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Ethostech
Tinkerer
Username: Ethostech
Post Number: 125
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, August 19, 2009 - 07:09 am: | 
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I have a Sigma F8 500mm mirror lens (which I believe was marketed in USA under the name Spirotone). It is cosmetically mint and functionally perfect. Through the front end I can see that the primary mirror at the lens-mount is in perfect condition. Similarly - through the lens mount I can see that the secondary mirror as underside the central disc of the front-end glass, is also perfect.
However - there is an apparently flat glass as about half an inch beneath the front glass which carries the secondary mirror disc and it has some wipe marks which need cleaning off. At this time I cannot judge whether the marks are on its front face or its underside - but the facts will emerge once I can find a way into this monster.
Over the years I have variously stripped and rebuilt dozens of lenses - including some pretty tricky zooms - but this mirror-lens has me challenged. I have a nasty suspicion that it is all put together from the lens-mount end but I am not about to be reckless in following such an assumption. Better to ask those who have been there before. Any contributions whould be most welcomed.
Stuart Willis - from downunder. |
Rick_oleson
Tinkerer
Username: Rick_oleson
Post Number: 969
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, August 19, 2009 - 08:30 pm: |
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There is a surprising amount of variety in cat lens design. I've had 5 of them, 3 of which I still have... I've only taken one apart, and that was just the very basic separation of the front and rear halves. Generally, mirrors have a simple single helical - essentially a front-element-focusing arrangement. On the one that I separated, the infinity and close focus stops were screws accessible from the outside of the focusing ring, and after pulling those out the front end just unscrewed. This exposes the primary mirror in the rear half, and the rear surface of the front corrector plate and the secondary mirror in the front half. I would not disassemble any further than this unless absolutely necessary, I expect that centering is pretty critical with all of the folds in the light path. I've seen some pretty nice performing mirrors and some really awful ones ... I wouldn't want to inadvertently turn one of the former into one of the latter. |
Ethostech
Tinkerer
Username: Ethostech
Post Number: 126
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, August 19, 2009 - 09:22 pm: |
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Thankyou Rick. There are some good leads within your reply - for which I thank you. I am sure you are absolutely right re the critical centering and that is why I was reluctant to access via the lens-mount end. I will remove all the rubber ring-grips and hope to find the focus-stop screw .. and proceed from there.
I note that the focusing ring "could" have had on its rear edge a simple engraved line as a focusing index but for some reason that indexer is a prominent cone which stands 6mm high and topped by a red spot. That's a complicated production expense if it is solely the focusing index so I figure it may in fact be the topside of the focusing-stop. It has no flats so would be easy to chew up in removal. Does this ring any bells with you ?
When I have done this job I will write an article for the Forum so that others might benefit.
Best
Stuart |
Rick_oleson
Tinkerer
Username: Rick_oleson
Post Number: 970
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, August 20, 2009 - 03:07 am: |
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My guess is that they just felt like making a big focusing index... I don't know though. I'd look under the rubber grip first. |
Ethostech
Tinkerer
Username: Ethostech
Post Number: 129
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, August 20, 2009 - 07:05 pm: |
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Yes. I suspect you are right.
Beneath the focusing rubber grip-sleeve I found a row of holes each 1cm apart and for the entire circumference. The focusing ring moves a full revolution from closest focus to infinity - and in one of those holes was a screw. Through the adjacent holes I could see the focusing ring projection as it approached and stopped. So disassembly was all plain sailing from that point.
It emerged that the grime/smears were in fact under the glass upon which the secondary disc mirror is mounted, so with some patience, Windex and cotton-buds I was able to restore that to crystal clarity.
I have still not worked out how one disassembles the extreme front plain glass - but it doesn't matter because the grime/smears were not in the airspace between the two.
All is now back together properly. I just have to remove the slotted screw to remove the rear element because of slight surface fungus on its underside .. and then replace the rubber grips. (I had to remove all the rubber grips because of differing diameters which would have triggered stretch during removal.)
It's been an interesting excursion and your initial contribution to my post was a great guideline and invaluable.
If anyone reading this thread happens to have a Nikon mount for this lens - I would be happy to buy it or, alternatively swap it for the present Pentax K Mount.
