Mirror vs. Sensor

I assume if it is only a small piece I should just leave it. And assume on the screen it will NOT show in the picture

You are correct in your assumption.

What I mean is............yup.

What material is the focus screen? some sort of relatively hard plastic or glass? if you use the correct brush, it shouldn't damage it. What reason makes you think it would damage it?

My common sense tells me that a softer material would not scratch a harder material. What does your's say?

FYI, I have moved the mirror with my finger, and my 10D still works fine.

The mirror is just a freakin' mirror that lets you see thru the lens. It not really that sensitive piece of a component in your camera.. Nothing like the sensor, something that you'd want to have minimum contact with..

What is the correct brush? Do you have a specific product in mind?

And to shock all of you alarmists, I have touched the sensor with my finger too, and nothing horrible has happened either.

Seriously, obviously you don't want to be touching these things if at all possible, but no reason to be so freaking paranoid about breaking them. Just be reasonable and be careful, but paranoia about equipment damage will prevent you from full enjoyment of these devices. These cameras were designed to work in the field, after all.

That said, it's time to clean my sensor again -- I have never been able to clean it completely to my satisfaction, I think I'm gonna have to scratch a few specks off with my pinky's nail.

I recommend a chisel and a 28 oz. hammer for that.

Any brush with bristle that is softer than the surface you're cleaning... this answer is obvious.

Cadenza: You actually never touched the sensor. There is a filter that sits on top of the sensor. If you had actually touched the sensor itself, then I think you are asking for troubles. The oils on your finger wouldn't help in the preserving the sensor's performance or longevity. I think you'll agree that touching the mirror is one thing, touching the sensor is another thing. Those two components are not in equal.

So, you recommend cleaning the focus screen with a tooth brush.
I think if you want to apply real common sense, the least invasive procedure, a blower, is what should be recommended as a first step. If that doesn't work, you can wiegh the risk yourself at applying something more aggressive to get the junk out.

I was thinking more like a painter's camel-hair brush. Call it thinking out of the box if you will, but that's my story and I'm stickin' to it!

I have used Q-Tips to clean the dust off of my Viewfinder. I haven't had any problems with it. Normally, I will just use my Rocket Blower.
Rocky

The focusing screen on a 20D is not just a piece of frosted plastic. It's a matrix of extremely tiny lenses (probably no larger then the microlenses in the sensor). It is, indeed, possible for something "softer" than plastic to destroy the extremely tiny edges of those lenses. In fact, even in the old days of manual focusing and relatively course fresnels on the bottoms of focusing screens, it was possible to make horrible scratches across them with a lens brush.

Don't touch the screen with anything solid. Blow on it gently with a blower (not your mouth). Nothing on the focusing screen will affect the image, but if it gets that annoying and won't blow off, send it for service. If you blow too hard, you may blow the stuff above the screen between the screen and the bottom surface of the pentaprism.

The mirror is even more sensitive than the AA filter covering the sensor. It's a front-silvered mirror, and that silver coating is extremely thin (it's partially transparent, after all) and has no protection whatsoever from physical damage. If you get in the habit of putting a brush or any kind of swab to it, you will eventually rub the silvering right off.

Those worried about lifting the mirror with a finger,please settle down and think before freaking out.
The mirror flies up and down,bashing against the top thin foam strip and the bottom hard stops tens of thousands of times and yet i've never heard of one wearing out.
Just be gentle with it and nothing bad will happen

Just be careful not to get any oil from your hand on the front surface and don't force anything, right?