We've had a lot of emails asking about diopter adjustment - that's how to set up your viewfinder for your eyesight. If you wear glasses I strongly recommend you do this because I know from experience how difficult it is to see the whole image and all it's data when you look through the viewfinder wearing glasses.
Auto focus is very accurate and provided you've got it set up properly you can trust it to get it right. So when the camera beeps to say it's focused and what you see is fuzzy - it's very worrying. By sorting out your diopter adjustment this will never be a problem again.
Diopter adjustment is pretty quick and easy to do. You'll need your camera, lens (I find a focal length of about 70 -100mm is best). It's really important you don't change the distance between your camera and the point of focus when setting your diopter so always use a tripod.
Whether you're long or short sighted, once you've got the diopter set up you won't need to wear your glasses when you look through the viewfinder regardless of how far away the thing you're photographing is. If you're both long and short you'll still need glasses with you so you can see the camera settings for reading the LCD on the back.
Before The 7 Building Blocks of Photography I didn't have any structure and used to delete 95% of my images. I have now learned to look at the qualities of light. I can spot a problem before I press the shutter. Composition has greatly improved and focal length is no longer just for making things bigger. A fantastic course which I definately recommend.