网上入手一个全新的canon 70－200 f/2.8 IS 2。发现镜头后组里面有一颗灰尘。白色，比较明显。经交涉，商家接受了退货。准备再进一个新的，又拿不定把握。上网查了一下。发现此现象不少。
上国外的摄影网站一查，才知道，原来这种现象还比较普遍。一般问题不大。就着有道看的，水平有限，不翻译了。如果有兴趣，可以看看原文。
原文有图：用手电光从后向前穿过镜头，可以看到大量灰尘和镜头中的气泡。
另外有视频：如何拆开镜头，吹走里面的灰尘。只能吹最外面一层的镜头组。（文章不推荐这样做）。

原文链接：

http://mansurovs.com/what-to-do-with-dust-inside-lens


What To Do With Dust Inside Lens
Apr
14
2011April 14, 2011
By Nasim Mansurov 22 Comments
One of the most frequently asked questions that I get from our readers, is what to do with dust inside a lens and whether it is something to worry about. I decided to write an article on this subject, because lens dust and flecks are a very common issue not only for camera sensors, but also for lenses. When I first discovered dust inside my brand new lens that I only used for a couple of days, I was very disappointed and I remember how I started searching for a solution online in panic mode. If you are frustrated with a similar issue and do not know what to do, keep on reading.

1) How to Inspect Lens for Dust
So, how can you find out if you have dust inside your lens? Actually, let me rephrase this question – how can you find out how much dust you have inside your lens? Because even brand new lenses normally do have some foreign particles in between lens elements. A quick visual inspection of the lens front will often reveal large dust particles behind the first lens element, if there are any. Just make sure that the front is thoroughly cleaned beforehand and any protective filters are removed. Look straight and then inspect the lens at an angle and you might see some dust behind the front glass element. Now if you really want to see dust, and I promise you will, here is the best way to do it. First, find a very bright LED flashlight. You can find those pretty much anywhere nowadays, even in a grocery store. Next, you will need to open up the lens aperture (the lens obviously needs to be dismounted from the camera, rear lens cap should be removed). If you have an older lens with an aperture ring, you just need to set the aperture ring to the smallest value (which is the largest aperture) like f/1.4 or f/2.8 and you are ready to go. If you have a modern lens like Nikon “G” type AF-S lenses, then you will need to push up a small metal lever to open the lens aperture as seen below. To keep the lens aperture open, you will need to keep pushing it with one finger:



Once the lens aperture is fully open, turn on the flashlight and point it towards the rear of the lens with the front lens cap off. Do this in a dim indoor environment with lights turned off. Look at the front element of the lens at an angle and see how much dust you have inside the lens. If you have never seen any dust, you will certainly see it now. Better yet, now you can see dust in between pretty much every lens element, because it will be visible when a bright source of light goes through the lens. Now here is a word of warning – as I have pointed before, don’t be surprised to see dust even if you have just bought your lens. Some of those particles might be dust, others might be small bubbles and other glass imperfections. Why? You guessed it right, no lens is perfect! But don’t panic, every lens I own has dust in it, even the brand new Nikon 35mm f/1.4G prime that I have recently received from B&H. Take a look at how much dust my Nikon 24-70mm f/2.8G accumulated over the years of abuse:



And here is how the Nikon 50mm f/1.4G looks:



Looks scary, doesn’t it? But I don’t really care, because both lenses produce excellent results and I am sure will continue to do so for many more years.

2) How and Why Lenses Get Dust
You might be wondering how and why lenses get dust inside. Let me explain a couple of things about lenses. Every time a lens focuses or it is zoomed in and out, it “breathes”. And no, I am not talking about the effect of lens “breathing”, when an image appears smaller or bigger when focus is adjusted – I am talking about the process of inhaling and exhaling. Lenses have to breathe, due to lens elements constantly moving inside them when focus is adjusted and/or when zooming takes place. Remember what happens with pressure inside a closed plastic container? If you try to reduce the container size, the pressure inside the container will only let you reduce it to a certain level before it pushes back. A simple concept of air pressure in physics. Now take the same concept and apply it to lenses. What would happen if lenses were completely sealed from all sides? You would only be able to zoom in a little before the lens would force you back to its original state due to pressure, especially on lenses that extend in size. A similar thing would happen with lens focus. Hence, there was no other way for camera manufacturers to design lenses – lenses with moving lens elements must inhale and exhale air. Some lenses are better than others in managing the air flow. While some expensive lenses are sealed against dust (which does not fully stop dust from entering the lens) and will only suck the air in and out of the camera chamber, cheaper consumer zoom lenses are the worst in this regard – they might suck the outside air and blow it out right into the camera chamber. Let’s take a look at which lenses are worse than others in handling dust.

