Did they show you these scratches? On all my years of being in photography, I've never known anyone to receive a factory-new camera with a scratched sensor. I'm not saying it cant happen... but it would certainly be exceptionally rare. The most common way to get a scratch on the sensor is through improper cleaning.
Let's test it. Do the following:
Find a "plain" white wall (or ceiling). The most important thing is that it be plain - no colors, no pattern. For this test you don't even have to focus the lens.
Set the camera to Av mode, then crank it to your highest f-stop (which on the Canon EF 24-70mm f/2.8L - that's the lens I think you have - is f/22).
Take a few shots of the wall.
Import them onto your computer. If there is anything "on" the sensor (and btw, this is technically one of the filters in front of the sensor) then when you shoot at highest possible f-stop, it's shadows and optical effects will show up in the image. It doesn't matter what the camera was focused on or if it was even focused at all ... because you're really just using the plain white wall as a flat illuimination source and basically what you're really doing is trying to get an image of the filter.
Canon can replace that filter... it's a bit of work to get at it (it can only be accessed by disassembling the back of the camera.)
If you go to this page and scroll about 1/4 down you'll see the "sandwhich" of filters that are stacked inside the camera just in front of the sensor.
http://www.imaging-resource.com/PRODS/E5D2/E5D2A4.HTM
I do just occasionally have to clean the sensors on my cameras and since this is sort of like occasionally needing to change the oil your car -- it's something every owner should either learn to do themselves or find a reputable shop to handle it (Canon actually offers this service.)
If there is anything on the sensor and you want to simply shake it loose, the easiest thing is to remove the lens (in an appropriately clean location of course -- you would not want to do this in the middle of a sand-storm), point the camera at the ground and navigate to the self-cleaning menu and let the camera have a go at cleaning itself. Often times this is all it takes.
If that's not enough, I have a hand-squeezed blower (such as a "Rocket Blower" brand blower). I discourage the use of those cans of compressed air because if the can is shaken or not held perfectly level, then the can will spit out it's own propellant material. This leaves a foggy residue and if that got on the camera sensor (really filter) it would have to be "wet" cleaned - no getting around it.
When you use a hand-squeeze blower, keep in mind that you don't want the tip of that blower touching the sensor and scratching it up -- so take appropriate care.
if that doesn't work, sometimes a _very_ soft bristle brush will do the trick. I have a small cleaning brush that I use and it doesn't get used for anything else. Avoid touching the bristles with your hands as this can tend to leave skin oils on the bristles. They do actually make versions of these brushes with a grounding wire - the idea being that if the dust is clinging to the sensor due to a static "cling" effect, the grounding wire will let it release that charge and it should be willing to let go.
If that still doesn't work, I ultimately go on to "wet" cleaning... but before I do this, I want to get a magnified view of what I'm dealing with. You can get various types of magnifying loupes to inspect the sensor. They make some with lights inside and which rest on the lens-mounting flange, giving you a nice magnified view so you can carefully inspect the situation before you have at it with anything more aggressive.
This is important because if there is a bit of debris stuck on, you don't want to be using strong pressure with a swab and then "dragging" that debris around the surface.
I use "Eclipse" solution (made by "Photographic Solutions") and their "Sensor Swabs". The sensor swabs come in sizes. A "Type 3" swab is sized for full-frame sensor cameras. The "Type 2" swab is for APS-C sensors (the difference is the width of the swab.)
Rather than following typical instructions and taking a swipe across the full surface of the sensor, I put 1 to 2 drops of solution on the swap and then lightly "daub" the spot with the offending material. I want to see if I can't tease it off the sensor and get it to cling to the cleaning swab.
Throw that swab out! Do NOT re-use (don't be cheap... if you think the swabs are expensive, consider the cost of the filter replacement.)
Once the offending material is gone, then I'll take a fresh swab, put 2-3 drops of solution on it, and then give it a gentle swipe across the surface -- no need for any pressure here.
Eclipse solution in near pure methanol and will evaporate on it's own and leaves no residue behind.
That's it. You're done. Switch off the camera and put everything back.
I can tell you that with regular care, the camera's self-cleaning or the hand-squeezed air blower will take care of whatever is on the sensor in the vast majority of situations. I almost never need to go any farther than that.
Tim Campbell
5D III, 5D IV, 60Da
