View Full Version : convertable len

Clay Turtle
5-Mar-2006, 08:42
I noted the mentioned "Your choices are Nikor T- (360), 500, 720 (interchangeable rear lens elements)" directly relates to my question. I have been looking for some information on a lens I just acquired a 300/500mm Convertible Symmar. I would think that these types of len came with an interchanging rear element to use when changing telephoto?

Ernest Purdum
5-Mar-2006, 09:12
Convertible Symmars are different than than telephotos with interchangable rear cells. The Symmar is "converted" by removing the front cell. Using the rear cell on its own gives the longer focal length. Symmars are not telephotos. They require bellows extension roughly equal to the focal length.

N Dhananjay
5-Mar-2006, 09:16
Nope, the Symmar convertibles did not come with interchangeable elements. The idea with the original convertibles was that you could combine two reasonably well-corrrected elements of differing powers to get a combined les of a different power, thereby having three reasonably well-corrected focal lengths to use. Obviously, you thus have a bunch of design restrictions that result in various compromises being made.

Most original convertibles tended to rely on symmetry between the elements to control some of the aberrations. Used with both elements, a well-designed lens will be remarkably well corrected. When you remove one of the elements, you obvioulsy lose some of the corrections (typically spherical and chromatic), which is then controlled by using a strong monochromatic filter to restrict the spectrum (thereby managing the chromatic aberrations) and stopping down well (to manage spherical).

The Schneider convertibles have one reasonably well corrected element that can be used on its own and the other element is used to balance out the design so that the combined lens performs really well (that is, you could think of it as giving you a really good performance combined and somewhat good performance using one of the cells, as opposed to getting three somewhat good performance focal lengths).

The Nikkors approach is to use interchangeable rear elements to provide good performance at all focal lengths (since you are always using combined lengths). What you lose is much of what makes the original convertibles attractive (less number of cells to carry, less expense etc).

Convertibles perform surprisingly well, but that is the idea - the performance is surprising but not really the best you can get, which is not surprising given the design constraints. In my experience, they work fine for contact printing or minor enlargements but as we migrated away from these requirements, they fall out of favor.

Cheers, DJ

Stephen Willard
5-Mar-2006, 18:18
I have the Nikkor T Series 360mm, 500mm, and 720mm lenses. I have used all three combinations extensively and have achieved excellent results. I have made enlargements up to 30x40 and 20x50 and have not been able to tell the difference between the T convertible lenses and other lens. They have produced very sharp images for me.

Clay Turtle
5-Mar-2006, 21:30
Thank you for your replies, I set up the tripod & camera & started checking the focus with different arrangements of len elements. Hmm . . . interesting, I wonder hmm?
I guess the thing that bothered me most is having the rear element removed leaves the shutter exposed within the bellows . . . but I had noted some strange effects noted in my exploring this avenue. I will let you know if anything works out of them. Using both front (500mm) with rear (300mm) produces aperture of 5.6 which is sharp focus but as I noted when I used the front (500mm) len (f12) seems soft & rather hard to focus. At f12, I guess I should accept it but I just can't help to think that a rear len could effectively control the softness. Oh well can't have everything so ya live with what ya got!

N Dhananjay
6-Mar-2006, 06:28
Just to clarify. With the Schenider convertible Symmars, you get the shorter focal length by using both elements and the longer focal length by using the rear element alone. So, on your 300/500, you get the 300mm fl by using the complete lens with both cells. You get the 500mm fl by removing the front element - remember to use the appropriate aperture scale (typically marked in green). If you use the front element alone (i.e., remove the back element), you should get a longer focal length but I got the feeling this element is not as well corrected as the back element for use alone. Might be useful if you wanted to do some exploration in soft focus stuff. Good luck, DJ

Clay Turtle
6-Mar-2006, 08:30
Well I 'll be! Hmm, I bought the lens online via EBay. The wording of the seller's dialog lead me to think that it is the rear lens that was removed? Oh, well I need to work with this lens (take photographs) to learn it (strong/weak suits) better. Thanks for the tip, I will give it a try and see what developes (haha).