210 Apo-Symmar as convertable?

[Mark_437]

(I also posted this to the Schneider board but thought someone here probably has some experience w/ this.)

I have a 210 Apo-Symmar and I was wondering - what would happen if I unscrewed t he front element and just used the rear "cell"? I played with it (set up and foc used - no photos) and would estimate the focal length at approx. 14" or so. This seems like a good focal length for headshots on 4x5, and if it's not fully corr ected for spherical abberation then so much the better, right? (After all, we're talking headshots of women here!) Has anyone done this and if so, what were the results? Would this also cover 8x10? What focal length would just the front cel l yield, and would it be even more uncorrected than the rear one? (That would be my guess.) And why does the focal length seem different if I flip the rear cell around?

[Duane]

1.) Theoretically as a 14" lens it should cover the 13" diagonal of 8*10. Practically it may not because the shutter and lens barrel may cause vignetting.

2.) The net focal length of a compound lens with the elements close together is: 1/(1/flf+1/flr) where flf & flr are the front and rear focal lengths. Since you've measured the rear focal length as 14", the front must be abut 20" to give the 210 mm (8 1/4") net focal length.

3.) Since symmetrical lenses use the complementary abberations in the front and rear elements to cancel each other out, I'd suspect that they would have similar abberations. However, since the focal lengths are different, the net abberations as imaged may be different. You'll have to check & report!

4.) I think the focal length appears to change as you flip the element because of the nodal point is not in the center of the element. That is, the reference point for the focal length is not halfway between the front and rear surfaces of the element.

Cheers,