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06-13-2010, 03:17 PM   #1
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yellowed lens or SMC coating

hi. i have a SMC PENTAX-M 1:1.7 50mm lens that appears clear when looked at head on, but when viewed from the side has a gold tint. is the color i am seeing the SMC coating or is it the yellowing of old pentax glass i see some people posting about? ive bought a fair number of k mount cameras from thrift stores and have some duplicate glass to sell, and if this lens is yellowed i want to be able to describe it correctly if i post it for sale. thanks!

btw, i did take several picutres with this lens, and the images turned out fine.

06-13-2010, 03:41 PM   #2
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Lens yellowing is really noticeable when looking directly into the lens itself. And ususally when shooting your images will have a warmish tone to them.

I have seen a number of angled shots of lenses with different coatings and I can definitely say that it'd be the SMC coating causing the Gold-glow ... I have seen it with my FA50 on different angles as well.
06-13-2010, 05:54 PM   #3
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Set it in bright sunlight for a couple of days. If the yellow goes away, it was yellowing.
06-13-2010, 07:08 PM   #4
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must be SMC

had the lens sitting in the windowsill most of the week, and no change, so it must be the SMC coating. ive never noticed such a pronounced color with my SMC Pentax A glass

06-14-2010, 04:08 AM   #5
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QuoteOriginally posted by mnseawa Quote
had the lens sitting in the windowsill most of the week, and no change, so it must be the SMC coating. ive never noticed such a pronounced color with my SMC Pentax A glass
SMC is a name that Pentax used for decades. But the coatings applied, changed over time. So the SMC on you9r M-lens is probably very different from the SMC on your later A-lenses.

Also, the yellowing of some lenses was to my knowledge limited to a few old Takumar lenses with Lanthanum glass. Later M-lenses don't use that type of glass and thus won't get yellow over time.

Ben
06-14-2010, 06:23 AM - 1 Like   #6
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It is also difficult to know who often and how much Pentax changed that formula over the years. Its unclear if those records exist. I also suspect that the formulation of the coatings may have been different for certain lenses as well.
06-14-2010, 06:50 AM   #7
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This is from another thread.
AFAIK K-Mount lenses are not subject to yellowing. You can't tell if a lens is yellowed just by looking at it as the coatings add some color to the lens. You can tell a lens is yellowed by mounting it to a camera and pointing it at a white surface. If it looks yellow instead of white it has yellowed.

QuoteQuote:
The following screw mount Takumars are known to yellow:

Part No. Model
--------- -------
43931 1:2/35mm Super-Takumar (49mm filter)
43932 1:2/35mm Super-Multi-Coated Takumar (49mm filter)
37800 1:1.4/50mm Super-Takumar
37801 1:1.4/50mm Super-Takumar
37802 1:1.4/50mm Super-Takumar
37902 1:1.4/50mm Super-Multi-Coated Takumar
37982 1:1.4/50mm SMC Takumar

The earliest 1:2/35mm Super-Takumar (part nos 368, 43680, with 67mm
filter) and 1:1.4/50mm Super-Takumar Model 1 (part nos 358, 35800)
with 8 elements) are NOT subject to yellowing.

The 1:1.4/50mm Super-Takumar Model 1 8-element lens can be easily identified by:
• Looking at the position of the red infra-red focusing mark which
lies between the f4 depth of field marker and the infinity mark.
On the later 7-element lenses the infra-red focusing mark lies
between the f8 and f4 depth of field markers.
• The protruding rear element which lacks the metal retaining ring
found on the later models.

There are many more 7-element lenses than the 8-element version.

No other screw mount Takumars are known to suffer from yellowing.

Last edited by boriscleto; 06-14-2010 at 06:55 AM.
06-14-2010, 02:13 PM   #8
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The reflection from the front coating varies, depending on shape of the front element, what it's reflecting and the coating formula. These photos are typical of what I get from the M50/1.7 and the M or A 50/2:



To check for yellowing, skip reflections. Look through the lens at a white piece of paper. Yellowing will be visible, and obvious when compared to a similar non-yellowed lens.

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