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Ezio
Tinkerer
Username: Ezio
Post Number: 18
Registered: 09-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, April 18, 2009 - 01:06 am: | 
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I've a pre-war uncoated Sonnar 1:1.5 with what I assumed was oil on the inner surface of the rear element. Dismantling this was intuitive enough, but what looks like a thiny veil with an irregular central clear surface cannot be cleared. Thus, the problem clearly lies between the cemented element.
Shall I asssume this is separation, or is it possible for oil to seep between cemented elements? If the former, shall I perhaps try the baking trick at 200° that has been recommended by some in this forum? Thanks |
Gez
Tinkerer
Username: Gez
Post Number: 68
Registered: 09-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, April 18, 2009 - 03:46 am: |
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If the elements have been cemented with Canada Balsam, most likely considering the age of the lens, then soaking in acetone should readily separated them.
If a stronger cement has been used try soaking for a few days in dichloromethane solvent. I have found both approaches work well with cemented eyepieces, so I haven't tried it, yet, with expensive optics.
Of course once the doublet has been separated the problem remains of re-cementing and re-centering the elements. Probably fresh Canada Balsam is the easier option, but I can't give any advice on accurately re-centering the glasses. |
Rick_oleson
Tinkerer
Username: Rick_oleson
Post Number: 863
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, April 18, 2009 - 07:34 pm: |
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This is common in postwar Sonnars from the late 50s, very uncommon in prewar examples. Separation usually looks like a reflective rainbow-patterned area in the lens. Its effect is largely cosmetic, it increases flare a bit but does not affect sharpness. Decementing entails a risk that the lens may be poorly centered after reassembly, which will have worse effects than the separation. If you want an experimental project in lens cementing this is a good opportunity ... if you want a usable Sonnar you might be better off leaving it as is. |
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