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I've just realised that I have been taking photos with the wrong settings. In the Canon 30D, next to the Mode Dial there are two symbols. One looks like a circle with a line across, which I took as the mark to select a program. I did not see the hidden square next to it, which seems to be the real marker. I have looked in the instruction books, but I have not seen any explanation of what it means or it is used for. Do you know?

On the 300D, the mode dial (and rectangle) are on one side and the mysterious circle with line through it is on the other side - so I've never been confused by it, but at the same time never questioned what it is!
I'm also interested to know what it is. I just looked at the picture of the 30D and see why you are confused :O Did you not notice that the modes weren't responding how they should when you selected them?

I just looked at the picture of the D80, it's there too What could it be? A marking to show where the hinge for the flash is? Now I'm wondering what it is too Anyone can tell me as well so that I can sleep tonight?

I'm pretty sure it marks where the sensor plane is in the camera. Its called the focal plane mark.
You can use it to determine the exact distance from the subject to the sensor. Apparently useful for doing precise macro work.

Murray1006 wrote:I'm pretty sure it marks where the sensor plane is in the camera.

oo thanks!
zafra52 - no shots lost by the wrong mode right?

You need to line up the program you want with the square. The circle with the line though it is the focal plane marker. Canon probably should have put the marker somewhere a little further from the mode dial to avoid confusion.

I have taken half decent pictures... but I did not see the half-hidden square till tonight. No wonder I had problems! I blamed the quality of the lens and my lack of familiarity with the camera. You see the 30D is the first SLR I ever had, except for an old Russian Zenith that I used 30+ years ago and sold recently in a garage sale. So, I better start all over again and take more pictures to see what I come out with. Thank you all for your help. I feel such a fool...

Thank you all! Today I've taken the camera for a walk and also my glasses. I took care that the Av setting in Dial Mode was next to the square semi-hidden symbol and I can honestly say it is the first time all lenses behaved like well behaved children, as they should - no erratic behaviour. So, I will keep practising and again thank you all for your help.

From the circle with a strike through line, you measure it with a line parallel to len's axis. That is your real focus distance.

Say, your lens focus down to 30cm, it is 15cm long from the sensor, so your object can be placed just 15cm in front of your lens front element.

Thanks Yi-P. I looked in the manual and the website to learn what it meant, but to no avail.