Focusing question, diopter adjustment and glasses

[petrus]

Hi,
I guess I did not understand the physics of optics quite well, but it seems I have a focusing problem with my DRebel, especially on my long zoom, Canon EF 75-300. First of all, the longer lens I use, the worse is the autofocusing working - on 300 the autofocus seems dead!
So I switch to manual focus instead, and even then I cannot get sharp pictures (using tripod and remote release) in daylight. Maybe the diopter adjustment is wrong, but assuming the glasses I wear are OK, the diopter wheel should be in the "0"-position (middle) - right?
Someone, please give me aclue what I am doing wrong!

Is there maybe even something wrong with the lens?

[WalterK]

. First of all, the longer lens I use, the worse is the autofocusing working - on 300 the autofocus seems dead!

In what way is the AF not working? Is it focusing in front of or behind where you want to focus, or does it not activate? You have to describe the problem better. Also, post an example

Maybe the diopter adjustment is wrong, but assuming the glasses I wear are OK, the diopter wheel should be in the "0"-position (middle) - right?

If you are going to shoot with glasses, you have to adjust the diopter while using the glasses. Put the lens in MF and put it at infinity. Then, put a white sheet of paper close to the lens. Turn the diopter wheel until the focusing rectangles and exposure circle are sharpest. Go back and forth a bit to make sure.

If you can't get them sharp, you'll have to get an auxilliary eyepiece lens (they come in +/- 1, 2 and 3 diopters, I think), which Canon sells for about $12, to use instead of the eyepiece in the camera, or adjust the diopter without your glasses and shoot that way.

[petrus]

Thank you Walter for the advice on calibrating the diopter - it works OK.
Also the autofocus problem has been solved, it was not a problem of the lens but of the polarizer filter I had in front. Taking the filter away resulted in sharp pictures. As a matter of fact nothing in the picture was sharp, not behind, not in front.

Thanks for your good advice!