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Cleaning lens elements and de-centering / collimating?
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PostPosted: Sun Jul 25, 2010 10:13 am    Post subject: Cleaning lens elements and de-centering / collimating? Reply with quote

All,

I spent the past few hours reading about cleaning lens elements and many people advised against taking lens elements out of a group because they claimed the elements will be de-centered when replaced and the groups would need collimation.

Any truth to this? My goal is to clean an SMC Pentax 50/1.4 front & rear element and judging by the 50/2, the elements are placed into "holders" with rings keeping them in place. I'm confused how de-centering can occur since the elements fit cleanly into the holding assembly.

Would love to hear your insights on this!

Thanks,
autotak.


PostPosted: Sun Jul 25, 2010 1:36 pm    Post subject: Reply with quote

Many people are afraid to open lenses in general, and will come up with any excuse they can to avoid it. Neutral

How badly does it need cleaning?


PostPosted: Sun Jul 25, 2010 2:58 pm    Post subject: Reply with quote

Collimation is a concern primarily with elements cemented surface to surface when the mating surfaces are not flat. Imagine two bowls, one inside the other, how it is possible to tilt them in relation to each other while maintaining rounded surfaces in contact. Proper collimation makes the lips of the bowls (lenses) parallel. Modern lens designs use flat mating surfaces between group component lenses to avoid the labor intensive collimation step.

Your 50mm iirc has one cemented group; I don't know if the mating surfaces are flat -- check a lens diagram. If they are curved and get separated, collimation will be necessary on re-assembly to insure alignment.

You are correct -- camera lens rings and seats typically can hold collimation between individual elements within acceptable limits.


PostPosted: Mon Jul 26, 2010 2:04 am    Post subject: Re: Cleaning lens elements and de-centering / collimating? Reply with quote

autotak wrote:
All,

Any truth to this? My goal is to clean an SMC Pentax 50/1.4 front & rear element and judging by the 50/2, the elements are placed into "holders" with rings keeping them in place. I'm confused how de-centering can occur since the elements fit cleanly into the holding assembly.

autotak.


If you must open it for cleaning, then you must.

Here are some tips that will reduce the risk, if not remove it altogether, of misconfiguring the lens during reassembly:
1. Do not work in a dusty environment.
2. Use a paper and pencil and take note of each disassembly step. Better still, take photos.
3. Be sure to use the proper screw drivers and tools.
4. It is a good idea to work in an area where there is no chance of losing any bouncing screws or flying springs. You have a shoe box like enclosure and open the lens in it, for example.
5. When you disassemble the elements, draw clearly (photos can be confusing!), and with exaggerations (especially curves), how the elements were arranged. By drawing, you can depict the order and orientation of lenses quite nicely and without confusion.

Clean the elements and reassemble. With the above tips, it is not that difficult a process if you know your way around mechanical things (it is not easy either, but demands care and attention to detail).

Good luck!


PostPosted: Mon Jul 26, 2010 6:14 am    Post subject: Reply with quote

Thanks all for the tips. I've been playing with a junker SMC Pentax 50/2 and have learned some of the advice the "hard way".

If I get a lens wrench, is there any benefit in taping the end so if I do slip with it, I don't gouge the lens or delicate rings? I'm guessing the tape would have to be thick like duck tape or quality packing tape so the sharp edges can't poke through.

Thanks,
autotak.


PostPosted: Mon Jul 26, 2010 7:29 am    Post subject: Reply with quote

Might be better to tape the lens and the trim rings instead. If you put any tape on the end of the tool, you would probably just make it more likely to slip out. Wink


PostPosted: Mon Jul 26, 2010 8:13 am    Post subject: Reply with quote

OK,

Maybe this is a wild idea, but what about using liquid latex to coat the lens element area before using the lens spanner?

I would only worry if the latex is too thin, but it *sounds* like a good idea.

Thanks,
autotak


PostPosted: Tue Jul 27, 2010 12:52 pm    Post subject: Reply with quote

I don´t dink thats a wild idea.
I read such things from people who filed down the rear element of a lens mount, without dissmanteling. One used record cleaning solution (-> sensor film) for that.