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The Apocalypse of Lens Dust

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Apocalypse (from the Greek apokálypsis; "lifting of the veil" or "revelation"):

  1. An event involving destruction or damage on an awesome or catastrophic scale.
  2. A disclosure of something hidden from the majority of mankind.

How appropriate the word apocalypse is for this little article. At least once a week I see a post somewhere from a fairly inexperienced photographer who thinks a definition 1 apocalypse has occurred: "OMG, my lens has dust inside!!!! How did it get there??? What will I do?? My shoot is ruiiinned!!!!!"

Pretty soon a more experienced photographer posts a definition 2 apocalypse to try to calm the first photographer down: "Lens dust is insignificant, you can never see it, it has no effect on your pictures."

Well, the second post is far more realistic than the first. Every lens has dust inside and you can almost never see it in a picture (99% of the time you can't). But obviously there must be some point where dust is either big enough or there's just enough of it to become visible, otherwise we could fill our lenses with mud and then go take pictures.

Before we get into that, I'll answer a few questions for those who are not experienced in the ways of lens dust:

How do I tell if my lens has dust inside?

You don't have to tell. It does. Every SLR lens does (there are some airtight medium format and rangefinder lenses I believe). Some have more than others, of course, and you may have to look really hard, maybe even with a focused light to always see it. But if you've used it more than a couple of times, it does. If you look with a slightly angled light source at the front and back elements and through the lens will usually show you more than you want to see.

How does it get there?

What? You think the lens is assembled in a clean room and then hermetically sealed? Well, it is assembled in a clean room, but lenses aren't sealed, air moves in and out ... and air carries dust. Dust prefers to stick to something rather than float around (I've proven this definitively with the shelves in my office - I wasn't just being lazy, I was doing an experiment) so once in the lens it tends to settle on an element. People assume that zoom lenses with extending barrels will "pump dust." They do to some extent, but the dustiest lenses we see are actually some primes and short range zooms.

Can I clean it out?

Generally not, without partially disassembling the lens (and depending on exactly where it's located, maybe not even then). Some lenses, like the Canon 17-55, are easy to open up and clean. Most aren't. We clean out lenses when dust gets heavy for cosmetic reasons (it certainly looks nicer) but it's amazingly rare for us to see any hint of lens dust in a picture.

So When Can Dust Have an Effect?

Obviously if there's so much dust that it interferes with light transmission or contrast, (a thick coating of dust would do that, not a few specks) there would be a problem. But today I'm focusing on when that big old dust spec right in the middle of your lens might show up in a picture. Multiple internet posts say the following (but never seem to have pictures to demonstrate it):

  • Dust is more likely to show up if it's near the back element (Makes logical sense, the light beams have finished refracting by then.)
  • It might possibly show up if the aperture is very small, say f11 (Makes logical sense. It's certainly true for sensor dust.).
  • It is more likely to show up if the lens is wide angle.
  • It might possibly show up if you're focusing very close, rather than at infinity (The last two make sense, particularly if you consider the depth of field is going to be closer to, or even include, the lens elements on a wide-angle lens focused closely.)

According to internet wisdom, then, a wide-angle lens, stopped down, and focusing on a nearby object might show some evidence of dust in the image, especially if the dust is near the rear element. Probably won't, but since I've never seen anyone try to demonstrate these things and we had a lot of dusty lenses waiting for Aaron to open them up and clean them out last week I thought I might try.

Let's See Some Dust Already

OK, here you go. This is a Canon 85mm f1.2, a lens notorious for getting dust in front of the back element where, according to the above theories at least, it is most likely to show up in a photograph. It's a difficult lens to open, and I have to admit we let the 85 get dustier than most before cleaning them, and this was by far the dustiest of our 60-odd copies. All that dust is inside the lens under the glass, which had just been cleaned.

Rear element of a Canon 85mm f1.2 with Megadust

 

Hoping to accomplish something that we've rarely been able to do (make an image showing lens dust) I stopped the lens down to f/16, focused at near focusing distance on a gray wall, and shot away. I think I may have gotten one area of dust to possibly show up in the image below, but I marked it with a red arrow because otherwise you'd miss it. (BTW - this is a crop of the left half of the image, if I shrunk the whole image enough to fit here, the sort-of spot disappears entirely.) I double checked that it wasn't sensor dust with another lens, and could reproduce it with this one, so I think it's real. But it certainly is not visible at f/11 and not very impressive at f/16. I can't imagine you could really notice it in a photograph.

Probably, possibly a dust spot showing up from the image above. I think.

