Schneider Kreuznach Convertible Lens

[enrico scotece]

Hi all,

Can anyone direct me to a site/s that discuss Schneider Kreuznach Convertible lenses? I can't seem to find much info on these.
Alternatively, If you have some knowledge of these then please share. I have no idea of their performance. Im thinking of buying one to use with 4x5. It seems a clever design.

Thanks!
Enrico

[John Jarosz]

Enrico:
This link has a lot of other links.

http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=00AuNB&tag=

John

[N Dhananjay]

If you do a search on this site, you should find lots of discussion about these lenses.

Convertible lenses were designed such that you could use elements singly to provide some focal lengths or combine them to provide other focal lengths. The Schneider convertibles were certainly not new. Some rapid rectilinears were convertible. The Dagor was touted as convertible and the Protar VII series were/are considered as good as convertibles can get. Some of the Protar VII are triple convertible - for example, you could combine an approx. 16" element with an approx. 24" element to get about a 11" combined focal length. Thus, you could use both elements to obtain about a 11" focal length lens, or use the individual elements singly to obtin two more fical lengths. Casket sets often gave you a choice of four or five elements for even greater variety of focal lengths.

The thing is that convertible lenses were always a compromise between cost, weight and performance. When you remove one element, you upset some of the corrections. Typically, the problems with using single elements is spherical, lateral chromatic and distortion. Distortion is perhaps less of an issue, at least with landscape kinds of work. Lateral chromatic can be reduced by using a strong monochromatic filter to restrict the spectrum. Spherical can be reduced by stopping down. But trying to optimize over so many factors often means some compromises are necessary. Thus, the performance is certainly not near modern glass. Convertible lenses, especially when used converted, work well for contact printing and mild enlargements

The Schneider convertible utilizes two elements that combine to provide the primary, combined focal length (e.g., 270mm). By removing the front element and dropping it in your pocket (i.e., by using the rear element alone), you obtain the second focal length (e.g., 370mm). The typical Schenider convertible will have two aperture scales, the one in red being used for the combined lens, the one in green being used for the longer focal length obtained using the rear element alone. You can try using the fron element alone also, although it provides far from stellar results. It looks like the design decision was to get only two focal lengths. Thereby, the designer would have had more degrees of freedom since the front element could be tweaked to get a better performing combined lens. The lens works well - you will get a very good lens when using both elements and a reasonable performance when using only the rear element. The usual convertible rules apply - using a filter and stopping down.

Cheers, DJ