View Full Version : Cooke XVa triple convertible flange distance

Collin Orthner
11-Aug-2004, 21:36
I was just looking over the spec sheet on this fine lens, interested mainly in the flange focus distance. Usually large format lenses flange focus distance is around the same as the focal length or slightly less and with telephoto designs quite a bit less. This new Cooke lens is only really close when using both front and rear group together but are quite dramatically more when only using the front or only the back group. Is this the way convertible lenses are or am I not understanding the spec sheet? I was hoping this lens would work nicely on a Phillips Compact II which has about 26" of bellows allowing up to a 660mm flange focus distance.


David A. Goldfarb
12-Aug-2004, 07:00
The Cooke lens is designed to be used with the single cells behind the stop, so the optical center of the lens is going to be behind the shutter, requiring more bellows extension than with a conventional lens with a stop in the center. Convertibles that work this way, like the Symmar convertibles all have this issue.

So if you need a bigger camera with more bellows, rail, focusing track, etc. (or alternately an extension lensboard) to use the lens that might cut into the weight advantage you would get from using a convertible.

Steve Hamley
12-Aug-2004, 08:14

It's maybe even worse than David implies. I had talked to Dick Phillips about this camera a while back and he stated that 18" or 19" lenses were about the usable max if you wanted to focus reasonably close, and that the 600mm Fuji C that's my longest lens is somewhat marginal on this camera from a closer-than-infinity standpoint (as would be expected from the numbers). Even if the Cooke FTF was close to the 646mm focal length, 14 mm won't get you much.

But all this is well known; Phillips cameras are not designed nor intended to be long-lens cameras, although I do recall seeing a picture of Michael Fatali with his sporting a big Schneider Apo Tele-Xenar.


N Dhananjay
12-Aug-2004, 08:48
Most convertible lenses tend to be symmetrical (or nearly so) designs - the nodal point will tend to be located at or near the aperture. When one of the groups (e.g., the front cell) is removed, the nodal point of the lens shifts and now lies well outside the glass (i.e., somewhere inside the bellows between the lens and the ground glass) - in other words, the design does become something like a retro-focus one. This is why the flange to flange distance is so much more than the marked focal length. Now, obviously if you placed the cell in front of the stop, it actually acts as something like a telephoto lens (i.e., the nodal point lies out in front of the lens) and there is a considerable reduction in the flange distance.

The position of the stop helps in correcting some aberrations, which is one reason most convertible designs place single cells behind the stop. However, placing the single cell in front of the stop does not compromise image quality too much (probably because the stop is not in the ideal position, to begin with, and this is convertible image quality, anyway). And placing the cell in front of the stop reduces the flange distance considerably.

Cheers, DJ

Collin Orthner
12-Aug-2004, 13:13
Thank you all for your answers. It is much appreciated. I would have to say this lens would definately not be a good choice for use with the Phillips camera.