Exposure adjustment with a convertible lens

Greetings all you helpful folks. I have recently acquired a Schneider Symmar 5. 6/300 lens (born in 1973. It's a so-called convertible lens and thus by removin g the rear element it converts to a 12/500. Great, two lens for the price of on e! Since the f-stop is still read off the shutter I'm confused by the f12 thing when I remove the rear element. If I let my imagination go, I figure that the f12 thing is due to the bellows extension and that all I really need to do when using the 12/500 version is to read the actual f-stop off the shutter and then a pply a normal bellows extension correction factor (500^2/300^2 = 2.8). Unfortuna tely, the conversion from 5.6 to 12 is more than the 2.8 that I calculate (it's more like 4.1) Please explain how to adjust exposure when using the 12/500 lens. Many Thanks in advance.

Keith: The front cell is the one to remove to get the longer focal length. The aperture scale should have a second set of numbers (often in green from that vintage) and all you do is use the second scale at the longer focal length. If yours only has one set of numbers, then it's been switched from one shutter to another, best bet would be to have Steve Grimes makes you a new scale.

Once you learned that the front element is the one removed if you recalculate the f/stop you should get much closer to the marked #. You're basically dealing with a pinhole that has a weak diopter behind it.

"Great, two lens for the price of one!" - Well, no.One good lens, and a barely useable fuzzy bit of glass for the price of one, actually.

As someone else said, you remove the front of the convertible Symmar, and the second aperture scale should be marked in green, next to the normal one.

Hi Keith,

Another bubble burster I'm afraid. You will need to focus the converted lens (rear elemnets only) at the shooting aperture as there will be some focus shift as you stop down from f/12.

The lens used with both front and rear cells is a good performer but not so with just the rear elements.

Joe Dickerson

I agree that the single cells of the convertible Symmars are not as good as the combined lens, but they do fairly well at taming coma and astigmatism, the two aberrations which look most obviously ugly in images. If you are using it on 8x10 for small enlargments or contact prints the performance will be perfectly adequate.

Try it, and see if you can live with the effects. Shooting in B+W with a yellow filter helps to optimise the performance. If things are too bad, don't forget that f9 process lenses in that sort of focal length can had for only $100-$200.

Keith I've got the Symmar 240/420, and as has been said, the 2nd scale is in green. B ut listen to this; when the long configuration is used, the coverage doesn't change ! I would have thought the 420mm length would have covered a bigger neg, but both lengths are made for 5x7 or 8x10 close-up work. I like the 240 a lot though.