Convertible lens question

[avandesande]

Any reason you are supposed to put the front element in the back of the diaphragm?

 

[ilya1963]

Hi Aaron,I hope someone comes forward and answers this question, I was told in the other thread to do that exact thing with Bush and lomb that I just bought mounted in a Copal 3 , but when I tryed to mount the front element to the back of the shutter I found out that it has a different thread then in the front????
ILYA

P.S. I tryed it in the front and was able to focus with no issues

 

[David A. Goldfarb]

In principle, it is supposed to produce a sharper result, all things being equal, but I confess I don't know why that is.

I believe that the early Rodenstock Sironar lenses were sold as convertibles designed to use the front element up front, rather than behind, as the Schneider convertibles.

 

[Pinholemaster]

I do as I'm told with my Cooke XVa. When I first got the lens, I shot with the front element in front of the shutter once before my actions were corrected by reading the paperwork provided by Cooke. Frankly, didn't notice any difference when the front element was on the front instead of in the rear of the shutter. Probably because Cooke glass is so excellent that it makes quality images even when I screw up.

I don't know the physics, but yes your image will be sharper when you mount the front or rear element behind the shutter when using one lens group. I do as I'm told.

Now if the front element won't fit, then I think the solution is to simply use that lens group where it does fit.

 

[MikeM1977]

Any reason you are supposed to put the front element in the back of the diaphragm?

I own an Ilex 215/360mm convertible. I always remove the rear element instead of the front when converting to 360mm. The reason is that this acts as a tele-lens such that I have enough bellows extension. The lens performs quite well when converted. Here is a pic scanned with a 4990 flatbed:

Dead Link Removed

 

[Ole]

Generally speaking the image will be sharper, aberrations less objectionable, and distortion a lot less when the single cell is behind the aperture.
Very few lenses have more "power" in front of the aperture than behind; the only one I can remember offhand is the very old Schneider Xenar Typ D. It is said to be soft wide open, and not much better when stopped down...

Of course you also need quite a bit more extension with the cell behind. And then there are the pesky unsymmetrical shutters. I tro to stick to 00, 0, 2 and Compound 3 (not Copal) for convertibles.

 

[avandesande]

I tend to think that it would have less flare because there won't be any direct light on the glass... I was hoping someone had done a qualitative analysis to answer this. My tired old agfa is much stiffer when it isn't racked out all the way, and I suspect this will contribute to sharpness as much as anything. I guess I will have to test it for myself...

 

[Ole]

... I was hoping someone had done a qualitative analysis to answer this. ...

I have. And it confirmed the findings from a century and a half ago - a single cell is much better behind the aperture than in front of it.

An additional thing is that many convertible lenses only give the focal length for the whole and the rear, not the front alone. Like (most) convertible Symmars. Even if the focal lengths for both cells are stated, the aperture markings won't be correct for a single cell in front of the aperture. With most LF lenses the difference is small enough that it can safely be ignored, though.

If you have a Symmar lens in a symmetrical shutter, the front cell placed behind the shutter will give you an f:14 lens.

 

[avandesande]

I wonder if it makes a difference with a protar vs a plasmat.

 

[ilya1963]

Just got this from Adam at SK Grimes, I don't know if this info is something you can use:

"Some Protars were design for only the rear to be uses independently. However, you can experiment with just using the front optic on the front of the shutter. I’m not certain what type of image this will produce, but it may be worth a shot"

"Copal #3 shutters have a M58x.75 mounting thread. However, on later versions there is a substantial thread relief which prevents a filter from engaging the threads."

 

[Vaughn]

I have a Turner-Reich, I belive it is 12-21-29...at least the 29 part is right. I can use it at 29" fine on the Kodak 2D, but I only have a 5x7 back for it. My ZoneVI 8x10 is stretched at its limit at 29" -- the standards want to walk off the geared tracks.

This is with the 29" element on the rear. IF it could go in front of the shutter, I would be less likely to run the stadards off the end of the focusing tracks.

But from the conversation here, it sounds like it needs to go on the back of the shutter for a sharp image (or at least the sharpest image the lens in capable of). I have heard that a yellow filter helps with sharpness (getting rid of the mis-focused blue light).

Any confirmation or otherwise of these two point?

Vaughn

 

[David A. Goldfarb]

Yes, generally a strong monochromatic filter will improve sharpness with a converted lens by reducing chromatic aberration.

 

[jimgalli]

Just got this from Adam at SK Grimes, I don't know if this info is something you can use:

"Some Protars were design for only the rear to be uses independently. However, you can experiment with just using the front optic on the front of the shutter. I’m not certain what type of image this will produce, but it may be worth a shot"

"Copal #3 shutters have a M58x.75 mounting thread. However, on later versions there is a substantial thread relief which prevents a filter from engaging the threads."

Lots of Protar VII's had identical lenses front and back making it pointless to move the front one to the back.