Fixing yellowed lenses

[Photopathe]

Fixing yellowed lenses:
Is it possible to get back the original speed of the lens ? I have used UV light to eliminate the yellowing on a Takumar 50mm 1.4 (7 elements). The yellow tint is gone and I got back some speed but not all of it. It is still a stop slower than it should be. I keep exposing the lens to UV light but no success. I tried the sun as well as 2 different UV bulbs. Is it normal not to be able to get back the original speed? Thanks!

 

[Nitroplait]

Fixing yellowed lenses:
Is it normal not to be able to get back the original speed? Thanks!

Probably not more than what you have achieved by now.
Maybe some commercial service can improve transmission further but keep in mind that F1.4 is not and will never become T1.4 - in other words; the F 1.4 is a calculated value, not actual measurement of light transmission - like it is the case with cine lenses.

 

[Donald Qualls]

I agree, if it's visually cleared, it's as good as it's going to get. I've been okay with my Super Takumar 50/1.4 on the rare occasions when I've shot it wide open, using the calculated geometric f stop for exposure. It's only about 1/3 stop absorption, once the yellowing is cleared, so unless you're shooting slides, film latitude will cover you.

 

[Photopathe]

One thing I did is compare it with other 50mm lenses at f8 with an adaptor on a digital camera. I do use those lenses with film as well but for convenience I used the mirrorless to test. After removing the yellowing, the takumar is still exactly 1 stop slower than the other lenses at the same aperture (under controlled light conditions). Does it mean it was probably like that even when it was brand new or the thorium degradation made it so even with the yellow tint gone? To my knowledge the 8 elements 50mm 1.4 Takumar doesn't have thorium. I am thinking of switching. I am fine with the tolerance for calculating exposure and so on but my concern is that I would like to have one very fast lens that would be close to a real f1.4...

 

[NB23]

One thing I did is compare it with other 50mm lenses at f8 with an adaptor on a digital camera. I do use those lenses with film as well but for convenience I used the mirrorless to test. After removing the yellowing, the takumar is still exactly 1 stop slower than the other lenses at the same aperture (under controlled light conditions). Does it mean it was probably like that even when it was brand new or the thorium degradation made it so even with the yellow tint gone? To my knowledge the 8 elements 50mm 1.4 Takumar doesn't have thorium. I am thinking of switching. I am fine with the tolerance for calculating exposure and so on but my concern is that I would like to have one very fast lens that would be close to a real f1.4...

You still get the depth of field (bokeh) of f1.4, regardless of the actual shutter speed. If you like how the lens draws, shutter speed is secondary.

Might be food for thought in your case.

 

[Donald Qualls]

One question that comes to mind (which I haven't really examined in my Super Takuma 50/1.4, which does have a thoriated element) is whether the yellowing turns to a gray neutral density with the UV treatment. Since there haven't been any of those lenses in new condition in half a century, it may not be possible to be sure, though disassembly to compare the thoriated element against the others (which I'm not going to do with mine) might be a way to check.

 

[Donald Qualls]

compare it with other 50mm lenses at f8 with an adaptor on a digital camera.

If this is just T factor (lots of glass in one of those), losing 50% of the light at f/1.4 means you'll still lose 50% at f/8. I wouldn't expect a full stop of correction on this, however.

 

[xkaes]

It's probably easy to find the lens "un-yellowed" -- one that's been unused and kept in the case all its life. I have an early Minolta Rokkor 58mm f1.2 that is subject to yellowing, but I don't see it in mine. OK, maybe a little, but not enough to worry about.
If the yellowing is gone -- even if it does turn to ND -- the TTL meter will take care of it.

 

[Donald Qualls]

It's probably easy to find the lens "un-yellowed" -- one that's been unused and kept in the case all its life.

Given that the yellowing is believed to be due to radioactive decay of the thorium dopant, such a lens will be at least as yellow as one that's been used regularly. Probably darker, since it won't have received any UV to offset the discoloration.

 

[albada]

You might wish to keep the yellow.
B&W film is too sensitive to blue, and yellow cuts blue enough to match human perception of the spectrum, darkening skies to match how we see them.
For color film, yellow counteracts the excess blue in open shade.