I'm thinking fungus too. It looks like you may have a fungus growing on the "sensor". Note that there are actually a couple of filters in front of the sensor... the UV/IR cut filter as well as the "low pass" filter.
If it is fungus, it is HOPEFULLY on the outermost filter surface and not on the inside. If this is the case then you can likely clean it using the "wet" cleaning method.
I use a cleaning solution called "Eclipse" made by Photographic Solutions, Inc. You can find it at most real camera stores (e.g. stores in business primarily to sell cameras and other gear to photographers... not "big box" stores that happen to sell camerass.) You can also order it online (Amazon, B&H Photo, Adorama, etc. will all have it.)
Eclispe is nearly pure methanol -- so it evaporates very quickly and it leaves no residue.
To clean ... you also need cleaning swabs. Photographic Solutions makes those too and they call them "Sensor Swabs". It's a plastic paddle with a width that matches the width of your sensor (along the "short" edge). For an APS-C camera, you wan tthe "Type 2" size. It's covered in a lint-free cloth.
(NOTE: They also sell something called "Sensor Swabs Plus" - which is basically the sensor swab pre-moistened with the Eclispe solution. So it's "all in one" making them more convenient to leave a few in a camera bag for travel purposes. But at home I prefer the seperate swab & solution so I can control how much solution I need to use.)
You put the camera into manual cleaning mode and remove the lens (you do this in a CLEAN lint-free environment to avoid getting dust particles on the sensor while you're trying to clean it.)
Put 2-3 drops of solution on the cleaning swap and give it a smooth wipe across the sensor... then discard (do not reuse the swabs.)
Tim Campbell
5D III, 5D IV, 60Da
