PDA

View Full Version : Doubt with convertible lenses



lungovw
17-Dec-2007, 08:06
Dear group,

When you have lenses like a Dagor or Zeiss Convertible Anastigmat and you want to use only one group, do you simply take out the other and leave the remaining one in place in its original position or do you need to place it always as front or rear element? When it is not a symmetrical arrangement. Is there a right order like the shorter element in the front or rear position?

Rgds

WL

Jim Galli
17-Dec-2007, 08:10
Dear group,

When you have lenses like a Dagor or Zeiss Convertible Anastigmat and you want to use only one group, do you simply take out the other and leave the remaining one in place in its original position or do you need to place it always as front or rear element? When it is not a symmetrical arrangement. Is there a right order like the shorter element in the front or rear position?

Rgds

WL

Usually you want the element you are using behind the aperture. Combined, the longer element is up front and the shorter in rear.

Walter Calahan
17-Dec-2007, 08:30
Like Jim said, with my Cooke XVa when using a single lens group, the group goes to the rear of the shutter.

Kevin Crisp
17-Dec-2007, 10:30
If you have the bellows draw for it, a single element on the rear is better. Significantly less bellows extension is needed for the same element on the front. (Actually it is fairly hard to see any difference in performance rear or front, but there is some improvement on most I have tried.) At least with Protars a focus shift for the single element is much more of a problem. Recheck your focus stopped down to the taking aperture which is better in the f:32-45 range. There is definitely some fall off in sharpness at the edges with the single elements, but if you are using only the center of a good sized image circle (as so often happens) this is usually not a problem. If you are contact printing it is certainly not a problem at all. The single elements can be fantastically sharp away from the edges if you adjust for the focus shift.

Bernard Kaye
20-Dec-2007, 19:26
If you use the shorter focal length in front, longer focal length in back, the shorter focal length has a smaller diameter and acts like an iris or diaphragm to reduce the size of the circle to stop down the lens; you will not be getting all available light for viewing and f stops will be off.

All the manufacturers' instructions I have read call for using one element only in rear, concave surface facing front (toward the object being photographed) never in front; even with this there may be a tendency to pincushion when using one only in rear; using one only in front may produce barreling; these are important if you do architectural work.

When using one only in rear, bright metal threads in shutter front should be covered with a screen ring that is screwed into shutter front, extends out front like a short hood that you may attach a longer hood to. B & L instructions cover this; they provided a lens screen ring with their Convertible Protars.

Bernie

Bernie

Struan Gray
21-Dec-2007, 02:17
An exception: the original Rodenstock Sironars were sold as convertibles, and the manufacturer's instructions were to remove the rear element. Two motivations for using the element in front is that it protects the shutter and you need less bellows draw.

If you are fiddling with cheap lens elements, like spectacle blanks or scavenged bits from dead 35 mm objectives, nine times out of ten the image will be better with the aperture in front of the glass. But lens designers can get more sophisticated than this, so there is no hard reason why a lens designed as a convertible has to use the aperture out front.

Richard Raymond
21-Dec-2007, 16:43
I have a 240mm Caltar II Type S lens that has an additional scale for 700mm. Almost three times focal length change. Do I reverse the cells to get this?
Thanks
Ric

lungovw
24-Dec-2007, 04:38
Thank you guys for your valuable input. I never tried with Sironars or Dagors but it happened that I received as a gift and old Ross lens that is really inviting the user to go for 3 options of focal lenght. It has different aperture scales for each case (first time I see something like that). Case you want to have a look... it is a beatiful piece.

http://www.lungov.com/wagner/Divers/Ross%20London

Best regards,

WL

Ole Tjugen
24-Dec-2007, 04:51
That is a nice lens - I wonder if it's a series VIIa or a series IV? :p