Lens separation repairs?

[cooltouch]

I have a Tamron 500mm mirror lens that appears to have a separation between the secondary mirror and an element immediately behind it.

Dead Link Removed

In the above image, the separation appears to be between two of the elements in the group on the left side, either between the first (which is the secondary mirror) and second, or the second and third.

I've dismantled the lens and even with this group out and holding it in my hand, it's difficult to tell where the separation is occurring. I've removed a shroud that surrounds the third element and can detect no crack, gap, or opening between the second and third element. I've even tried dripping a bit of alcohol along the seam and there appears to be no penetration, so I'm figuring that the separation is most likely between the first and second elements.

Problem is there's a plastic cap covering the secondary mirror, which actually protrudes from the front of the lens. And I've tried everything I can think of -- short of a pair of pliers, or heat -- to get that cap off. I don't want to break it, obviously.

This lens uses rear-silvered mirrors, so the interface between the elements where the delamination is occurring won't be subject to desilvering.

I've never tried recementing a lens, but it might be something I'd be willing to try. I mean the lens is useless the way it is, so I don't have anything to lose. But if I can find a service that will recement it for a reasonable sum, I'd also be willing to consider this route. Provided they'll be able to remove that plastic cap without breaking it.

So, if you were in my shoes, what would you do?

 

[ChuckP]

I have no ideas about fixing it yourself but you could ask here: http://www.focalpointlens.com/fp_intro.html about the cost of them doing it. Assuming they will handle it. Probably will be cheaper to buy another lens.

 

[lxdude]

I think you're better off replacing it than trying to fix it yourself. Tamron says it's a 7 element/4 group design, meaning the element which also constitutes the secondary mirror is bonded to the large element. Even if you get that cap off, which is probably glued on, I doubt you can adequately recenter the element.

You could see about getting it repaired, but I think it will be less expensive to replace it. My Tamron 500/8 (first model, 55B) cost me about $115 with case, caps and filter set complete, in beautiful shape. Hardly more than those lousy Phoenix, etc., ones! An equivalent 55B from KEH is going to be less than 200 bucks.

 

[ic-racer]

How bad are your negatives?

I know of no web page or forum post that shows before and after images that demonstrate improvement in prints after lens separation repair. I'm not saying it won't do any good, but if it does, I have not found any pictures on the internet that show the improvement (of course if someone has some links, or before-after images, that would be great to share).

Personally I have been using a severely separated 90mm LF lens for about 6 months now and have yet to detect anything in the negatives that would cause me to get a different lens.

 

[John Koehrer]

I'd agree with lxdude about replacing rather than repairing the thing.
If you get in touch with focalpoint sit down before you check the price. You may be a little shocked. Reassembling these things properly isn't all that simple.
I've recemented some simple lenses with OK results but used the lens housings for aligning the groups. It was OK but likely far from perfect.(didn't notice any degradation of quality) but I'm not that critical.
My byword tends to be " close enough"

 

[cooltouch]

Thanks for the feedback, guys. I have some comparison photos. This is a favorite scene of mine for testing long telephotos -- a water kiosk about 400 meters down the street from me. I focus on the small writing on the kiosk's sign.

And a comparison at 100% crops between the Tamron 500mm f/8 and an old Century Precision Optics 500mm f/5.6 T-mount refractor set to f/8:

I used my DSLR's Live View function to insure focus accuracy and had the camera's self-timer set to 10 seconds to quieten any vibrations.

Here's a pic of the front group, showing the rainbow "smear" that indicates the element separation:

Lxdude, I wish I could find one of these lenses for $115. About the cheapest I see them going for on feeBay is close to $300. KEH is a good idea -- all too often I forget about that place. Just checked -- they have a BGN for $150. A good price. Maybe I should wait for an UG with good front glass, and combine the two.

Yeah, I figure even if I can get to the element and recement it, recentering it will be an issue. I've wondered about thinning and then injecting a glue with an insulin syringe (very fine needle), then clamping the elements until the glue dries. In other words, not removing the element at all. Of course, if this doesn't get rid of the rainbow, then it will have been for nothing.

 

[Ian Grant]

Over on the Large Format Photography Forum there's been threads of successful repairs where lenses have separated, both old balsam cement and modern light cured compound. Go search and read, the posts are from people I trust.

I have a Rodenstok lens with separation, and when an errant Italian APUG member eventually returns it I'll do my own re-cementing. Nothing lost if it fails but everything to regain.

Ian

 

[lxdude]

I just checked ebay and was amazed to see the prices, and almost all were BIN. I got mine off there a couple of years ago; it took a little while to find the one I wanted to bid on, but they were generally going for less than $150 regardless.The 55BB's were maybe 20% more than the 55B's. I did get a good deal, but part of that was there were usually a couple or three up at auction at the same time. What made it a really good deal was that it came with the case and filters, and showed little use. It's a great lens. I'd pay $200 for it today, no sweat, if I didn't have mine.

Good luck with it. I suggest, so to not overlook anything, that you talk to a few repairers. Maybe email them the picture. You never know-maybe you'll find a repair charge which is within reason. Tamron has a list of official repairers on their website, too.

 

[ic-racer]

Thanks for posting those pictures. I suspect that since you cannot stop down like a LF lens the effects are quite prominent. If you go ahead and tackle this post pictures please.