Originally posted by lensnut Not quite so scientifically complicated is the manner of clearing the yellowing/ browning. Ultra violet from the sun will do it and leave your lens crystal clear again. Wrapping foil round it reduces the amount of radiant heat hitting the black surfaces but also reflects the light back out again, giving double the action! Here is my f1.4 Tak in the kitchen window.
Yes, that will work, though many of us have had good results using the JANSJÖ LED desk lamp from IKEA. The JANSJÖ can clear a lens in less than two days rather than the week or more often required with direct sunlight.* In addition, the JANSJÖ does not subject the lens to the risk of excessive heat (causes oil/grease migration).
JANSJÖ LED work lamp - black - IKEA
I have cleared five badly yellowed thoriated lenses using the JANSJÖ and highly recommend it or something similar.
The method goes like this:
- Remove both front and rear lens caps
- Set aperture wide-open
- Unless sure that the yellowed elements are toward the front of the lens, cover the front element in aluminum foil, shiny side toward the glass
- Position the head of the JANSJÖ a few millimeters from the rear element (this is where the flexible "neck" comes in handy)
- Evaluate after 24 hours
Evaluate by looking through the lens at a sheet of white paper in good light. The paper should appear quite white when viewed through a new or properly cleared lens.
Steve
* UV light is often a part of the discussion, but make take from multiple sources related to radiation-induced "color centers" is that light energy is more important than wavelength
per se and that visible spectrum white light works nicely. Heat may also be used if the glass is being treated by itself.