Well i had noticed a dark blob through the viewfinder when out in the forest the other day, wasnt on the photos so cleared it by whipping the lens off and blowing into the camera up toward the focusing screen..

all was well..

but i decided to clean it properly..

result my mirror is more dusty and having removed and refitted the focusing screen its now filthy as well despite using tweezers.. arghhh lol i need some proper kit to do this kinda stuff.

I have not touched nor will i go near the sensor as its clean and I cant be dealing with that hassle. How long it will stay clean now the mirror isnt remains to be seen.. might be sending this off for a service at this rate!

I'd get it properly serviced mate, not sure where in Dorset you are, but if you are near Bristol Black on White photography are some experts in this kind of stuff...

And next time you want to blow in to your camera, use one of these


http://www.mymemory.co.uk/Camera-Accessories/Giottos/Giottos-Rocket-Air-Large-Camera-Cleaning-Blower

2nded, and for future reference the focusing screen and mirror are FAR more delicate than the sensor. I've seen both utterly destroyed by well meaning attempts to clean them.

The most I do is blow with a good rocket style blower, but I'd take on cleaning the sensor much more happily than the mirror.

At least you didn't do what I did the first week I had my first ever SLR, and blow directly into my camera body with my mouth....

Ok so with some cleaning kit I managed to remove all but 1 spec from the viewfinder sans focusing screen.

As soon as I add the screen though theres a ton of marks, i have cleaned it best i can but even my untrained eye can see that its been damaged arses!!

Still camera works fine, just got a few marks in the view finder, will get a replacement focusing screen (sub £30) and carefully install that, hopefully will remove the worst of the issues.

Mirror looks clean as well.

Also the sensor appears to be clean as well.. theres no obvious dust i can detect on the images i have tested with (blank white)

http://www.ebay.co.uk/itm/Genuine-Canon-Ef-A-Focusing-Screen-EfA-Digital-Camera-EOS-40D-50D-60D-/300626237236?pt=LH_DefaultDomain_3&hash=item45feb85734

Its only a few tiny specs, what harm can you do...