Hey! My Takumar actually IS yellow...

I'd heard rumors before that Super Takumars (or maybe it was just the 50/1.4?) go yellow sometimes with age, but I always thought the rumor was just a product of people being confused by seeing a yellow lens coating rather than a purple one.

But, in the wake of my last 50/1.4 being stolen last spring, I've just got a new one, and looking through it it looks like I've got a yellow filter on the front.

So how's that going to effect my photography? Like a yellow filter? Including having to factor in the filter factor, I suppose, as I haven't got a TTL on any of my m42s? Anyone know what exactly causes this little phenomenon?

I believe it is radiation from a thorium element in the lens that turns it yellow. I also believe you can effect a (relatively) short-term "cure" by putting the lens in direct sunlight for several days. Over time the yellow would come back, because of continuous radioactive decay of the thorium in the glass.

If my memory is correct and this is indeed the source of the yellow color, just don't sleep with the lens between your thighs or you'll have funny-looking children. Otherwise the radioactive output is weak enough your desk drawer or closet door or whatever else you keep between you and the lens is enough to block the radiation.

Can't vouch for the degree of accuracy here, it's the internet after all:

http://www.pentax-manuals.com/repairs/yellow.htm

Lee

I used a strong UV lightsource on both my 35mm 2.0 and 50mm 1.4 Super-Takumars. The yellow color was gone after one day´s treatment. This was at least two years ago, and the lenses still look good to my eye.

Trond

I had some yellowing on a SMC takumar 50mm f1.4, and what I did was to wrap it in aluminium foil, uncovering only one side of the lens, and leaving it on a windowsill for a few days. Somewhere I've read that the foil supposedly bounced the UV rays back, thus accelerating the cure, but who knows. At any rate the yellowing was gone after a few days.

Yeah, mine is too. I use it only for B&W anyway, so I didn't bother with the sunlight treatment. Rough guesstimate say that it knocks about 1/3 to 1/2 stop worth of the light passing through it.

Luckily mines in a new home, it fell out of my backpack halfway up the Glyders 18 years ago. (Welsh mountains)

Ian

Ian Grant said:

Luckily mines in a new home, it fell out of my backpack halfway up the Glyders 18 years ago. (Welsh mountains)

Ian

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Not necessarily Ian-you may have lost a magnificent lens with a built in yellow filter. Among my lenses is a 35 f2 which is one of the sharpest 35's I've ever used,
Mark

TheFlyingCamera said:

I believe it is radiation from a thorium element in the lens that turns it yellow. I also believe you can effect a (relatively) short-term "cure" by putting the lens in direct sunlight for several days. Over time the yellow would come back, because of continuous radioactive decay of the thorium in the glass.

If my memory is correct and this is indeed the source of the yellow color, just don't sleep with the lens between your thighs or you'll have funny-looking children. Otherwise the radioactive output is weak enough your desk drawer or closet door or whatever else you keep between you and the lens is enough to block the radiation.

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Or funnier looking children, whichever the case may be.

Some of the FSU lenses have the same issue, supposedly because of lanthanum. I would think that if shooting black and white the 'problem' may be worth retaining for contrast.

my Chinon f1.4 50mm lens that only recently replaced it is far better anyway.

Actually I had a great pair of Chinon's and an f1.4 lens until they were stolen and I went back to Pentax equipment

Mark Layne said:

Not necessarily Ian-you may have lost a magnificent lens with a built in yellow filter. Among my lenses is a 35 f2 which is one of the sharpest 35's I've ever used,
Mark

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Thorium, Bruce, not Lanthanum. See http://home.earthlink.net/~michaelbriggs/aeroektar/aeroektar.html

Another misconception that just won't die.

Dan Fromm said:

Thorium, Bruce, not Lanthanum. See http://home.earthlink.net/~michaelbriggs/aeroektar/aeroektar.html

Another misconception that just won't die.

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The Briggs piece is brilliant but deals only with Aero-Ektars. There is a later generation of 'hot' glass where the radioactive component is (according to Dr. A. Neill Wright, one of the authors of the original BJP paper) the result of using a radioactive salt to bleach ferric and ferrous salts to colourless ferrates; these lenses, including several Takumars, are primarily alpha particle emitters.

At least, that is my recollection, but on checking some past correspondence with Michael Briggs I find that he disagrees. As both he and Neill know vastly more about this than I, and base their observations upon actual research, all I can do is step back and provide both points of view. I had forgotten, but apparently it was a piece I wrote in Shutterbug, about Neill's observations, that got Mr. Briggs interested in 'hot' lenses. I've always said that a good teacher wants his pupils to be better than he is, so I must have been a good teacher on this occasion!

I once got a convincing autoradiograph from standing an Aero-Ektar on a sheet of Polaroid -- in less than 24 hours, as I recall, though I've forgotten now (it was a very long time ago). Neill also found that some Apo-Lanthars are 'hot' and others aren't, which is quite intriguing.