View Full Version : fuji 65mm swd film to flange distance

Darcy Cote

23-Mar-2010, 09:03

Hello:

I am building a camera and I require the film to flange distance at various distances for this lens. Any idea where I can get this information? If not, what is the best way to determine this? I do have the lens in my possession.

thanks in advance.

Darcy:)

aduncanson

23-Mar-2010, 09:27

http://www.cameraeccentric.com/html/info/fujinon_1.html

Shows the second principal point to be 8.4mm behind the flange plane and the focal length as 65.0 for the "New SWDS 65". Flange to film distance would be the sum (75.4mm) at infinity for this lens. If your not sure that this is your lens (or even if you are) I would make at least a rough measurement to confirm this.

Peter K

23-Mar-2010, 10:16

Hello:

I am building a camera and I require the film to flange distance at various distances for this lens. Any idea where I can get this information? If not, what is the best way to determine this? I do have the lens in my possession.

Darcy, in the link given by aduncanson on page 4 the flange focal lengt (Ff) is 73.4mm. For other image scales you can assume your lens is an infinte thin lens with the center at the rear plane of the shutter and calculate with the formula

b = f' * (M +1)

b is the flange to image distance; f' = 73.4mm; M = image scale

E. G. for an image scale of 1 : 2 the flange to image distance (b) is 73.4 * (0,5 + 1 ) = 110 mm.

To estimate the distances you only need the lens, a white cardboard and a room with a view into a bright landscape. Open the shutter (T) and point the lens in the direction to the bright subject. Focus the image projected on the cardboard and measure the distance between flange and cardbord.

It works fine, I've tested it one minute ago with a SA 65mm ;)

Have fun

Peter

aduncanson

23-Mar-2010, 13:26

Peter is correct 73.4mm. My arithmetic skills failed me (and I overlooked that entry in the table.)

However, I thought that after finding the second principal point all image distance measurements would be from that second principal point to the image plane.

That would imply that the change in image distance from the infinity focus would be M * F. That yields the often quoted result that when focusing from infinity to 1:1 the lens moves exactly 1 focal length, and for Peter's example of a 1:2 scale the flange to image distance becomes 8.4 + 65 * (1 + .5) = 105.9.

Am I wrong again?

Peter K

23-Mar-2010, 14:35

However, I thought that after finding the second principal point all image distance measurements would be from that second principal point to the image plane.

But for this one has to find at first the rear nodal point as shown here (http://www.largeformatphotography.info/forum/showthread.php?t=60087&page=3). Because the information given in datasheets changes from batch to batch.

That would imply that the change in image distance from the infinity focus would be M * F. That yields the often quoted result that when focusing from infinity to 1:1 the lens moves exactly 1 focal length, and for Peter's example of a 1:2 scale the flange to image distance becomes 8.4 + 65 * (1 + .5) = 105.9.

Am I wrong again?

Yes and no.

A lens moves exactly 1 focal length when focussed from infinity to 1:1. But lenses from the Biogon-type are double-ended reversed-telephoto lenses, so the flange-focal-lenght is larger as the focal-lenght defined by the angle of view and image scale.

So if one only wants to know the flange-focal-lenght for different focus distances it's easier to measure between flange and image plane as described before.

Peter

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