Most of this information (in particular, the recommendations for adjusting settings) has been summarised from the instructions supplied with a Kodak Gray Card. For complete details, you are advised to refer to material supplied with this card. (There is some additional stuff on metering in dim light and determining lighting ratios. I didn't want to infringe their copyright too much, so left some bits out.)

The gray card can be used with a light meter or the in-camera meter to take reflected light readings for the purpose of determining aperture and shutter speed settings.

Position the card so that it is at an angle of approximately 30 degrees to the main lighting or sun and angled towards the camera at about 15 degrees.

Make sure there are no shadows, coloured reflections or glare on the card.

If you can, position the card near the subject. But always make sure the card is illuminated by the same light as the subject.

Make sure you take the light reading from the actual card. With a light meter, this means getting close enough not to pick up light from beyond the edge of the card. With the 10D, you can see exactly what you are aiming at. It's probably best to set the camera to partial metering, and make sure the centre circle on the focus screen is covered by the card. Take care not to throw any shadows on the card as you are taking the reading.

Adjust the meter readings [See Note 1] as follows:

• For subjects of normal reflectance, increase the measured exposure by half a stop. For example, if the meter reads 1/125 second at f/8, set the camera to 1/90 @ f/8 or 1/125 @ f/6.7. [2]
• For light subjects, use the metered settings. For very light subjects (like a snow scene), decrease camera settings by half a stop, so 1/125 @ f/8 would become 1/180 @ f/8 or 1/125 @ f/9.5 [3]
• If the subject is dark to very dark, increase the camera settings over the meter readings by 1 - 1.5 stops. [4]

Kodak recommends bracketing exposures, if you are uncertain, but, of course, with DSLRs we have the advantage of being able to check the histogram between shots to take out some of the guess work. Unfortunately, there's no easy way to know what "normal reflectance" actually looks like, but another advantage of the digital cameras is that it doesn't cost any money to do a few tests.