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? thanxThe 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

[Bob Salomon]

Rodenstock recommended removing the REAR cell to protect the shutter. Many of us who shot with other convertible lenses when they were current also did this and we were taught at the USAF Photo School at Lowry AFB to also remove the rear cell only. Try it both ways. By removing the rear cell you protect the shutter. Still have the ability to use filters (especially rotating ones like a polarizer or a circular grad and have glass protecting the shutter blades that would be exposed when the front cell is removed. You also eliminate those shiny threads in the front.

Lastly you should not shoot at the smallest aperture. While this will increase DOF it will also create diffraction. You can use tilts and swings to increase the plane of sharp focus.

[Ole Tjugen]

... But if you do remove the rear cell, remember that

a) The focal lengths are different, so the aperture will not be correct if originally made for rear cell alone, and

b) Those convertibles which were intended to be converted by removing the front cell are likely to have much better corrections in the rear cell alone than in the front cell alone. The sharpness and coverage of a rear cell aren't all that great; with a front cell alone they are (generally) even worse. You will end up shooting at f:32 or smaller to get acceptable sharpness with a converted Symmar. Diffraction and DoF don't matter at all here; astigmatism and chromatic aberrations will settle the matter for you.

[Vick Vickery]

On rechecking focus after stopping down...Yep, its really dark on the ground glass when you are shooting at f-32 or f-45, but use your focusing magnifer on an important highlight that you want in focus...you'll be able to do it after working at it awhile! :)

[Arne Croell]

Rodenstock recommended removing the REAR cell to protect the shutter. Many of us who shot with other convertible lenses when they were current also did this and we were taught at the USAF Photo School at Lowry AFB to also remove the rear cell only. Try it both ways. By removing the rear cell you protect the shutter. Still have the ability to use filters (especially rotating ones like a polarizer or a circular grad and have glass protecting the shutter blades that would be exposed when the front cell is removed. You also eliminate those shiny threads in the front.

Lastly you should not shoot at the smallest aperture. While this will increase DOF it will also create diffraction. You can use tilts and swings to increase the plane of sharp focus.

That is certainly better from the practical point of view, like impact of a dirty/dusty environment and the other things you mention. Using the rear cell is recommended from the optical correction point of view (except for the Sironar, see below). The usual rule for convertibles is that if you have two cells with different focal lengths, the longer focal length should be on the side of the longer conjugate, which is usually the object side, not the film side of the lens. "No lens cell" is equivalent to a cell with infinity focal length, so it should be on the object side -> then the remaining cell goes behind the shutter. This was the case for the Symmars, the Cooke convertible, the original Plasmat, the Protar and many others. The one exception where the manufacturer recommended using the front cell alone, and the 2nd scale being calibrated for that was the original Rodenstock Sironar. I can only assume that the front cell was better corrected for single use on purpose.

[seawolf66]

Why not place a clear uv filter in place of the lens or a 4x4 uv filter clear in front to replace the lens Just a thought

[Bernard Kaye]

I suspect Ole & Ian are correct but I (the messenger) wonder why Zeiss, B & L and others provided screen rings and recommended they be used? Bernie