 

Let's Try Something Worse

Well, that was the dustiest lens we could find and it wasn't very dramatic. So we started trying more dramatic things. Blowing dust onto lenses, putting dots of marker ink on the element - nothing really showed up. Finally we found something that would show up: pieces of sticky notes about 3/8 inch in diameter. So here's the takeaway message: if the dust is much smaller than the pieces of sticky note on the front element of the lens below, chances are pretty much 100% you can't see them.

But of course, having spent half an hour finding something that what would show up in pictures, we put some of those on the front element of that same 85mm f1.2 and took some pictures.

 

Three sticky note pieces on the front of an 85 f1.2

First let's see what effect changing the aperture had:

 

Images taken with the Sticky note 85mm at f2, f6.3, and f16

At f/2.0 there may be a subtle effect on image quality, but I can't see it. About f5.6 we started to see a bit of shadow and by f/6.3 (middle shot above) you can definitely see some shadows from stickies in the center, upper left and lower right corners.  As you can see I took the f/16 picture against a gray wall to clarify how poorly defined even a large sticky is on the front of the lens. Nothing like the sharp dot you might see with sensor dust. OK, this seems to prove that stopping down will make a foreign object on the front element more visible.

What if we put the sticky on the rear element?

 

3/8" Sticky in center of rear element

This time it's clear there's something wrong at f4, and by f/7.1 it's very clear there's an object blocking light. Compare how you can tell even the rough shape of the object on the rear element at f/7.1 but on the front element the shape was very vague even at f/16. This certainly seems to agree with what everyone says: a foreign object near the rear element is more likely to show than one near the front element. But remember that's a big piece of sticky. You sure aren't seeing any of the 20-odd dust specs on that rear element from the first picture.

One other thing made us know putting the sticky on the rear element was an issue, though: even wide open the camera would not autofocus with the sticky on the rear element and center focus point chosen. DUH, the sticky was enough to block the light being reflected down into the autofocus box. It was easy to focus the lens manually, but center point autofocus didn't work at all.

Does the Lens Focal Length Matter?

Well I didn't want to make this an exhaustive test, but I thought I could at least compare a Canon 24 L to the 85. We did that, both shots at the same distance from the target and at f6.3 below:

Comparison of the same stickies on a Canon 85L and 24L @ f/6.3

Interesting: The stickies on the front element are far more apparent on the 24 than on the 85, which goes along with common wisdom that a wide angle is more likely to show it than a longer lens. But on the rear element the 85mm lens shows up much worse (although both are obvious).  So perhaps front element dust is more critical for wide-angles, and rear element dust more critical for normal or short telephotos?

My guess (and I didn't do enough of these to do more than guess - we have a fairly short attention span here at Lensrentals) would be that the reverse-telephoto construction of a wide angle lenses will make the rear element less critical, since light is still bending at the rear element. The depth-of-field argument doesn't make as much sense, since the rear element is only about 5 cm behind the front element.

I won't waste a lot of bandwidth posting more images, but we did find that front element stickies were a little more evident at closeup distances than far away, but the difference was pretty subtle. Not nearly as pronounced as a change in aperture with either of these lenses (or even with a 100 Macro, just to see if really close up made a difference). So focusing closer did make the foreign body a bit more obvious, but not nearly so much as stopping down the aperture did.

So what did we learn today?

  • The biggest surprise I got (it shouldn't have been, I just didn't think about it) was putting a fairly large foreign object on a rear element may keep the camera from autofocusing. Does it have real world implications? I don't think so, although perhaps heavy rear element dust could interfere with autofocus accuracy.
  • Individual dust particles have to be HUGE before they can show up under almost any conditions. By HUGE I mean bigger than dust. Think an insect part or a small screw (I've seen both of those in lenses. It happens.)
  • Even something large probably won't show up at wide apertures (f4 or wider probably).
  • A wide angle lens shows front element objects more than a moderate telephoto.
  • The moderate telephoto showed rear element objects more dramatically.

Remember, though, that what we really did here was look at big objects on the front and rear element of the lens. Internal dust might behave a bit differently if it was near the focal plane of the lens internally, or perhaps just in front of or behind the aperture. I'm not so curious that I'm going to take a lens apart and put a sticky note on an internal element and reassemble it to take test shots. Not yet anyway. Maybe in the winter when things are slow.

Does any of this have any real world implications? Nah. Unless you get a bug inside your lens there's not much need to worry about dust. Until there's so much dust that it interferes with contrast or light transmission. When is that? Well sounds like another article coming up, doesn't it? But it would be a lot of dust.

