convertible lens question

[Bryan Harvey]

Trying to get my lenses together... Just got a nice 8x10 dorff on ebay.

The convertible lenses like the G Clarons sound great in theory - two lenses in one, but Im leery. I dont mind paying a little more for two lenses if Im going to get superior results.

So, a quick survey...

Is removing the front element on a convertible to increase focal length just something a hobbyist can "get away with" or is this something that a professional might use on a commercial shoot.

Assume 30x40 color enlargements to be made.

thanks,
Bryan

[Walter Calahan]

Ansel Adams used a convertible lens.

I'd have no problem using my Cooke XVa on any commercial shoot. Grin.

[Mark Sawyer]

As Walter mentioned it was good enough for Ansel Adams, who used a single element from his convertable Cooke to make "Moonrise, Hernandez", argueably his best known image, and one he enlarged well beyond 30x40, albeit in b&w. Edward Weston also used a triple convertable Turner Reich.

But color is another story, and others have argued that converted lenses are a step down in quality. But since most modern convertables are plasmats, and will be "at their best" when all-together, you can buy one, try it in its converted state, and if it's not up to par, you'll still have it as a normal plasmat.

[Vick Vickery]

Mark mentioned that "others have argued that converted lenses are a step down in quality", but I've always suspected that alot of that is from folks who didn't understand how to use these lenses. You must remember when using only one lens group of a converted lens to refocus after stopping down...much of the "softness" some folks complain about when using converted lenses is, I think, due to focus shift after stopping down. This is very common with convertible lenses. I use a 160mm Turner-Reich triple-convertible and a 215mm Ilex double-convertible regularly and have always been well pleased with them. Remember to refocus after stopping down...yeah, its dim, but you'll get used to it!

[Kevin Crisp]

Bryan: The G Claron to my knowledge was not marketed as a convertible lens. The front and back groups will form a longer focal length and per some the image is pretty decent. With the possible exception of the new Cooke convertible, I have not seen a credible claim that a single element creates images that are just as good as the lens with both elements in place. I have had T/R lenses where the single elements were simply unusable. My experience with Protars (both B&L and Zeiss) has been that the single elements can be useful. They can be very, very sharp in the central portion of the image (if you adjust for the focus shift) but the more of the image circle you use the more you can see the fall off in sharpness. With a large image circle and a smaller negative like 4X5 the effect of the fall off on the edges becomes much less pronounced on the film. So if, as you say, you are willing to carry more lenses if the unconverted ones are better, then I'd have to say carry more lenses, because the single elements will not match the complete lenses. This has been true with Protars, T/R, Symmars, rapid rectilinears, and ever other convertible lens I have tried.

[N Dhananjay]

Once you remove an element, you remove some of the corrections. So, theoretically, a single element will perform worse. The typical problems are coma, spherical and chromatic aberrations. Spherical aberrations are reduced by stopping down and using small apertures. The single elements of some lenses such as the Protars are corrected for coma, but some lenses that have been marketed as convertibles (e.g., Dagors are not). Chromatic aberrations are corrected by using a strongly monochromatic filter to restrict the spectrum, which is obviously only a solution with B&W. Chromatic is also arguably more problematic in color in that you will see color fringing (rather than poorer sharpness that you would see in B&W).
The key is how much resolution do you need. If you are contact printing or making 2X enlargements, you will probably be quite happy with the single elements of convertibles used with careful technique (especially if you like their other image qualities).
Cheers, DJ