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Shortest and longest possible focal length

Martin Czermak , Jun 10, 2003; 05:26 p.m.

I have a Linhof Super Technika V 6x9, built in 1996 and use lenses between 58mm and 210mm. My question: Is there anybody who can tell me what is the shortest focal length and the longest focal length, possible to use it with my Technika? I ask here because the tech's at Linhof, the tech's at Schneider and the tech's at Rodenstock gave all three different answers.

Greetings from Vienna,Austria

Martin Czermak

Responses

Michael Briggs , Jun 10, 2003; 11:41 p.m.

The answers to the questions "what are the shortest/longest usable focal length" are somewhat matters of opinion. For example, is a long lens "usable" if it can be focused on infinity but no closer?

With the camera in hand and some information that the lens manufacturers provide, you should be able to figure out whether a lens will be useful to you. The lens parameter that you need is the "flange focal distance". This is provided on most lens datasheets and is the distance from the image to the back of the shutter when the lens is focused on infinity.

For a short focal length lens, use a ruler to position the front of a lens board the flange focal distance from the ground side of the ground glass. If the bellows and camera can do this, and if you are satisifed with the movements that are possible, then the lens is useful. For a short focal length lens, this is the worst case -- focused on infinity. As you focus closer, the lens will move out and the bellows will be less compressed, etc. You might be able to go to a shorter focal length with a recessed lens board. If you only have a non-recessed lens board, you can estimate how much further forward the front standard would be with the recessed board and make your judgement.

For a long focal length f, again position a lensboard the flange focal distance from the ground glass. This would focus the lens on infinity. Now measure how much further forward the front standard can move while still allowing for any desired movements, without excessively stretching the bellows, etc. Call this change in distance Delta_d. Define the maximum optical extension for this lens as Si_max = f + Delta_d. (This differs from the mechanical extension because the flange focal distance isn't the same as f.) Then you can use the lens formula 1/So_min + 1/Si_max = 1/f to determine the distance So_min to the closest object that you can focus on. If you find this distance satisfactory, then the lens is usable. A true telephoto lens has a small flange focal distance compared to its focal length f so that the lens can focus closer for a given mechanical extension.

There is more information in the Photo.Net Lens Tutorial and in related recent postings: LF lenses, bellows length & Closest Focusing distance, how to calculate closest focus distance for max bellows draw and FL of lens?, and Macro lens: Calculating subject distance with film distance.

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