 

Roger Cicala

Lensrentals.com

 

 

 

 

 

 

 

 

 

 

 

26 Responses to “The Apocalypse of Lens Dust”

Mark Olwick said:

Great article, Roger! Right up there with the one where you shot through the lens with the shattered front element.

FYI: Typo in this subhead: How dDes it Get There?

Mark

LensRentals Employee

Roger Cicala said:

Thank you Mark. I knew I'd have a typo if I put this out at night. Never fails.

Phyl said:

Thanks for another great and interesting article. I had wondered how much internal dust would affect photos. I've suspected that a lot of internal dust might affect the ability to focus.

Nikhil Ramkarran said:

I think you should just give up renting equipment and live off the inevitable millions that would result if you were to start writing more regularly :) Another great article.

Bob said:

I once had my own dustocalypse when using a 25-year-old Minolta 70-210 f/4 zoom. At f/32.

I had forgotten my ND filters, but I wanted to try some panning of a bicycle race anyway. At f/32, about a half-dozen dark spots were noticeable in the photos, but they were almost entirely gone at f/16. At f/8, I never had a problem with the lens -- and the dust looked much worse than the 85mm posted here. Even with those f/32 shots, it was easy to clone out the dust spots in lightroom anyway.

Bernard Ortiz said:

Fantastic article. My 135L has a fairly large dirt spec on one of the internal elements. I never noticed it until I tried some extreme macro with extension tubes and a macro bellows. THERE, it shows up. Any other time? Not at all.

Aaron Whiteman said:

Roger, I've successfully gotten dust to show up in a shot. Like Bernard, it takes macro work to do it.

Tamron SP90 on a D40, SP800 flash. Crank the aperture down to f/64 (!), focus in to the minimum working distance to get 1:1 macro, fire away. You'll see dust sitting on the back element, and it'll be pretty well resolved shadows too.

You can see it in the upper left corner here: http://www.flickr.com/photos/aaronwhiteman/4202062032

Tyler McCall said:

Roger, thanks for the great info. I made the mistake of taking my Canon 17-55 on a dusty horseback ride (shooting with one hand while holding the reins with the other was much harder than I had previously thought) and now it's got noticeable dust on the interior elements. Thanks for the reminder that a bit of dust doesn't affect image quality.

Robert Todd Wilcke said:

Thanks for taking the time to write this great article!

Mario said:

I swear Dust must work for the CIA because for the life of me i cant' understand hwo it gets iside lens sometimes. Great post

Tony Siu said:

Did you make a test about how dust affecting Bokeh and lens flare?

LensRentals Employee

Roger Cicala said:

Not yet, Tony, but that's a good idea, and I think something we can do. I'll look into that.
Roger

Richie said:

Hi Roger. This is a very informative article. Thanks for taking the time to write it. I made the mistake of looking to see if there was any dust in my Nikon 60mm AFS macro lens today. It hasn't been used much yet so I assumed it would be pretty much spotless. Ouch, I was wrong! Where did all those white specs come from? I was freaked out. I mean, it's a pro grade lens, not a cheap consumer zoom. I can only assume most lenses aren't assembled in a clean room. Anyway I hit the net to see what I could find, and found this article, which has made me feel a bit better. Thanks again.

Richie

Jacek Z. said:

Another great article! Thank you. With so much ado (on so many forums) about FF/BF, sharpness, bad filters, dust inside etc. it works like an antidote.

By the way, speaking about my own experience: Several years ago I bought a point & shot camera. It had some dust particles under the front element of the lens from the start (I bought it thinking it was just a light reflex). Later on it collected some other dirt (mostly cotton fibres) while carried, almost daily, inside its bag in my backpack. Last but not least, one day a single grain of sand happened to enter and block the zoom mechanism. Finally, the grain was crushed somehow and... sucked into the lens. The camera works fine: 5 years, over 15,000 shots, now with another user. Effect on the photos (considering this was a 1/2.5" sensor and the zoom wasn't a fast prime): NONE.

Ville said:

Great article. Thank you!

I think the biggest effect of the dust can be seen in out of focus highlights. The bigger dust specks appear in highlight circles as black spots in such a way that every highlight contains a spot in exactly same position. At least I think this is what caused the spots I saw. It was pretty annoying but luckily the dust speck moved and is not visible anymore :)

Ben @ EnglishPhotographer.com said:

Another great article Roger. Also worth considering that the dust in the lens might make its presence known through increased flare when shooting into the light - could be worth testing?

William Cowan said:

G'day Roger

Thanks for the great article.

