reversing a lens

[jasonikon]

Hey guys...I got a 240mm schneider which has two pieces of data on the inside rim...the first is what you'd expect 1:5.6/240mm. The other is in green and runs 1:12/420mm. Is this what happens when I take the thing apart and reverse it on my lens board?

Thanks guys

Jasonikon


BTW save the benign sarcasm by not saying "why dont you just try it and see"...I have, but Im not sure if I can see a difference and the thing is a bugger to take apart and get back together again, so thought Id save myself the hassle and ask you lot instead...;0D

[Ole Tjugen]

No, that's what you get whan you unscrew the front cell and shoot with the rear cell alone.

Reversing the lens doesn't change the focal length, but makes it a lot better for macro work at 2:1 and above.

[jasonikon]

Oh, ok, but that would leave a bare leaf shutter? Could I actually shoot like that? I wonder what is the point of putting that piece of data on the lens? interesting...thanks.

[Vick Vickery]

You have what is known as a "convertible" lens; both elements = 240mm/f-5.6; rear element alone = 420mm/f-12. When desiring the longer focal length, simply unscrew the front element, put it in a safe place, and focus/shoot with the rear element in its normal place...it won't hurt a thing for the diaphram and shutter to be bare in front. After focusing and stopping down to the desired shooting aperture, RECHECK your focus...focus shift after stopping down is common in these lenses. I use a 160mm tripple-convertible and a 215mm double-convertible and find them to be very good lenses when used carefully.

[jasonikon]

wow, never knew any of that...thank you. One Q though, how can I recheck focus without opening up again? I basically shoot at the minimum aperture for obvious DF reasons and checking at this Ap is far too dark...Thanks though,very useful info...cheers

[jasonikon]

another thought...it says 1:12...does this mean i have to allow for this when setting the aperture or do I just use it normally? thanx

[Dan Fromm]

another thought...it says 1:12...does this mean i have to allow for this when setting the aperture or do I just use it normally? thanx

The two cells together make an f/5.6 lens. The rear cell alone is f/12. If you have the original shutter, it will have two aperture scales, one starting at f/5.6, the other at f/12.

If the shutter has only one aperture scale, starting at f/5.6, you can have a second scale engraved or you can make your own. But if you shoot the rear cell wide open, set the aperture to f/5.6 and expose for f/5.6 the negative will be underexposed by a bit more than 2 stops.

[Bernard Kaye]

With front cell removed, light will hit the exposed shiny front threads and scatter onto negative: you can use a screen ring that is black and screws into front threads; then you attach a lens hood onto frond part of screen ring. Convertibles were provided with screen rings for this purpose, to cover shiny front, now exposed threads. Camera specialty shops make the rings; they are scarce.
Bernie

[Ole Tjugen]

The "exposed shiny front threads" are so close to the aperture that at the usable shooting apertures for a converted lens (f:32 to f:64), no reflections from these can possibly reach the film.

[IanG]

The "exposed shiny front threads" are so close to the aperture that at the usable shooting apertures for a converted lens (f:32 to f:64), no reflections from these can possibly reach the film.

Are they ? They are very close to the aperture, probably too close.

They won't because the faces of the threads you see reflecting light are actually reflecting light back at you, the thread faces reflecting into the camera may have an effect but it's almost certainly insignificant.

Ian