View Full Version : Computing Bellows Extension for 300/360mm lens

Yo' Vinny

12-Sep-2009, 12:58

How do I compute the bellows extension required for infinity to 10 ft focus points for a 300 and 360mm lens respectively? I'd like to get one of these lenses for longer distance scenic photos and very close portraits (frame filled almost entirely by just the face).

I think my Sinar F2 4x5 will handle the infinity focus for both of these lenses, but I'm concerned I have enough bellows for closer photography focus.

The lens charts posted on the large format site show image circle sizes, but they don't indicate required bellows lengths for focus.

It would seem that this shouldn't vary between cameras and it would be specific to a focal length. Will the amount of extensions vary between lenses of equivalent focal length?

- Vince

Bob Salomon

12-Sep-2009, 13:26

It will vary with the design of the lens. An Apo Sironar N, S or W or Apo Symmar will require a lot more extension then a Tele Arton. But then the Tele has a smaller circle of illumination.

Yo' Vinny

12-Sep-2009, 13:43

It will vary with the design of the lens...

That's what I figured, it was specific to the lens. So how do I find this out without purchasing the lens, putting it on the camera and then finding out if I have enough bellows?

Bob Salomon

12-Sep-2009, 14:22

If you go to the Linos web site (they own Rodenstock) then you can see what the flange focal lengths are for their lenses. Same on the Schneider site. The FFL is the distance that you need from the back of the shutter to the image plane at infinity. So if the FFL is 360mm you will need 14" of extension just to reach infinity with that lens. More if you will do some movements. However, your camera only has a 12" long rail and not that much bellows. So you will need another rail (18" and leave the original one off or buy an additional 12" rail, possibly another bellows and an intermediate standard to join the two bellows together). How much total rail will depend on the maximum magnification - how close you want to focus - of the image that you want. If you are sure that you will only go into 10' with a 360mm then you probably can stretch your bellows enough and just need a 6" rail extension.

Leonard Evens

12-Sep-2009, 16:51

The bellows extension factor is

One plus the Magnification

The Magnification is obtained by dividing the distance to the subject (from the front principal point) by the focal point, subtracting one, and then taking the reciprocal.

The actual bellows extension, as measured from the rear principal point, is the bellows extension factor times the focal length.

To find the rear principal point, you look up the rear flange focal length and subtract that from the focal length. The principal point is that distance in front of the front of the lens board.

Of course you have to use common units.

Let me do an example !0 feet is 120 inches or 3048 mm. For a 300 mm lens, the magnification is the reciprocal of 3048/300 -1 or 1/10.16 ~ 0.098. So the bellows extension factor is 1.098 and the bellows extension is about 329 mm. For a normal lens, the rear flange focal length is very close to the focla lenth, so the distance from the film to the front of the lens board would be about 329 mm. For a lens of telephoto design, you would have to know how fhe rear flange focal length.

Note tha t in either case, to focus at 10 feet you would have to extend the lens an additional 29 mm beyond the position at which it is focused at infinity.

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