Bosun Higgs
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I recently bought a lens that unexpectedly answered a couple of questions that I have had for a while.
The lens is a Super Rosskote 133mm, a projection lens that would have shown 35mm film in cinemas. I got the lens quite cheaply as it had some separation issues. Not long ago I dealt with another lens from this series , the 121mm, which had identical issues.
Although these lenses can show some typical separation artefacts, they mostly show a unique kind of milkiness in the cement, in this latest lens the front doublet had a large perfect ring of this and the rear doublet had a central circular area half the diameter of the lens.
As I knew from the previous Ross that the lens used synthetic cement, I put it straight into a methylene chloride mixture. All previous lenses have separated in 2-3 days, this one took nearly two weeks.
As I usually do not know what cement has been used, I usually heat the doublets first, if they are balsam, this will separate them quickly, if not, I just cool and drop them into the witches brew.
With this latest Ross, when it had not parted after five days in the brew, I took it out and heated it. The heating had no visible effect, I suspect the solution had already released a circumference around the doublet and softened the remainder, consequently, the cement was able to absorb the expansion of the glass.
I had wondered if heating synthetic cemented doublets before the chemical bath helped, it certainly increased the usual visible separation artifacts.
So the answer to the first question is, always heat the elements before chemical treatment, this physically disrupts the cement and allows quicker penetration of the methylene chloride.
The answer to the second question came when I was disassembling the lens for cleaning/separation. I noticed a strange screw inserted in the collar of the lens and realised I had seen this before.
A while ago I bought a large piece of projection apparatus that contained a partial projection lens that I later Frankensteined into a rather nice optic .
The mystery lens, top, and the Aperful, bottom. I used these two disparate parts a while ago to construct Frankenlens.
The front section of this lens was missing, and this must have borne the manufacturer's engraving so the identity of this lens was a mystery. Now I realise that this lens too, had the strange screw, and the unusual double threading on the front section is also a perfect match for my new Ross. To clinch the identification, both lenses have an additional fifth element, which is very unusual for a Petzval.
The screws in the new Ross, left, and the Frankenlens, right. Looks like we have a match!
The donor lens had been a Super Rosskote, and from the length of the empty front section, I think that it was probably a 150mm.
Both doublets from the new Ross are now fully recemented and the lens reassembled. I am always concerned that a radical procedure like recementing might affect IQ, but when I tested the Ross it gave excellent central sharpness.
Pretty good IQ, also note the in-focus twig far left, low field curvature for a Petzval - that fifth element seems to be doing its job!
So, it was quite satisfying to get these answers, but of course it is the lot of the antique lens adapter to never have the full story, so it got me wondering "what were those strange screws intended for?".
The screws are brass, countersunk, and penetrate through to the interior of the lens, they do not protrude at either end, and they do not secure anything.
The only thing I can think of is that they are a blanking screw for the hole that mounts an optional accessory. Projection lenses generally have an "industrial" construction and the Ross is on the heavier side of this, so it is hard to imagine attaching anything to a single hole this small.
I am totally mystified by the screws, but also quite grateful for their presence, as they allowed me to finally idenitify my partial lens.
The lens is a Super Rosskote 133mm, a projection lens that would have shown 35mm film in cinemas. I got the lens quite cheaply as it had some separation issues. Not long ago I dealt with another lens from this series , the 121mm, which had identical issues.
Although these lenses can show some typical separation artefacts, they mostly show a unique kind of milkiness in the cement, in this latest lens the front doublet had a large perfect ring of this and the rear doublet had a central circular area half the diameter of the lens.
As I knew from the previous Ross that the lens used synthetic cement, I put it straight into a methylene chloride mixture. All previous lenses have separated in 2-3 days, this one took nearly two weeks.
As I usually do not know what cement has been used, I usually heat the doublets first, if they are balsam, this will separate them quickly, if not, I just cool and drop them into the witches brew.
With this latest Ross, when it had not parted after five days in the brew, I took it out and heated it. The heating had no visible effect, I suspect the solution had already released a circumference around the doublet and softened the remainder, consequently, the cement was able to absorb the expansion of the glass.
I had wondered if heating synthetic cemented doublets before the chemical bath helped, it certainly increased the usual visible separation artifacts.
So the answer to the first question is, always heat the elements before chemical treatment, this physically disrupts the cement and allows quicker penetration of the methylene chloride.
The answer to the second question came when I was disassembling the lens for cleaning/separation. I noticed a strange screw inserted in the collar of the lens and realised I had seen this before.
A while ago I bought a large piece of projection apparatus that contained a partial projection lens that I later Frankensteined into a rather nice optic .
The mystery lens, top, and the Aperful, bottom. I used these two disparate parts a while ago to construct Frankenlens.
The front section of this lens was missing, and this must have borne the manufacturer's engraving so the identity of this lens was a mystery. Now I realise that this lens too, had the strange screw, and the unusual double threading on the front section is also a perfect match for my new Ross. To clinch the identification, both lenses have an additional fifth element, which is very unusual for a Petzval.
The screws in the new Ross, left, and the Frankenlens, right. Looks like we have a match!
The donor lens had been a Super Rosskote, and from the length of the empty front section, I think that it was probably a 150mm.
Both doublets from the new Ross are now fully recemented and the lens reassembled. I am always concerned that a radical procedure like recementing might affect IQ, but when I tested the Ross it gave excellent central sharpness.
Pretty good IQ, also note the in-focus twig far left, low field curvature for a Petzval - that fifth element seems to be doing its job!
So, it was quite satisfying to get these answers, but of course it is the lot of the antique lens adapter to never have the full story, so it got me wondering "what were those strange screws intended for?".
The screws are brass, countersunk, and penetrate through to the interior of the lens, they do not protrude at either end, and they do not secure anything.
The only thing I can think of is that they are a blanking screw for the hole that mounts an optional accessory. Projection lenses generally have an "industrial" construction and the Ross is on the heavier side of this, so it is hard to imagine attaching anything to a single hole this small.
I am totally mystified by the screws, but also quite grateful for their presence, as they allowed me to finally idenitify my partial lens.
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