50D viewfinder problem

I've just noticed that there is a problem with the viewfinder in my 50D. It seems to have somewhat of a 'zoom burst' effect on the image as seen through the finder - see the photo below for my rendering of it! I've had the camera for a while now, and it's the first time it has behaved like this. The photos themselves are unaffected; the issue is just within the finder. The camera has never been dropped / abused in the slightest, so I'd be surprised if the prism had come loose or something, but it's a possibility...
Has anyone had a similar problem / know of a solution? It's not a fatal flaw as I can still use the camera reasonably well, but it is getting annoying...

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woah, that would be weird to look through.

have you checked to see if the screen has been bumped out of place?

it may even be a prism issue....

I've checked the focusing screen, and it seems to be in place normally - I couldn't move it with the tip of a rocket blower so it's not loose either. There's no unusual rattling or anything either, just the ususal mirror noise if I gently shake it...

All lenses or just one?

All lenses, it's in the body itself. The photos themselves are fine.

Clean the eyepiece on the viewfinder.
My guess is that it's just sweat/oil from your face that's acumulated over time.
Just give it a wipe and you'll probably find it fixes it.

or very carefully wipe down the mirror with a microfibre soft cloth (the ones that come with eyeglasses are perfect). remember to apply very light pressure. it could only be four things: the eyepiece, the prism, the focusing screen, or the mirror. SOL if its the prism, you will have to send it in to Canon to have that cleaned. Good Luck!!

Viewfinder is clean, so's the mirror.
New development; if I completely turn the dioptre adjustment knob counterclockwise, it rattles when shaken. If it's done up all the way clockwise, it doesn't.
Hmm...