Exposure adjustment with a convertible lens

[keith_baker1]

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 removing the rear element it converts to a 12/500. Great, two lens for the price of one! 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 apply a normal bellows extension correction factor (500^2/300^2 = 2.8). Unfortunately, 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.

[kcrisp]

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.

[armin_seeholzer]

Hi Keith

 

<p>

 

The f 11 will be f 22 now with only the 3 back elements.Thats how easy

it is!

 

<p>

 

Good shooting.

[dmin-99]

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.

[pete_andrews]

"Great, two lens for the price of one!" - Well, no.<br>One good lens,

and a barely useable fuzzy bit of glass for the price of one,

actually.<p>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.

[joseph_dickerson]

Hi Keith,

 

<p>

 

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.

 

<p>

 

The lens used with both front and rear cells is a good performer but

not so with just the rear elements.

 

<p>

 

Joe Dickerson

[struan_gray]

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.

 

<p>

 

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.

[gary_meader]

Keith

I've got the Symmar 240/420, and as has been said, the 2nd scale is in green. But

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.