Radioactive/Yellowing K-mount SMC Pentax?

Steve Roberts · Oct 11, 2016

Hi,
Whilst swapping lenses between cameras a few days ago I noticed that my 50mm SMC Pentax (non-M) consistently required half a stop more exposure than either of two 50mm SMC Pentax-M lenses. This was consistent with various bodies and only when I placed the lenses on a piece of white paper did the reason become clear (no pun intended!). The 50mm non-M lens displayed a very obvious yellow tint which was not present on the two M versions. I had previously only known of the radioactive glass/yelowing issue affecting screw mount Pentax lenses, but it would seem this is not the case. The lens came with the KX that I bought a little over 30 years ago. The KM/KX range were in production for only a couple of years between the screw mount range and the M series, though the K1000 continued for long after and the K2/DMD for several years. Has anyone any knowledge of this effect as regards K-mount lenses?

Best wishes,

Steve

Paul Howell · Oct 11, 2016

I thought the M 42 50 1.4 had thorium elements, but that the K mounts had a different design, if it is a thorium lens leave it in sun light for a day or so and it will clear.

Chrismat · Oct 11, 2016

If you're going to leave it in the sun wrap rest of the lens in aluminum foil so the interior of the lens does not over heat.

ciniframe · Oct 11, 2016

Use it for B&W, no yellow filter required! It's built in!
Sorry, not really helpful but I couldn't resist.

pentaxuser · Oct 11, 2016

Chrismat said:
If you're going to leave it in the sun wrap rest of the lens in aluminum foil so the interior of the lens does not over heat.

At this time of year in Tavistock, Devon, England I wonder if overheating is likely to be a problem or even if the sun's angle gives sufficient UV to have any effect?

pentaxuser

Svenedin · Oct 11, 2016

pentaxuser said:
At this time of year in Tavistock, Devon, England I wonder if overheating is likely to be a problem or even if the sun's angle gives sufficient UV to have any effect?

pentaxuser

I don't know how much UV it needs but I made a cyanotype (needs UV) in Southern England last week at it was done in 10 minutes.

Kirks518 · Oct 11, 2016

I did this with a couple of the Super-taks that were yellow. I did wrap them in foil, more to avoid whatever was below it from getting fried then anything else. The lens will get warm (hot) on occasion, but they all seemed to weather it quite well. It did take quite a while for the yellowing to go away, so don't expect to use the lens for a bit. In the Florida sun, I left them for about 6 months.

Theo Sulphate · Oct 11, 2016

Oh... it takes 6 months to clear the lens even in Florida sun... that long? My 50/1.4 might never clear then because it may take 12 years to get the equivalent amount of sun here.

Yeah... built-in yellow filter.

Maybe take it to a tanning booth and tell them to turn it up to 11?

Kirks518 · Oct 11, 2016

TBH, it may have taken less time, but I didn't really pay attention, and essentially forgot about the lenses. I had them on a south-facing window sill behind curtains, so they weren't 'in my face'.

To some degree, I like the yellowing, it gives a warm look to the image.

Poisson Du Jour · Oct 11, 2016

Yellow Pentax lenses are not confined to the M-mounts.
Early Takumar lenses and right-angle finders for the Pentax 6x7 / 67 system featured elemental thorium as a glass tint until technology was sufficiently developed to provide high quality multicoating. There is no harm in the yellowing, but there is a lot of harm being asked for in leaving a lens in the hot sun. This is probably the biggest shibboleth to ever do the rounds: "put the lens in the hot sun to neutralise the yellow". It will not. This "treatment" does however effectively soften the cement binding the elements, causing slip. Neither Pentax nor any repair facility has made any recommendation about "treating" yellowed lenses. Leave them as they are as a novelty rathe than something that must be fervishly dealth with no matter what.

Kirks518 · Oct 11, 2016

I have to disagree with you, Poisson, that the yellowing doesn't fade/disappear. I've done it with 3 Super-tak 1.4's, and the yellowing did go away. I'm sure it will come back in 20 or 30 yrs, but for now it's gone. I'd take a before/after photo, but I've sold them all.

Chrismat · Oct 11, 2016

I had a yellow tak that only took about a week for the yellow to clear, and I live in Maine. I wrapped the lens in foil (but not the affected lens, of course) faced the yellowed lens toward the sun for a about an 1-2 hour each day, that did it. Where I lived at the time, the angle of the sun that was most direct in helping the lens lasted only 1-2 hours.

Paul Howell · Oct 11, 2016

I have a 35mm F2 in M42 that cleared in just a couple of days, but I live in AZ. Or by a UV lamp.

ransel · Oct 11, 2016

I thought Dead Link Removed for clearing the yellow's...

Steve Roberts · Oct 11, 2016

Thanks for the comments. I knew about the sunshine treatment and successfully cured a Super-Takumar f1.4 a while back. I wrapped it in foil and left it in the April sunshine for a week and the yellow was gone. The nature of the sun here is that it doesn't often get hot enough for long enough to bother the lens! Pentaxuser's observation is probably very close to the mark and I may have to wait until next spring before we see any sunshine worth having! My post wasn't intended so much as being about the cure for the problem as about the fact that I believed it only affected screw mount Pentax lenses. A couple of replies above stack up with my experience. I may try to photograph the yellow cast before I give it the UV/sunshine treatment.
Best wishes,
Steve

Gerald C Koch · Oct 11, 2016

You really can't just leave it on a windowsill indoors and expect any fast clearing. Ordinary glass absorbs quite a bit of UV radiation. You need to take it outside. Covering the front element with a piece of saran wrap will protect it from air blown crud. Remember to take it in before it rains.

faurefan · Oct 11, 2016

I used an Ultraviolet LED flashlight (about 15 bucks) on my thorium Canon FD 35mm f2.0, 24 hours on the front and 24 hours on the back and it cleared the yellow out entirely.

I think the brand was "Blacklight" but there I'm sure there are others.

The lens needs a redo every six months or so, but this was far more effective than sunlight, the various Ikea lamps recommended etcetera.

MikeDahlke · Oct 15, 2016

I used a cheap IKEA led lamp for about 24 hours and cleared 95% of the yellowing off a 1.4/50 super tak. Apparently it just needs energy to clear, not necessarily UV light. I'm no expert on this, I just followed the instructions from a YouTube video and it worked pretty well.

I tried the sunlight and foil method at first for a bit but I wasn't comfortable with how much the lens body was heating up.

Radioactive/Yellowing K-mount SMC Pentax?

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