionados, are not familiar with the (image / object) magnification ratio, but a rough estimate of the magnification is very easy to know when you can approach your subject to measure it; for example a person 1m80 tall ( approx 6 feet) framed for a "portrait" image including feet and head with a 4x5" camera, image height is about 120 mm for the 4x5 in portrait orientation, hence the magification is 120/1800 = (1/15) = 0.06667

...

Le last objection will be: eh, I cannot approach my subject, hence the previous technique fails since I have no means to estimate the magnification ! The answer, then is : probably your subjet is, say, not at infinity, but not very far from infinity and your bellows draw will be very close to the infinity-focus setting ;)

As you figured out yourself the landscape photographers have a good reason not to know the magnification ratio...;)
To say that they can go and measure their objects if they are close enough doesn't help either. Try to measure 30m high tree that is just 20m ahead of you. Good luck.
Or try to measure its width just 3m over the terrain when the tree is "just in front of you". Good luck with it also. And if you're at it try to measure the hight of the house 10m in front of you in a garden, or how tall the waterfall you see 20m ahead of you is etc. etc. Nothing beats practical advice...:)

Ed Richards
25-May-2011, 05:19
Struan is exactly right. I am trying to figure out if I could use a Fuji 600mmT on my Ebony using a top hat board. The Ebony has a 365 bellows (maybe a little more), the T600 needs 384 at infinity, and Ebony makes a 35mm top hat, giving me 400. Where will that put my focus point? There are longer top hats available on Ebay, but stability will begin to be a real problem as the extension gets longer. I can add a Bogen brace, to make it a little more stable. Or maybe I should just get rid of the 600 and stick with my 300.:-)

Struan Gray
25-May-2011, 05:51
The Ebony has a 365 bellows (maybe a little more), the T600 needs 384 at infinity, and Ebony makes a 35mm top hat, giving me 400. Where will that put my focus point?

10.9 meters (35' 8'') in front of the lensboard.

Do you already have the 600? If not, it might be worth taking a look at the Nikkor 500 mm tele. It has a flange focal distance of 350 mm, which would let you focus to 56' on a flat lensboard, or to 18' on a 35 mm top hat. The difference between a 300 and a 600 is quite severe, between a 500 and a 600 much less - equivalent to cropping 1/4-1/3" round the edge of your negs. (And you'd still get to buy a new toy :-)

Ed Richards
25-May-2011, 07:24
I doubt that I would ever use a 600m for anything closer than about 40 feet, so that would work. I do have the 600, but I am within the return period. I like the larger image circle on the 600mm.

25-May-2011, 07:47
I suggest that you review The Fuji Lens Catalog (http://www.cameraeccentric.com/html/info/fujinon_1.html)on the Camera Eccentric site.

It gives the design focal length as 589.6mm. Use this value in the lens equation attributed to Ansel Adams above, rather than 600mm.

Flange Focal Length (FFL) is the 384mm given above rather than the 600mm assumed in your original post.

Measure Image Distance (v in Adam's equation) from the front of the lens board to the ground glass and add 205.7mm, (the distance that the second principal plane is ahead of the front of the lens board.)

Measure Subject Distance (u) from the front of the lens board to the subject and subtract (205.7 + 66.4 = 272.1)mm, (the distance that the second principal plane is ahead of the front of the lens board plus the separation between the two principal planes.)

It is rarely necessary to bother with the correction to the Subject Distance measurement unless the 272.1mm correction is a significant fraction of the Subject Distance or if you really are focusing by means of a tape measure or ruler rather than the ground glass, range finder or focusing scale. Can you really tell the difference between 50 feet and 49.11 feet?

Good Luck

GPS
25-May-2011, 12:23
I doubt that I would ever use a 600m for anything closer than about 40 feet, so that would work. I do have the 600, but I am within the return period. I like the larger image circle on the 600mm.

Go for it - often it is so that once you have the lens then you see and you find the pictures for it...