20D front and back focus?

HI all,

Can anyone tell me what the front and back focus are. I have read a little but not sure how to do it. Also can anyone recommend a good focus screen. I have managed to scratch mine. Don't ask. Let's just say don't ever use q-tip to clean screen!!!

Thanks Britt

if you autofocus on a subject with a very shallow depth of field e.g. tip of someone's nose, back focus means that your depth of field would stretch behind where the AF landed - so up the nose towards the eyes would become sharper. front focus, therefore, means the area in front of the AF selected point will become sharper.
basically its a way of define where abouts your range of depth of field is when you take a picture. obviously too much front or back means you'll get consistently out of focus images and could be a result of the AF sensor in the camera or a problem with the lens...
hope that sort of makes sense.

Britt,

Here's something that might help you: http://md.co.za/d70/ch​art.html It's regarding the Nikon D70, but it explains some detail about what it is and how to check it.

As he states in his article: "There are many things that can cause this problem and almost all of them can be chalked up to operator error. Occasionally though, the camera and/or lens is to blame and that’s what this test is for."

Be careful Britt,

You have some nice lenses, remember that at close focus distances with you lens wide open the depth of field is tiny. I did a test with a 24-70L wide open and was confused to see nothing in focus. Until I did the dof calculation, only to find out the shot had 0.01 in of DOF. So I suggest starting with f/4.

dof calculator
http://www.dofmaster.c​om/dofjs.html

A simple way to do this without all the fancy targets is to set down a ruler long way (point at camera) and then set a cereal box front edge at 6". Focus on the box front edge, take the picture. Oh put camera on tripod, use mirror lockup.

If the sharpest focus point is in front of the box edge, its front focused and of course if it is in back of the box edge it is bask focused.

Britt - Here is a link to a good test to see if you have a focus problem.
http://www.canon-dslr.com …/Canon_SLR_Focu​s_Test.htm
I was not happy with the quality of picture I was getting from my 20D and 100-400L. Yesterday I did this test and it clearly showed that I had a back focus problem. Yesterday I also packed up my 20D and 100-400L and sent them back to Canon to be calibrated.

thanks for all the suggestions and tips. Irondad. I did yours and seems like everything is dead on. Thanks so much. Learned something new. In the photography world that is always a great feeling. You know your that much better at achieving a great photo

Britt,
If you havn't noticed any problems with your pictures, don't worry about this front back focus stuff.

As IronDad says, be carefull. All this hype of bad focus can get us over sensitive and expecting what is not realistic. I fell in this trap when I first got my 10D. I sent it to Canon for focus problems. I got it back and I could not tell any improvement. So called them and they said if I wasn't happy to send it to them again and they would have a senior tech check it out special. So I did and when I got it back, it was still doing the same thing.

I later lerned what to expect from auto focusing and how to use it within its capability, pay attention to depth of field and my poorly focused images diminished, to very few. Also veiwing the pictures at realistic magnifications on the computer is important in evaluating results. We can look at a crop area at a magnification that is equal to a print of a few feet wide. We rarely would expect a billboard to be as sharp as an 8 x 10, or 20 x 30 either.

IronDad, neat link. Thanks for sharing