schneider convertible symmar question

[bill kehoe]

i have a 150mm f5.6 and i know if i remove the front element, i get a 265mm f12 focal length. it's been a while but i'm pretty sure that i once noticed i could get a 3rd, even longer focal length if i removed the rear element (having replaced the front one, of course). what's more, i'm pretty sure i read about this too, someplace i can't find again. does anyone know if this is correct, and if so, what that 3rd focal length and max aperature would be? thanks.

[Paul Ewins]

Using the front element alone gives even worse performance than just using the rear element which is why Schneider dropped the triple convertible feature of the previous range of Symmars. It is something to do with the placement of the aperture, so with a lens that is in a Copal 0 or Copal 3 you could swap the cell to the rear but as a Copal 1 has different thread sizes front to rear you are basically stuck.

I couldn't tell you what the other focal length will be, but it will be in the same ballpark as the rear cell (265mm) and will also be close to f12.

[dave_whatever]

Doesn't the formula for convertables go something like 1/150mm = 1/265 + 1/X where X is the focal length of the front cell alone? That makes the front element along something like 346mm.

There's a couple of things to watch for though. Firstly using the rear element alone means you lose the abilty to mount a filter (unless you've got somthing that fits the shutter thread). Also the rear-only setup is very retrofocal, i.e. you'll need bellows much longer than 265mm to focus it. On the other hand the front-only setup is very telefocal, i.e. you'll need a lot less bellows to focus it, probably around the 200mm mark.

[mikec]

see http://www.cameraeccentric.com/html/info/schneider_7.html for the Schneider brochure. Plenty of technical info. Only use of the single rear element is indicated.

[Peter K]

The convertible Symmar is nearly symmetrical with a ratio of 1 : 1.75 : 1.65 (whole lens : rear element : front element) so using the front element alone makes no sense. Also as Paul has mentioned before with a diaphragm behind the lens the spherical abberation isn't tolerable and mounting the front-cell behind the shutter isn't possible with a shutter size #1.

So the front-cell of your Symmar 150 would give a appr. f/10 247mm lens.

[Mark Sampson]

Like many people, I once tried using the front cell only of a 180mm Symmar (being ignorant of the correct method). Very poor resolution, didn't look good at all. Years later, I tried the proper setup (rear cell only), and was pleased to find it quite usable, better than its reputation. But not better than the 300/9 Nikkor-M that I owned by that time, so I went no further with it. One has to wonder, though, why Schneider didn't provide an adapter ring to allow filters on the front of the converted lens, and a rear cap for the front cell.

[bill kehoe]

thank you all very much. bk

[aduncanson]

Dave,

Peter's answer contradicts your supposition because for lenses where thickness and spacing are appreciable, these parameters enter into the equation. Wikipedia has a good, simple explanation.

http://en.wikipedia.org/wiki/Lens_%28optics%29#Compound_lenses