This likely dust on the sensor -- don't be alarmed it's ordinary and easily cleaned.
There are actually two filters in front of the sensor -- so "dust on the sensor" is really dust on the filter (nothing can actually touch the sensor.)
Most Rebel models have a self-cleaning mode -- but this uses a piezoelectric effect to vibrate the filter and hopefully shake the dust loose. It wont always work.
If it does not work, you have two options... take it in to a camera store to be cleaned (some stores might charge you a few bucks... although I have heard of places that charge customers $50 -- which I think is pretty excessive.) The other option is to clean it yourself. If you do a lot of photographry, you may as well learn to do it yourself because it's something you are likely going to need to do from time to time.
I have levels of agressiveness that I use to clean a sensor... starting out by merely using puffs of air (not very agressive) and work my way up to the point where if nothing else works, I might eventually have to use wet cleaning (but that almost never becomes necessary.)
Remove the lens, point the camera down so that the lens opening is facing the ground (or resting on a table). Power up the camera, use the menus and navigate to the option to self-clean.
See page 184 of your manual. Basically you'll hit menu, navigate to yellow wrench #2, and navigate down to "Sensor cleaning". Invoke the option to "clean now" (you may have to do this more than once... but if it doesn't clear after 2 or 3 attempts, then time to move to the next step.)
Incidentally, to test whether it's clear, put the lens on. Set the camera to Av mode, dial in a high aperture value (f-stop) -- say f/16 or f/22. Now find a plain white wall (or ceiling) and take a photo.
It's VERY easy to spot sensor dust when you have a plain white non-contrast surface like a white wall or ceiling AND use a high f-stop value.
If it's clear, great. You're done.
If not... next step is to get a hand-squeeze blower -- such as a Giottos "Rocket Blower". I do NOT recommend you use cans of compressed air. These cans will spray a residue (the propellant) onto the filter if you're not careful and then the filter WILL have to be wet-cleaned.
Navigate to the same sensor cleaning menu, but instead of "clean now", pick "Clean manually". This causes the camera to flip the mirror up and open the shutter exposing the sensor (really the filter). Keep the camera pointed downward and squeeze several puffs of air into the camera. DO NOT let the tip of the blower hit anything inside the camera (otherwise you could mark or scratch the filter and make things worse.)
If that STILL doesn't clean it, I use a magnifier to inspect the sensor to see what I'm working with. You can buy these from brands such as "Sensor Scope" and they are inexpensive.
My next step is to try a VERY soft brush to see if it will brush off. Use no pressure... just a few very gentle and very light strokes should do it.
If none of that works, then it's time for wet cleaning.
Buy Eclipse brand cleaning solution (made by "Photographic Solutions, Inc" and carried at most real camera stores (not big box stores that happen to sell cameras). You could also order it online.
You will also need the correct size cleaning swab. That same company (Photographic Solutions) makes "Sensor Swabs" but they come in sizes to fit your sensor. For cameras with APS-C size sensors (which is what you have), you want a Type 2 size sensor swab.
Ok... the goal of this next step is to NOT damage your camera buy wasting money (no.... I'm not actually kidding.)
You will place approximately 2 drops of Eclipse cleaning solution on the tip of the swab. You will use the swab to sweep from one side of the senor to the other using almost no pressure. Eclipse solution is almost pure methanol, evaporates VERY quickly, and leaves no residue.
You will now IMMEDIATELY throw that swab away. (that's what I mean about the "wasting money" bit.) There are 12 swabs in a box. You might use a couple of them to clean the sensor. They are cheap. Your camera is not. Do NOT try to save money on cheap disposable swabs and risk damaging your camera -- because if that swab picked up a bit of sand, you would scrape that bit of sand across the sensor by re-using it. This is why I suggest using almost no pressure AND why I suggest using the sensor scope to inspect the sensor and see what I'm working with.
Cleaning a sensor isn't difficult and, if done right, it's not even dangerous. But if you grab just anything and have at it like you were cleaning a window, then you could make things worse (and potentially expensive to fix.) Just go slow. Inspect everything. I operate with the assumption that whatever is on the sensor is going to scratch it if I'm not careful (even though most of the time that's not true) and use very light pressure to "tease" the offending bit of dust off the sensor.
When you are done, just power off the camera and put the lens back on. It will automatically close the shutter when you hit the power switch.
Tim Campbell
5D III, 5D IV, 60Da
