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12-21-2012, 07:19 AM   #1
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Dust in used lenses ?

Hello,
Santa is brining me a new K-30 for Christmas and I'm looking for some used lenses. I have noticed that a lot of lenses advertised say " light dust" or "small spec of dust". Is this something I should be concerned about? Or does it matter where the "spec" is? Almost all of the 55-300's I have looked at say this. Is this lens prone to dust? Any feedback on buying used lenses over the Internet is greatly appreciated!
Thanks ~~ Yale


Last edited by Heelers; 12-21-2012 at 07:20 AM. Reason: Spelling
12-21-2012, 07:48 AM   #2
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What happens is there are people who are pixel peepers/camera weenies that look for perfection in their used devices - especially in their *used* purchases. They want a used purchase to be such a great deal that it eclipses a new one. By telling people there is some dust or other insignificant thing, it protects the seller from having to deal with one of these weenies.
12-21-2012, 07:54 AM - 1 Like   #3
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I find that the closer the dust is to the sensor, the greater the effect. However, in most cases it takes a very light image (sky shot) at a very narrow aperture (f22) to make it show up on the final image. A quick clone in PP and it's gone. Old manual prime lenses can be great fun and are easy to clean. They can be bargains if you choose carefully.
12-21-2012, 07:54 AM   #4
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So, for the most part, look at the lens, see if it's a big deal to you. In my experiences buying over the internet (here and eBay) usually goes pretty well. Lenses are pretty stable over the course of their lifetime. Older lenses might have fungus or serious dust, but that's rare. A rule of thumb for me is 50-60% of retail, and I will buy used and take the risk of getting a lemon. If you are not comfortable buying used, then you answered your own question.

12-21-2012, 08:10 AM   #5
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Some suggestions for lenses: Pentax K-mount "A" or "M" type in 28 and 50 mm sizes. Check the database reviews before you buy. You should be able to find lots of these fairly cheap. Let us know how it works out.
12-21-2012, 08:40 AM - 1 Like   #6
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I started using Pentax in 1968 with a spotmatic and over the course of the next ten years I managed to acquire 16 genuine Pentax- Takumar, Super Takumar and Tele Takumar prime lenses, which I still use very commonly today. (In those days no reputable photographer would be caugh dead with a zoom lens in his/her kit, zooms were strictly for amateurs)

Quite often when someone is examining one of those old lenses they complain that their is dust inside the front element and I laugh. Pentax explained that very carefully back in the 60's. The old vintage Takuma lenses were made with what Pentax called "Rare Earth Glass". The rare earth glass was much harder than ordinary optical glass so it allowed Pentax to grind and polish some rather intricate element shapes which could not be duplicated by other optical companies. During the late 60's and early 70's it was a continual year to year competition between Pentax Takumar lenses and Nikon Nikor lenses for who had the best lens quality and more often than not Pentax won the day,,,however, even when they were brand new, if you looked closely at the lenses you could see what appeared to be dust on the inside of the lead element. According to Pentax, what appeared to be dust was actually microfine bubbles in the glass. The bubbles resulted from the glass making process because the rare earth glass formula produced a melting temp for the glass which was about 500degF hotter than normal optical glass melting point. The excessive heat resulted in causing some of the glass chemicals to actually gas off while it was being poured.

According to Pentax, and further supported by my own personal study of Optical Physics, light is refracted by the shape of the surface of a lens, both front & back, and the physical distance between those two shapes, but the actual core of the glass has very little to do with optical quality. Pentax then developed a method to measure the actual diameter of those bubbles and if they were under a specified diameter they would have no impact on the optical quality.....In fact, In the early 70's there was a company in with China or Korea that was making fake Takumar lenses and Pentax put the word out that looking for the bubbles was a way to tell the real Takumar lense from the fake ones.

Takumar lenses were also made airtight and filled with nitrogen gas to prevent corrosion, so if the lense is sealed air tight, pray tell, how would dust get in there?

So when I see the dust in the old Takumar lenses I just snicker to myself as I point it out to the seller and get a reduced price on a great lens.
12-21-2012, 08:54 AM - 1 Like   #7
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for the most part, i don't worry about internal dust and know that i can usually open the lens and clean it if need be.

that said, if you shoot wide open bokeh circles like this pic below, you can often see all the internal dust inside each little circle, so even a single contaminant becomes pretty obvious...i wish i had the OOC version because i cloned out several bits of dust from the image before it was useable.

12-21-2012, 09:06 AM   #8
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QuoteOriginally posted by LaurenOE Quote
What happens is there are people who are pixel peepers/camera weenies that look for perfection in their used devices - especially in their *used* purchases. They want a used purchase to be such a great deal that it eclipses a new one. By telling people there is some dust or other insignificant thing, it protects the seller from having to deal with one of these weenies.
Exactly. :-)

I wouldn't worry about it unless there's a ton of dust in there, or a lot of dust on the rear element. Just about every lens has internal dust, especially push/pull zooms. I have yet to see dust show up in an image, and I've used some lenses that were pretty dusty on the inside.

12-21-2012, 09:09 AM   #9
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QuoteOriginally posted by mikeSF Quote
that said, if you shoot wide open bokeh circles like this pic below,
Wow that is a beautiful shot Mike!
12-21-2012, 09:13 AM - 1 Like   #10
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Almost all my lenses are 30 years old (or older) and almost all have dust or bubbles or other 'this is actually NORMAL' blemishes about the things. I have yet to take a shot that I can point at and say it was screwed up 'because the lens was dusty'.

Theres a site out there that shows just how mangled a lens can be before you actually start seeing anything in the image.
12-21-2012, 09:26 AM   #11
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QuoteOriginally posted by Sagitta Quote
Almost all my lenses are 30 years old (or older) and almost all have dust or bubbles or other 'this is actually NORMAL' blemishes about the things. I have yet to take a shot that I can point at and say it was screwed up 'because the lens was dusty'.

Theres a site out there that shows just how mangled a lens can be before you actually start seeing anything in the image.
It should be noted, the wider the lens, the more resolving power the front element has. For a Normal or Tele lens, even a large ding in the glass will produce decent results. Wide lenses though, if you even have a smudge on the front element, it will produce horrid results.

That said, my favorite Tele prime (135/2.8 Suntar) has two elements with nothing but bubbly glass. Produces some of my best images in regard to contrast and DoF play.
12-21-2012, 01:59 PM   #12
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Wow,
Thanks for all the great replies! This have given me more confidence purchasing lenses used off the Internet. Generally it sounds like dust closer to the sensor is more critical than the focal end. This has been a great help!
12-21-2012, 06:03 PM   #13
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QuoteOriginally posted by Sagitta Quote
Almost all my lenses are 30 years old (or older) and almost all have dust or bubbles or other 'this is actually NORMAL' blemishes about the things. I have yet to take a shot that I can point at and say it was screwed up 'because the lens was dusty'.

Theres a site out there that shows just how mangled a lens can be before you actually start seeing anything in the image.

I would say that is one darned good article.
12-21-2012, 06:44 PM   #14
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QuoteOriginally posted by Colbyt Quote
I would say that is one darned good article.
I would absolutly agree! I wish it addressed dirt on the lens area close to the sensor as well.
12-21-2012, 07:09 PM   #15
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QuoteOriginally posted by Heelers Quote
I would absolutly agree! I wish it addressed dirt on the lens area close to the sensor as well.
I have a 28mm that I was having crazy-go-nuts flaring on that I couldn't fathom the source of until I realized someone apparently licked the rear element or something (my son was 1 at a time, it was a possibility...).
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