Yes. This may be sensor dust. There's an easy way to test.
Use "Av" mode (you can also do this in Manual mode) and set the camera to the highest possible f-stop the lens can handle (e.g. f/22... f/32, etc.) and take a photo of an absolutely PLAIN white wall (you want something plain... absolutely no contrast.) Then adjust the camera to the lowest possible focal ratio and take the same photo again.
If it is dust on the "sensor" (I use quotes for a reason I'll reveal in a moment) then you'll easily see it in the very high f-stop shots, but you will only barely see it in the very low f-stop shots. That would definitely indicate it's dust on the sensor.
The reason I use quotes on "sensor" is because it's not really on the sensor. It's on a filter in front of the sensor. There are two filters in front of the sensor... one is a UV filter (and this is the same reason we tell everyone it's pointless to buy a UV filter for your lens... because the camera already has an internal UV filter) and the second (which is the one furthest from the sensor) is usually the low-pass filter and it also doubles as the filter with the dust removal system (a piezoelectric charge that causes the filter to vibrate rapidly in hopes of shaking loose any dust.)
Sometimes just removing the lens in a dust free environment, pointing the camera body toward the floor, and manually activating the self-clean cycle will shake that dust loose and let it fall out of the camera. Other times the dust needs more convincing.
I suggest getting a hand-squeezed blower (such as a Rocket Blower), then use the camera menus to put the camera into the "manual" cleaning mode (this causes the mirror to swing clear and tells the shutter to open to expose the "sensor" (really the filters)) and then give a few gentle puffs of air into the camera (I point my camera downward so that the dust will fall out of the body).
I do not suggest using cans of compressed air nor an air compressor... both of which can spew liquids onto the filter and make things worse than they were when you started.
If things still don't come clear, I use a sensor scope to inspect the sensor so I can see what I'm working with. Sometimes a very soft and pristinely clean brush can sweep away stubborn stuff (sometimes there's a bit of static clean so things don't let to fall loose without some encouragment). If things really get desperate, I use Eclipse brand cleaning solution with "Sensor Swabs" (both made by "Photographic Solutions" and sold at practically every "real" camera store you'd walk into.)) Eclipse is nearly pure methanol so it evaporates almost insantly and leaves absolutely no residue behind. But there are cautionary notes and instructions on how to use this properly so you don't just drag something scratchy across the sensor or smear things around... if you choose to go this route on your own, do read instructions, watch YouTube tutorails and read as much as you can before plunging in so you don't make things worse.
Many camera stores offer a cleaning service and prices vary. But most people find the price is usually more than the cost of everything you'd need to do it yourself. I've seen one repair guy who really doesn't want to be in business just by cleaning sensors (he's got better things to do) so he's nice enough to teach his customers how to do it themselves -- but I think that's probably uncommon.
You can, of course, send the camera in to Canon and they'll clean it for you, but hat probably involves mailing it away so you'll be without for a while.
Lastly... there's a possibly that what you have are really "stuck" pixels... but these are often in random colors such as red or blue... and not just in black -- whic is why I don't suspect a stuck pixel problem.
If in doubt, you can attach an image here -- or you can upload the image to another website and post a link. But images of a plain white (or white ceiling) wall make it easiest to spot the problem.
Tim Campbell
5D III, 5D IV, 60Da
