Triple convertible Dagor?

[peter_galea]

After reading posts about removing one cell from a Dagor to make it

longer, I tried it. I have a 7" Dagor f/6.8. When I remove the rear

cell I get infinity at 283mm. When I remove the front cell, infinity

is 325mm. Is it infinity, or close focus that counts? Am I going about

this in the right way, is it 3 lenses in one?

[Troll]

No, it's only a plain convertible. Whether you use just the front component, or the rear component alone, the focal length is the same (about 160% of the combined F.L.). The difference in extension required is a function of the nodal point location of the elements, not their focal length. Goerz recommends that it be used behind the shutter for best results.

[dhananjay_n]

The Dagor is a symmetrical lens i.e., both front and rear elements have the same focal length. The reason your flange focal lengths are different is because removing one of the cells shifts the nodal point of the remaining cell outside the cell. So when the element is behind the aperture, the actual nodal point lies behind the surface of the glass, somewhere inside your bellows. Similarly, when the lement is in front of the aperture, the nodal point is a little in front of it (like in a telephoto lens). If memory serves me right, I think the converted length of a Dagor is about 1.7-1.8 the prime length i.e., approximately 300mm for your 7" lens.

 

The position of the stop is supposed to help with correcting some of the aberrations that result in using a single element. The barrel distortion that occurs with the stop in front of the element is considered preferable to the pincushion distortion you get with the stop behind the element. So, it is probably preferable to use the rear cell i.e., behind the stop. However, if you have bellows limitations, go ahead with the cell in front of the stop. The Dagor single cells are not corrected for coma (unlike the Protar VII). So, you really do need to stop down when using the single cells. Performance can be quite grim at stops short of f/45. Having said that, some famous pictures have been made with single elements of Dagors (e.g., Cliffs and Ice by Ansel Adams).

 

Cheers, DJ