View Full Version : Back focal distance of large format lens

Ho Pei Jiun
5-Jan-2005, 19:13
Dear Sirs/Mdms,

Thanks for the replies from you guys out there.

I've one Rodenstock Apo Sironar-N 210mm f/5.6 Copal 1 Large format lens. This lens do not have any ring on it for adjusting the focal length like what other 35mm lens do, hence this lens has a fixed focal length of 210mm.

May i know what is the rear focal distance (i.e. the length from the tip of the
lens's furthest rear surface to the film plane in the large format camera where the image is
focused) for the above mentioned 210mm large format lens?

Please kindly advise. Thanks.

Ho Pei Jiun

Gem Singer
5-Jan-2005, 20:38
Hi Ho,

I'm really trying to understand what you are asking. The length from the tip of the len's furthest rear surface to the film plane is not the same as the rear focal distance. The measurement can vary according to the thickness of the lensboard, the spacers used to mount the lens, etc. The way I understand your question is: " What is the flange focal length of a Rodenstock Apo Sironar-N 210 f5.6 lens? " The distance from the mounting flange to the film plane. The measurement for that particular lens is 200mm. when it is focused at infinity.

Large format lenses have fixed focal lengths. They are focused by changing the distance between the lens and the film plane until sharp focus is achieved. They are not designed like the zoom lenses on your 35mm camera. It would help us to help you if you would please explain the reason that you need to know the exact distance between the rear surface of the lens and the film plane.

Thilo Schmid
6-Jan-2005, 04:35

only the exact mechanical specification of the lens can answer your question. However, the Flange Focal Distance or Flange Back - the distance between the mounting flange at the lens board and the ground glass at infinity - may help you as well. There is a table on the Ebony Website (http://www.ebonycamera.com/cam/lenses.html), telling that the Flange Back for the Apo-Sironar-N 5.6/210 is 200mm.