The original EOS 5D introduced the C mode to Canon DSLR cameras, and recent models such as the EOS 40D, EOS 50D and EOS 5D Mark II have included Custom modes, but few people use them to full effect.

There's a small 'wrinkle' with the C modes as they currently stand. The C mode registration takes a snapshot of the camera configuration including custom functions and actually quite a lot of other things.

First a common story of C mode usage is that of a wedding photographer who shoots a wedding in a church. Before entering the church for the ceremony he needs to set the camera to get shots of the guests and bride arriving so he sets the camera for a 'hopefully' brightly lit outdoor scene. At this point store the settings to a C mode. Now he enters the church to find it's lit by candles at the brides request and the stained glass window is blocking more light then an ND16 filter and turning it all a weird colour temperature. So indoors he needs a whole different low light setup which he sets on his camera in the normal way. So once the ceremony is over he's walking backwards through the church door, and simply switches to the stored C mode once he's outside to carry on shooting with his outdoor bright light setup.

If the tog had stored Av mode in his C mode then the camera will also store the aperture. Of course he can change it in the normal way, but if he is organising the formal groups and the camera powers down it will revert to the stored aperture once he touches the shutter, not the last aperture he was using so now all the groups are shot at f/2.8 instead of f/16! The best fix is to set the camera power down timer to off or at least say 15 minutes before storing the C modes. This setting is stored as part of the C mode registration. Now in C mode the camera won't power down, or will do so after 15 minutes, and he can make changes as needed without it sleeping. However use another mode and the camera reverts to it's battery miser ways and powers down after just one minute.

There is more you can do, a colleague bought an EOS 40D to shoot some indoor martial arts. It was fast, recommended for the job and fitted the budget. Only issue was that it seemed too slow and delivered poor results in the hands of the inexperienced photographer. He talked with a knowledgeable photographer about camera settings. The 'knowledgeable one' made several changes to the camera setting with high speed continuous shooting, AI servo A, small JPEG (for a website only), Tv mode, ISO auto and stored them in C1. Then the same settings with a custom function set to also allow safety shift in Tv/Av mode and this stored in C2. Now the martial arts photographer can quickly call back the wisdom of the 'knowledgeable one' directly on the mode dial.