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View Full Version : Infinity focus and bellows draw

jon.oman
25-Oct-2008, 16:27
I was wondering if there was a way to pre-determine the amount of bellows draw that a particular lens would need to focus on infinity. I currently have two lenses for my 4x5 camera. One is a Nikkor 210mm f5.6, and the other is an older 135mm f4.7 Wollensak from a Graflex camera.

I know that the 210mm lens can also be looked at as a 8-1/4 inch lens (210mm = 8-1/4 inches). I have found that from lens board to film plane, the distance appears to be 8 inches, for infinity focus. But, for my older Wollensak 135mm lens I measure it to be 4-7/8 inches, instead of the 5-1/3 inches that I would expect it to be.

So, I assume that this distance is more a factor of lens design, than it is about the focal length of the lens? Is that correct? If that is the case, for purposes of building your own camera, you would have to measure this distance for each lens that you have, to allow for the correct range of bellows draw. Or, is there some sort of math that can be used to calculate this, or a lookup table?

Thanks,

Jon Oman

ic-racer
25-Oct-2008, 20:15
If you can find a spec sheet for you lens the Flange Focal Distance would be what you are looking for.

Kirk Fry
26-Oct-2008, 00:04
For non-telephoto designs it will be close enough for government work to use the focal length of the lens, unless you really want it in focus. That is the reason they invented the ground glass and the focus mechanism. :-) It depends where the optical center of the lens is and that is not always right at the back of your lens board. For telephoto lenses you will need to measure the distance after you focus it. It will be about 2/3 rds the marked focal length depending on the lens. K

Nick_3536
26-Oct-2008, 00:32
Then you have retro focus lenses that need more then the focal length. This is for wides.

In either case [long or short] it tends to only matter at extremes. You need to make sure your shortest and longest work. The stuff in the middle will look after itself.

mikec
26-Oct-2008, 05:16
And for the short and long, you have the added help of using recessed and top hat boards.
Rail camera owners just add pipe.

Leonard Evens
26-Oct-2008, 08:40
As others have pointed out, you need to find out about the rear flange focal length. That is the distance from the ground glass focusing surface to the front of the lensboard when you are focused at infinity. The actual focal length is the distance to a point called the rear principal point, which need not be the same as the distance to the front of the lensboard. For most lenses the two distances are pretty close, but for telephoto lenses, the focal length could be significantly greater than the flange focal length, and for wide angle lenses, it is often less than the flange focal length.

Most view cameras don't give you any method to control the positions of the standards accurately enough for focusing because they assume that you will always be focusing using the ground glass image. The Graflex is a bit different since it was designed to be used as a press camera. So you have stops which you can set to position the lens properly. But the way to get that right is to determine the infinity position optically and then set the stop there. To do that, bring the focusing rail all the way in, pick some distant object and focus on it as carefully as you can by moving the front standard back and forth. Do this for several such objects and see if you get essentially the same position. That is where you should put the stop.

Eric James
26-Oct-2008, 08:43
This page lists the flange focal length of the Nikkor as 202.7mm.

http://www.ebonycamera.com/articles/lenses.html

jon.oman
26-Oct-2008, 11:21
Thanks everyone, this is very good information! I guess the best way for me to do this, is to just focus the lens on a distant object, and then make a measurement. I had a feeling that, for the most part, it was dependent on the lens design.

Jon