210 Apo-Symmar as convertable?greenspun.com : LUSENET : Large format photography : One Thread
(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 the front element and just used the rear "cell"? I played with it (set up and focused - 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 corrected 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 cell 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?
-- Mark Parsons (Polar@thegrid.net), December 03, 1999
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.
-- Duane (email@example.com), December 03, 1999.
Since 3 weeks I`m the owner of a old 1978 Symmar f 5.6 210mm and is also only without the front element a f 12 370 mm thies is stated in the front element in green! But I could not test it up to now because it needs a bellows of 430mm, but it was an older one thad was designed for thad Schneider told me thad!
-- Armin Seeholzer (firstname.lastname@example.org), March 14, 2001.