Plasmats

[PBrooks]

Hello All, I have gotten quite a good education in the last couple of weeks reading past posts about lens design. As some of you may know I am trying to assemble a variety of focal lengths that cover my 14x17. My first lens purchased was a none marked 21" Ilex lenses, I say unmarked because there is nothing factory engraved on the front or rear element, the only reason I say 21" is a hand engraved 534.5mm on the barrel of the rear set of elements and the fact that Tim Sharkey said so. He also said that after further inspection of the lens that it is a 6 element plasmat. My second lens is a 355 G Claron so around 14", its also a plasmat.
Questions?
1. Are all plasmats convertible?

If so, Jim says the rear element of the G Claron should be around 26.5"

2. What do you guys think about the 21" Ilex? ( coverage on this thing is like a little over 800mm) Do I have another lens here or no.

Any help would be great, Thanks in advance to anyone who posts.
PBrooks

[Mark Sampson]

Well, yes, any plasmat should be convertible. Whether you like the results or not is another matter. The general idea is that the 'converted' lens is not as sharp as the complete lens in the corners, and is best used for portraiture. That was my experience with a Schneider (convertible) Symmar. Schneider advertised in the early '70s that they had eliminated the convertible feature when upgrading that design to the Symmar-S, in order to improve performance at the marked focal length. But that doesn't stop people from converting them... Since you'll be contact printing, these issues may be less important... you're just going to have to shoot some film and find out for yourself what's "good enough".
As far as the Ilex lens goes, just try it and see. Perhaps someone else has used another lens like it, and can tell us what it's like.

[sanking]

Hello All, I have gotten quite a good education in the last couple of weeks reading past posts about lens design. As some of you may know I am trying to assemble a variety of focal lengths that cover my 14x17. My first lens purchased was a none marked 21" Ilex lenses, I say unmarked because there is nothing factory engraved on the front or rear element, the only reason I say 21" is a hand engraved 534.5mm on the barrel of the rear set of elements and the fact that Tim Sharkey said so. He also said that after further inspection of the lens that it is a 6 element plasmat. My second lens is a 355 G Claron so around 14", its also a plasmat.
Questions?
1. Are all plasmats convertible?

If so, Jim says the rear element of the G Claron should be around 26.5"

2. What do you guys think about the 21" Ilex? ( coverage on this thing is like a little over 800mm) Do I have another lens here or no.

Any help would be great, Thanks in advance to anyone who posts.
PBrooks

Plasmat type lenses generally cover about 70-80 degrees. If you really have a 21" plasmat lens that is quite a find because it should cover 20X24", which your lens apparently does since you mention coverage of over 800mm. I was aware that Caltar made some triplet type lenses of this focal length that were placed in Ilex shutters but never heard of a plasmat this long.

All plasmat type lenses are convertible, but whether the performance is adequate even for contact printing is debatable. If you want to use such lenses in convertible mode take off the front element and the rear element should be about 1.7X the combined focal length. You will get better results by using an orange filter.

Sandy King

[PBrooks]

Thank ya'll for the input.

[Gordon Moat]

I am a little confused on the issue of a plasmat design being convertible. My Schneider lens works that way, but my one Nikkor does not. When I tried either the front or rear group alone on my Nikkor-W 180mm f5.6, all I got was a fuzzy image that would not focus at all. So is there something really different about Nikkor-W lens design, like maybe not being much like a traditional plasmat?

Ciao!

Gordon Moat
A G Studio

[Gene McCluney]

I am a little confused on the issue of a plasmat design being convertible. My Schneider lens works that way, but my one Nikkor does not. When I tried either the front or rear group alone on my Nikkor-W 180mm f5.6, all I got was a fuzzy image that would not focus at all. So is there something really different about Nikkor-W lens design, like maybe not being much like a traditional plasmat?

Ciao!

Gordon Moat
A G Studio

I experienced this also with my Nikkor-W 180mm f5.6. Very soft, very very soft.
I can only attribute this to the fact that while the lens is a Plasmat in number and placement of optical elements, it has been optimized way out of being symmetrical in performance.

There is no "law" that says a Plasmat has to be a convertable, especially if the manufacturer does not put dual aperture scales on it. Probably the older Symmar convertable lenses work better as convertables than the newer Symmar-S lenses do.