I bought a second hand Tamron 28-75 that was very dusty inside and had low contrast. (the lack of contrast was clearly visible in the histogram) After having it professionally cleaned the lens performed like a new one. I don't believe this lens lived in a particularly hostile environment, it was just the nature of the lens that had it sucking dust for years when it was zoomed. The pro Nikon lenses appear much better in this regard.

Micke said:

As always a great article, Roger!

And it made me somewhat calmer also: I like to buy old severly used (i.e. cheap) 1930's to 1950's folders from the auction site. Removing fungus is sometimes necessary and straightforward but the dust!! No matter how careful I am it is there. Grrr. Anyway, I haven't made it show up on the 6x6's and I'm glad now it's official - it won't in the future either.

Sourav Chakraborty said:

Very interesting article indeed. And full of empirical evidence that anyone can easily understand! Keep up the good work!

Rob said:

While I haven't usually noticed dust affecting my photos, in my last trip I didn't have luck and there was a thin thread of fibre between the lens filter and the front element in my 18-55 kit lens, which I used under the rain. Photos taken at f/5.6 started to show it to the point where it was noticeable but not annoying; those shots taken at an smaller f/11 clearly showed this fibre and ruined some nice and cloudy skies. I can compare since I bracketed a couple of those photos. Focal length didn't matter in this case and the thread could be seen through the whole 18-55 mm range. Since you didn't cover filters in your test I hope this could add some information.

Dan said:

Hi Roger. I've just bought a Sigma 70-200mm f2.8 EX DG OS HSM which was described by Amazon as used buy "like new". I took the chance and I've recently got it. Now ... I was satisfied so far with the tests, the only issue being something that looks more like a small piece of plastic or something inside the lens, on one of the front elements. It can be seen when zooming out and it disappear when zooming in (gets to the side of the lens) - all this observed while looking straight to the lens from the bottom of it.
I haven't seen so far any signs of dirt on my pictures but in the same I was wondering if still, I should try to clean it of course, if this is possible with this kind of lens. Any opinion would be appreciated.

LensRentals Employee

Roger Cicala said:

Dan,

I'd leave it be. That lens is a PIA to get into.

Sailor John said:

Roger, You have just saved me a few hundred dollars, but more important, allowed me to sleep again. I spent all night with a knot in my gut thinking I'd screwed up all of my lenses by using them in the wind. Thank you for taking the time to share what you learned with us. Well written!

John Fechner said:

Folks. Very interesting. Thinking out loud here and not all confirmed at this point... I have a Nikon 60mm f2.8D Lens and the dust on the rear element at f11 and esp f16 when focusing 1:1 WILL show up blurry round discs on your image in midtones like blue sky (I am copying slides - and no, the dust is not on the slides). I found that if you PP an image (adjust to very low contrast and very high brightness) shot of just the blue sky (or some other bright object that can fill the frame) with the lens focused to 1:1 and the aperture set to f16 ish then all your dust specs at or just behind the rear element will show up horribly. I have heard of this happening to folks shooting product photos against consistent backgrounds. I have sent the lens to Nikon for repair. hope to have it back soon. My gut guess is that since the old non digital lenses have flat rear elements instead of rounded ones, that contributes to the problem a bit because the image is more coherent at the rear element at 1:1 focus esp. I hope to try a new 105mm micro lens as well and see what happens. I am using a D7000 body and have sent that in for cleaning as well just in case it is a contributor. I also have a Nikon 50mm f1.8D lens and it does the same thing but not as bad. The dust spots in the images are not in the same place with this lens as with the 60mm so I know it is not on the sensor. I use the 50 to copy flat art and for stitched landscapes and notice it on both. Frustrating to PP all of it out after the shoot. John===

L.P.O. said:

I've read this article when it was new and it is just as good and relevent as when you wrote it.

There is, however, one observation of my own that I'd like to add.

I used to own the Peleng 8mm f/3.5 circular fisheye for my Canon 5D. As I learned to my chagrin, with that lens of extreme focal length, and with certain circumstances, you could make front element dust particles consistently show up.

Set the lens to f/8 for infinite DoF, and locate it so that the sun reaches the front element (can't use a lens hood with a circular fisheye of course), and every single speck of dust would show up in the dark parts of the image (unless those dark parts had lots of detail).

Fortunately, blowing or gently wiping the front of the front element would get me rid of this issue - but I had to do a reshoot when I learned about the problem. As a user who normally shoots his 135/2L and other lenses mostly wide open, and thus never worries about dust, it was a bit of a shock that you could not only see _some_ dust specks, but easily see _each and every individual speck_ with the Peleng.

I believe this experiment is easily repeatable, so I dare you to go and find some dust when you have the time! :-)

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