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Author Topic: Dust, is it myth or fact?  (Read 3117 times)

unfocused

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Dust, is it myth or fact?
« on: February 07, 2012, 02:05:27 PM »
I know there has been much discussion about push-pull zooms and dust. But, this was a new one to me;

Quote
Internal zoom means no barrel extension to get dust in the lens.

This got me thinking (and I'm not trying to start a new "war' here) how much of the "dust in the lens" debate is fact and how much is fiction?  I guess I've always assumed that most lenses are pretty much sealed environments. You need to protect the rear element with a cap and the front with either a filter or a cap or both (although the front elements seem pretty well sealed to me.)

But, as for dust getting inside a lens either from focusing or zooming, is that really likely? Sure, no system is going to be perfect and dust is pretty persistent, but are lenses really designed in ways that can allow dust to get in like this, or are we just repeating urban legends? 

Aside from anecdotal evidence (I own the XYZ lens and it is a dust magnet or I own XYZ and I've never gotten a speck of dust in it) can anyone shed some light on this?
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Dust, is it myth or fact?
« on: February 07, 2012, 02:05:27 PM »

keithfullermusic

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Re: Dust, is it myth or fact?
« Reply #1 on: February 07, 2012, 02:21:46 PM »
There is no such thing as dust.  It is a complete myth.  It is conspiracy brought on by cleaning product companies.




In all honesty, I have the 100-400 and I don't really have any dust in mine.  If anything, it is comparable to my other prime lenses.
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Mt Spokane Photography

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Re: Dust, is it myth or fact?
« Reply #2 on: February 07, 2012, 04:18:24 PM »
A extending lens zoom has to let air in / out as it extends or contracts.  Just how well it filters dust as it does this depends on te design of the lens.  Most lenses seem to have a little dust in them, they are not assembled in a class 100 clean room.

Dust is not usually considered to be a problem unless it gets onto the rear element of a lens, so why worry.

NormanBates

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Re: Dust, is it myth or fact?
« Reply #3 on: February 07, 2012, 06:03:36 PM »
My best lens is a Leitz Elmarit-R 35mm f/2.8 from 1978

it doesnt extend at all: it's a prime, and when focusing the whole glass block moves forwards and backwards

even so, it has dust inside, including a shiny thin filament, about 5mm long, deposited on the inside of the back element

and yet it's sharp as can be, has absolutely amazing bokeh, and is, in general, just perfect (as I said, the best lens I've tried, and I'm the one who's tested all his lenses against a resolution chart and a standard bokeh test subject)

unfocused

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Re: Dust, is it myth or fact?
« Reply #4 on: February 07, 2012, 06:56:38 PM »
Quote
A extending lens zoom has to let air in / out as it extends or contracts.

Good point. I never thought about that, but I suppose if it were a totally sealed environment it would create a vacuum and wouldn't zoom.

BTW, I'm not really all that concerned about this in my own use. I'm pretty casual about dust (I don't go out of my way to get dust in my equipment, but I also don't sweat about it under normal circumstances.) I just thought it was an interesting subject and always like to get the perspective of "techies."
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neuroanatomist

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Re: Dust, is it myth or fact?
« Reply #5 on: February 07, 2012, 07:47:23 PM »
Yes, dust is real. Canon is well aware of the problem of dust in lenses, and the significant impact it has on IQ.  Many, many years ago, when they first decided to produce a professional zoom lens (the FD 80-210 f/5.6L), they designed and built a completely dust-proof prototype, hermetically sealed and assembled by engineering staff in a specially-constructed low-pressure chamber.  When the photographic staff tested the prototype, which was given to them in extended position, they were surprised to find that as they retracted the zoom barrel, it got more and more difficult, the shorter the lens got.  They talked to the engineers about the problem, and that team explained patiently that since the photography team had asked for - and received - a completely sealed lens, there was no way to equalize the pressure, so the lens needed to remain extended. They went on to say that they had the option to assemble the prototype such that the internal vacuum would tend to pull the lens either to the fully retracted or fully extended position, and they chose extended because 'more zoom is obviously better'. The photographic staff did agree that more zoom was better, but sent the engineers back to address the problem of not being able to keep the zoom lens at the wide end.  The engineers bowed and went back to work, soon returning with a Mark II prototype.  Their solution was a switch mechanism that would lock the zoom barrel in the retracted position.  The photographic staff liked the solution - after all, they knew that zoom lenses are never quite long enough and never quite wide enough, so they are always used at one end or the other. At that point, the marketing team got wind of the project. They looked at the prototype and balked, claiming they could sell an 80-210mm zoom lens, but not an 80-or-210mm lens.  They ordered the project scrapped, and the idea of a fully dust-proof zoom lens was abandoned.  Marketing did have one final comment: "Keep that zoom lock thingy, we like that."

Thus, today there are still no dust-proof lenses, and yet we do have zoom lock switches, even on the brand new 24-70 II. True story.







Or, maybe not.
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CatfishSoupFTW

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Re: Dust, is it myth or fact?
« Reply #6 on: February 07, 2012, 08:49:46 PM »
i can see it as being a problem... but i mean, how big of a problem? a misstreated lens maybe will have its dust, more than a more standard set up. however, if you treat it well i wouldnt see a problem. plus, each lens regardless of the design would have dust. not enough to see on a picture but its there. depends the person i suppose.
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Re: Dust, is it myth or fact?
« Reply #6 on: February 07, 2012, 08:49:46 PM »

D.Sim

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Re: Dust, is it myth or fact?
« Reply #7 on: February 07, 2012, 09:40:19 PM »
True story.