Right. Now to complete the job, mount on a Pentax K1000 on tripod and see if I can get some stunning girlie pictures on the beach. Well - maybe some birds in my garden :-))
Thankyou again Rick.
Best
Stuart |
Ethostech
Tinkerer
Username: Ethostech
Post Number: 131
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, August 21, 2009 - 07:12 am: |
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All is done now Rick and the lens is now as perfect as it gets.
Mostly I am incisive and intuitive with camera stuff - but sometimes totally blind and dumb ! Can you think of any logical reason why Sigma made the lens-mount rotatable and lockable by a milled thumbscrew? The mount is already secured by four set screws which ride in grooves and if not screwed up hard, enable the mount to rotate and then locked with the thumbscrew. But if those set screws are fully tightened at a preferred indexing point they make the thunbscrew totally redundant.
Either I need to eat more Shushi brain-food :-)
Or- Sigma does !
Stuart |
Rick_oleson
Tinkerer
Username: Rick_oleson
Post Number: 971
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, August 21, 2009 - 03:18 pm: |
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Not off hand ... does the lens have a tripod socket on it? If so, the rotating mount might be for vertical shots. Other than that I can't think of a reason for it.
Hope you enjoy the lens! Let me know how well it performs, I've developed a sort of low-budget interest in these. I did some comparison shots of the moon with 3 lenses on my Flickr page, but they don't tell the whole story: my Tokina 500/8 came in last but it's a very handy lens and I use it more than the winners. |
Ethostech
Tinkerer
Username: Ethostech
Post Number: 133
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, August 23, 2009 - 02:50 am: |
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Yes - I thought about the rotating mount in connection with vertical format shots but it didn't make much sense unless Sigma doesn't recognise that all modern tripods have the head capacity to achieve such. Regardless - yours is the only possible explanation. I will indeed report back to you on the performance of this lens. I am a Nikon F2 man but I have a mint condition Pentax ME Super to which I have now married this K-Mount Sigma Mirror Lens. The latter camera is an excellent companion in that it meters the lens in either AUTO or MANUAL SETTINGS. 400 ASA with a max aperture of F8 is totally useable in decent ambient light but the lens is not something one can whip out of one's gadget bag and snap away. Lots of care and a tripod needed.
If you send your email address to me at mailto:[email protected] I will send you a pic or two.
(I never post my regular ISP email address on any web forum and neither do I expect you to do so. The Spambots are definitely harvesting from Henry's Forum. I have proved it so many times.
Best
Stuart |
Glenn
Tinkerer
Username: Glenn
Post Number: 646
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, August 23, 2009 - 03:37 pm: |
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You have completely misunderstood the rational for a rotating tripod mount on a long lens, a technicality that clearly Sigma does.
If you change between vertical and horizontal formats using a tripod head, the optical axis of the lens moves in an arc across the subject. Clearly for technical photography this will mean that image centering will differ between the formats, unless the lens/camera are re-aligned on the center of the original subject field.
With the rotating tripod socket, the optical center remains 'locked' on the center of the subject as formats are changed.
This facility may not appear important to the casual user of long lenses, but to technical photographers it can be most useful. I have used both 500mm and 600mm cat lenses to record evidence/areas of damage in severely fire damaged buildings, so structurally unsafe that access to the areas was impossible by any other means. In cases such as this centering of the subject is important, especially so if you need to change formats to include all evidence. |
Petercat
Tinkerer
Username: Petercat
Post Number: 91
Registered: 01-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, August 28, 2009 - 07:08 am: |
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The rotating tripod mount is also very useful with monopods and shoulder stocks, not to mention the various clamp-on mounts for use on fences, auto door glass, etc. |
Rick_oleson
Tinkerer
Username: Rick_oleson
Post Number: 975
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, August 31, 2009 - 03:34 am: |
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I have a modern tripod (a mid-1990s Slik 444)whose head does not have the capacity to achieve such. My 500/6.3 Maksutov mirror lens has 2 tripod sockets at 90 degrees from each other (actually it has 4 - both 1/4" and 3/8" at each location) and the other lenses that I have with tripod sockets have them on rotating collars for this purpose. |