3) Lenses Prone to Dust
As I have explained above, some lenses are more prone to dust than others. Here is the list of lens types that are more prone to dust than others, in the order of “worst to best”:

Consumer zoom lenses with extending barrels – examples: Nikon 18-55mm f/3.5-5.6G VR DX, Canon 18-55mm f/3.5-5.6 IS.
Professional zoom lenses with extending barrels – examples: Nikon 24-120mm f/4 VR, Canon 24-105mm f/4L IS. Cheaper pro-level lenses with Red (Canon) and Gold (Nikon) rings often have similar weather protection as expensive pro-level zooms, but are generally more prone to dust due to significant changes in lens barrel length. Most come with rubber gaskets on the lens mount to prevent dust from entering the camera chamber through the lens mount.
Expensive/top-of-the-line professional zoom lenses with extending barrels – examples: Nikon 24-70mm f/2.8G, Canon 24-70mm f/2.8L. Top-of-the-line professional zoom lenses typically have better weather sealing all around the lens. Rubber gaskets are always included and other rubber seals are present in other parts of the lens such as zoom ring, focus ring, switches, etc.
Professional zoom lenses with fixed barrels – examples: Nikon 70-200mm f/2.8G VR II, Canon 70-200mm f/2.8L IS. Lenses that do not change in barrel size are generally better against dust and moisture. Since nothing moves, there are fewer places where dust can accumulate and then make into the lens. Rubber gaskets and other rubber seals are also present in all areas where dust can potentially enter the lens.
Prime lenses with extending front element – examples: Nikon 50mm f/1.4D, Canon 50mm f/1.4 USM. Prime lenses are generally less prone to dust than zoom lenses, because fewer parts move inside them. Prime lenses with moving front element that changes in length as you focus are generally better than zoom lenses, but dust can still make it into the lens through the front. Rubber gasket on the mount is sometimes absent (especially on older models), which can also contribute to dust making it into the camera chamber and the lens.
Prime lenses with fixed barrels – examples: Nikon 35mm f/1.8G, Nikon 24mm f/1.4G, Canon 24mm f/1.4L II. Prime lenses with non-extending barrels are typically protected best against dust. Some of the prime lenses with rear focus feature (such as Nikon 24mm f/1.4G and Nikon 35mm f/1.4G) might have a moving rear lens element as you focus, while others have a fixed glass element that never moves. The latter is typically better than the former. Many of the modern prime models are designed with rubber gaskets around the lens mount and high-end models have additional weather sealing in other parts of the lens.


4) What to do with lens dust
Once you spot lens dust, what should you do with it? The answer is – nothing. Don’t worry about it and just keep on shooting, concentrating on creating great images. As I have explained above, lens dust is a normal fact of life, just like dust on your camera sensor. Even if you take a good care of your gear on a daily basis, you will eventually end up with dust in your lenses and cameras, guaranteed. You can certainly minimize the amount of dust getting into your gear by storing it properly and performing regular cleaning and maintenance (which I will cover in an upcoming video tutorial), but you cannot fully prevent it from happening. Dust is inevitable and it does get into camera gear one way or another, so you should not be sweating over it if you have it. Try an experiment – come close to a dirty window in your house and look outside. When your eyes focus on the outside, can you see the dust or dirt on your window with your eyes? No, unless the dirt particles are huge. The same thing happens inside the lens, if there are small dust particles, it is not a big deal. So take a deep breath, chillax and stop worrying about dust.

The only case where you might need to call your lens manufacturer, is if you spot an abnormally large spec of dust more than several millimeters in size that moves when you rotate the lens. There are cases, when particles break off inside lenses, typically after lenses are dropped/damaged.

If you are a very brave soul, you can try removing dust from your lenses by doing something like this. However, there is a high risk of potential damage, so do it at your own risk!

有一颗灰尘又无所谓的～

太正常不过了

楼主 你不是生存在无**界里

虽生活在有污染的界里，但并不代表在某些领域有灰尘就算正常，镜头里有灰尘这得需要多大的使用量和损耗才能达到啊，何况买新镜头的人还说有一粒或几粒灰尘属于正常的？胡说八道！（先声明，我没有人身攻击的意思，只是就事论事。）

等老机来解读

这是越南文，我看不懂。

镜中有灰尘（可见型）肯定是不正常的，楼主要是自己把它拆开后清理灰尘，那就更不正常了。

不太影响吧.......

新镜头有灰尘确实不正常，佳能工厂 无尘环境不达标？

有空也用手电筒在自用的镜头照照，看看里面都有些啥