I read all the way to get that...
Quote
Or, maybe not.

Then this.=(

Well played Neuro...

On topic though, yep, have had it... even returned a sigma 50 1.4 with dust inside.... my old 55-250 has some dust in there too...

Does it affect IQ? not as much as dust on the sensor...

Hillsilly

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Re: Dust, is it myth or fact?
« Reply #8 on: February 07, 2012, 10:17:28 PM »
The story of my 100mm macro... 

The AF-MF and the focus limit buttons fell off a few years ago.  I didn't think much of it at the time as I was able to change the setting using a pen or other sharp object if needed.  What I didn't realise was that this was giving dust an easy entry point into the lens.  My lens is one of the early versions where the barrels extend a lot while focusing. 

When I did notice a bit of dust inside, I wondered if blowing some air into it would clear it (or at least move the dust away from the centre of the elements towards the edges).  Turned out to be a bad idea - a lot of dust had been sucked into the lens through the hole but hadn't yet made it onto the elements.  By blowing air into it, I ended up distributing a lot more dust over the elements.  When I saw what I had done, I was very disappointed.   But I still use the lens a lot and if the dust is causing any image degradation problems, it's not obvious to me.  I sometimes feel the photos lack saturation, but it could just be my imagination. 

Sorry about my anecdotal story, but I just had to warn everyone.  Blowing air into your lens to move the dust off the elements will probably make matters worse!  Who'd have thought that?  And to Canon - why can't you build decent buttons?

So, dust in lenses?  With an easy entry point, these things definitely suck in a lot of dust.  But if it is reasonably sealed, it's probably not much of an issue.

Is it a problem?  You'd have to have a lot of dust in there for it to be a problem.
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s66

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Re: Dust, is it myth or fact?
« Reply #9 on: February 08, 2012, 09:26:53 AM »
Unless it goes to extremes a bit of dust in your lens is not a problem at all. The tiny amount of light blocked by the dust particle is not in focus and hence will be absolutely invisible where the light is in focus: on your sensor.

Keep dust away from the sensor and you're fine.

It's the same a "oh my lens has a [tiny] scratch on it, now it' useless". See http://www.lensrentals.com/blog/2008/10/front-element-scratches for how bad a lens can be broken and still yield pictures.

ejenner

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Re: Dust, is it myth or fact?
« Reply #10 on: February 08, 2012, 11:26:40 PM »
I don't think the zoom function is as bad as the internal focusing leaving a gap at the rear of the lens.   At least on otherwise sealed L-lenses.   I suspect most dust gets in by the rear element, so I don't agree with the statement you posted from your quote.

I say this becasue the EF 85mm 1.2L is renowned for collecting dust.  lensrentals says that lens requires more cleaning by far than any other (which they do more because of aesthetics and not renting out super-dusty lenses than IQ)

My 24-105 got really dusty the first 6 months I had it.  It was getting silly and I suspected that somehow there was a blob of dust left in it during manufacturing that was gradually coating my lens elements.  Or it was not sealed properly.  I got it cleaned under warranty (not usual, but it was insane) and in the 18 months since, almost nothing.  I have to say that although the dust was so bad Canon cleaned it for free, I never did actually see an impact in the IQ, although I didn't do very extensive testing.

Lenses like the 24-70 and 24-105 are not renowned for getting super dusty as far as I know.  Lenses like the 85 1.2 are.  Moral of the story, be careful not to let dust in behind the rear element if you are worried about it.  Change lenses with zooms like the 24-105 and 17-40 zoomed out and at infinity focus and get the rear cap on quickly.  With primes, set the focus to infinity before changing.  I must admit since my 24-105 'experience' I'm anal enough to actually do this 80% of the time, but I've basically been dust-free for 18months of a lot of lens changes (not saying my lenses don't have some dust in them of course).

How much gets in a really 'sealed' lens like the 70-200, I'm not sure.  I haven't noticed any increase in mine in 2+ years since I bought it, but of course that has a fixed rear element as well.
« Last Edit: February 08, 2012, 11:40:30 PM by ejenner »

kennykodak

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Re: Dust, is it myth or fact?
« Reply #11 on: February 09, 2012, 02:53:42 PM »
no matter if you keep one lens on a camera, even a prime, at some point the sensor will need to be cleaned.  ever notice the film the builds up on the inside of your car's windshield? even non-smokers get it.  it forms from vapors of molecules that break down over time from rubber and plastics.  hot and cold changes can increase this process even more.

kubelik

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Re: Dust, is it myth or fact?
« Reply #12 on: February 09, 2012, 03:06:08 PM »
I haven't had a problem with dust inside my canon lenses, but I have had it be an issue with sigma lenses.  my old 70-300 ended up with a gigantic piece of carpet fuzz / animal hair inside the front element and for the life of me I can't figure out how it worked its way in there ... but it was definitely not there one day and there the next.

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Re: Dust, is it myth or fact?
« Reply #12 on: February 09, 2012, 03:06:08 PM »