Lens cell placement on classic convertible lenses

Is there a general rule of thumb for which lens cells are placed in

the front or back of an old convertible lens when they are combined

together? Specifically, if you have two lens cells, one with a

larger focal length, is it placed in the front or rear? Or, are

specific cells meant to be always be placed in front or the rear

when used in conjunction with another cell? (I know if you use the

single cell, it should go to the rear). Or does it matter?

 

If anyone's curious, I'm wondering about the lens placement for

Zeiss series VII and Turner-Reich convertible lenses. On the Zeiss

I've seen, the longer focal length is in front, and on the Turner-

Reich, if I remember right, the larger cell is in the rear.

My 12,19,25 Turner Reich puts the 25" element in front in the combined 12" lens.

In general the longer focal length is placed in front of the shtter while the

shorter is behind. Also single cells are used behind the shutter. Does it

matter? A lot depends on the particular lens set and the neg size you intend

to use with it. Depending on the circumstances the reverse of the 'rules' works

too for a lot of users. ie. shorter fl in front, longer fl in rear - single cell in front

rather than rear.

 

Best bet is to give what you have a try and see if you can detect any

differences. Good luck!

 

Cheers,

Roger...

cell placement: with only one cell in place, there necessarily will

be more distortion. both cells offer their own measure of

correction. most users prefer the type of distortion that typically

occurs with a single cell behind the shutter (barrell instead of

pincushion as occurs with the cell in front). however, there is

another consideration. with the single cell in front, the nodal

point is placed further from the film plane. this results (like a

telephoto) in shorter bellows extension at infinity.

sorry to be sloppy -- i should have said "puts the nodal point in

front of the mechanical center."

Normal design practise is to place the longer focal length in front (when using two cells together). This is to ensure that the light reaching the rear cell is as collimated as possible. When using single cells, place the cell behind the diaphragm - the position of the diaphragm helps in correcting some aberrations. However, if your bellows are too short, you can place the single cell in front of the diaphragm to reduce the bellows draw. When using single cells, the primary aberrations are spherical, coma and lateral color. The first two are reduced by stopping down. The third is reduced by using a sharp cutting filter to reduce the apectrum. Cheers, DJ

Has anyone compared the difference in performance of a convertible SYMMAR between using the front and the rear components? I think that the second aperture scale is based on the rear component, but I cam only use the front component due to limited bellows extension.

The Symmar convertibles are meant to use the rear component for the converted focal length. The front component will give you a focal length slightly longer than the rear (approximately 2 times combined focal length). It's a lot softer - lots of aberrations, sort of pleasing in its own way but hardly what you would lcall a sharp lens. It's the rear component that has been corrected better for single use - the front component seems to have been used to correct the remaining aberrations when the lens is used as a whole. If your bellows extension is limited, one option is to see if you can jury rig the rear component in front of the iris (not the best place, but it is probably going to perform better than just the front used alone in front of the diaphragm). Alternatively, you could try jury rigging an extension board. Cheers, DJ