Ok, so if you're an owner of a DSLR, you have either experienced sensor dust or sit in constant fear of exposing your sensor to this bane of photography. Digital cameras record an image by reading light that lands on the camera's sensor. Since no camera is air tight, it is possible for small dust particles to get inside the camera, land on the sensor and block the light from hitting the sensor. This results in typically dark spots on the final print.
sensor/mirror chamber on a typical DSLR
All digital cameras can get dust on the sensor, but DSLRs are more prone because it is so common for lenses to be swapped giving dust an opportunity to get inside the body and, potentially, on the sensor. Some high end DSLRs are weather-proofed via o-rings and single-body manufacturing techniques, like the canon 1D series, and certain lenses provide a sealed state when on the body of the camera, but, once that lens is off that dust can still get in as easily.
When is sensor dust a problem?
The first thing to note is that any camera that is used will most likely have sensor dust problems at some point. That's just part of the photography game, but dust is rarely a show-stopper.
Sensor dust is usually not seen in the field since camera displays are so small. It is typically after shots are downloaded that the presence of dust spots are detected. When I was in Italy last year shooting, I shot for 5 days before I even knew I had dust on my sensor. It wasn't until I had the combination of small aperture and solid blue background that I could even see the effects on my prints.
Florence skyline with dust spots
Sensor dust will be seen the most prominently in situations like the above where the background is one solid (typically light) color and where your f-stop is higher than say f-8. If your background is not solid and/or your f-stop is low, the spots will probably not even be noticed (even though you can bet they are still there).
How do I know if I have sensor dust?
The most obvious way to tell if you have sensor dust is to look. Lock the mirror up and examine the sensor visually. Sometimes you can see right where the dust is, but you risk getting more dust inside the body and smaller particles may not be visible.
The best option is to grab a sheet of paper. Set your camera to the highest f-stop and take the picture. That will be a good base to measure how clean your sensor is. That technique was used to illustrate the work-around below.
Working around the problem
The best thing to do if you have dust on your sensor is to remove the dust, more on that later. If you're like me and you don't realize you have a problem until it's too late, don't worry, there are a few things you can do to 'work around' the problem.
When I noticed the dust problems with my camera, I still had 6 more days of shooting before I could even think about taking an in depth look at cleaning my sensor. I had to come up with a plan to mitigate the impact the dust would have on my future shots.
My first step was to make sure that I only shot with low f-stops for the rest of my trip. By lowering my f-stops (increasing my aperture) I would have a narrower depth of field and, therefore, the dots on the sensor would be blurred-out and less visible on the shot.
dust spots showing with a high f-stop
dust spot impact is lower at lower f-stops
My second step was to be extra mindful about solid backgrounds in my shots. I had to pay special attention to shots where there was a solid blue sky since my dust spots were on the top of my sensor. I didn't let that force me to miss shots that I truly wanted to get, but I knew that I would have to spend time to clean up the shot later if I took it.
Fixing the problem & cleaning the sensor
So you can work around sensor dust for quite a while, but the best fix is to get that dust off of the sensor and there are several ways to do this.
First, let me mention that you should never touch your sensor! Whether it is with your finger, a brush, or anything else, don't touch your sensor. You risk taking the problem from annoyance to damage and the sensor is about the most expensive thing to replace on a camera. Instead try out these options to get rid of the dust.
Try blowing the dust off with an air bulb. DSLRs have a sensor cleaning mode that is typically activated from somewhere in the menus. This locks the mirror in the 'up' position so you can access the sensor. Be careful, because with the mirror up and shutter open, you risk letting even more dust in, so do this in a clean environment. Once the sensor is exposed, the air bulb can be used to blow the dust off of the sensor. I find this method to ineffective, but it's quick and free, so give it a try. Just be sure to not touch the sensor and don't use compressed air. Compressed air can contain chemicals that may harm your sensor.
air brush/bulb
There are plenty of DIY tools and kits to clean your sensor ranging from $30 to $100+. Most either try to blow the dust off or try to lift the dust off with 'sensor safe' chemicals. Here is a great list of sensor cleaning methods including pros and cons to each. The short is that they are usually cheap, the long is that they may not work, may damage your camera and will most likely void your waranty.
The last and probably best option is to send the camera back to the manufacturer for cleaning or taking it to a professional camera shop. This will give you the most liability coverage during the cleaning process and typically ensures that the cleaning is done right. The two drawbacks to this are that it will probably cost you a little bit to do and you'll have to be without your camera for a while. I decided that I would compensate for the dust a little while before sending the body in for a full cleaning.
One final option available to some camera owners is to use the camera's built in dust cleaning system. There are various techniques employed to actively clean sensors, but I'm only truly familiar with canon's implementation. Canon has a cleaning mode that actually uses tiny vibrations to nock the dust off of the sensor. This may not get all the dust, but the hope is that it gets most of the dust and lets you keep on shooting. Here is a clip from youtube:
Correcting prints with dust spots
Because we're shooting digital, correcting dust spots after the fact will make your prints usable and typically wont' take to much time...typically. For everyone, the ideal situation is to not have any dust to worry about, but we're not usually that lucky. Once a shot is taken and there are dust spots, you're stuck and there typically isn't any going back for a do-over. That's where post processing techniques come in.
There are many tools out there to accomplish dust removal, but my tool of choice is photoshop. With photoshop, I am able to mark the problem area and fill it in with a portion of the picture that is correct. With a large number of spots, this can be tedious, but a little time can turn a spotty image back into a pristine piece of art.
Some camera software automatically employs the method above when prints are downloaded from the camera. You actively mark out where dust spots show up and the camera fills that in with surrounding colors to mask the spot. I haven't ever seen this in action, but, in theory, this should save you some time in the long run.
Preventing the problem
The best solution is to avoid the problem all together, if only that were possible. Here are a few things to keep in mind.
First, be careful when changing lenses. If you swap from wide-angle to telephoto on a dusty road right as a car drives by you, you're probably going to get some dust in the camera. Lens changing should be done when the air is free of particulates and done only when needed. The more times you expose the gust of the camera, the more dust will get in.
Turn off the camera when the lens is off, this prevents dust from being electrostaticly attracted to the sensor. Some people say that's not an issue, and it is probably somewhat brand specific, but it's better to be safe than sorry.
Holding your camera body down may help avoid heavier particles falling into the camera. Light particles will still be able to float inside the camera, but since most things want to go down, they won't have the opportunity to go down into your camera.
Summary
Sensor dust is nothing to fear, it happens to the best. In my opinion, if you don't get at least a little dust in your camera, you're not using it enough. Just know that there are work-arounds, fixes and touch-ups that will reduce the impact dust has on your photography.
I still have dust on my sensor, but I haven't let that stop me from shooting one bit and after my little trial by fire, I know that it never will.
Here are a few more great sensor dust related links to look at:
