Getting the correct exposure is a little less critical these days with 16 bit RAW files and digital post-production software to provide correction. However they do have their limits. You will not recover image information from burnt out highlights or black shadows. If the scene is so contrasty that you are not going to be able to get both correctly exposed, then you need to use fill flash, graduated filters, or take at least two exposures — one for correct highlights, the other for correct shadow detail. You will need to mount the camera on a tripod so each image is exactly the same. Of course if something is moving in your frame it is not going to reproduce clearly when the too images are overlaid in layers during post-production to create a final image using tonal mapping from the two layers. I am not going to go into tonal mapping, or HDR post-production here, other than to say that it is an option.
Most meters use a program to try to get the correct exposure. This program looks at the whole frame, or at least the metered area of the frame to get a compromise between highlights and shadows. On printed material, it’s claimed that the half way point between black and white reflects 18% of the light. However it is said that meter algorithms use direct light (luminance), rather than reflected light, so these algorithms are not actually based on 18% grey at all, but a much lower number. For more technical details go to this site: http://www.bythom.com/graycards.htm .
Starting at pure white, there are usually 256 tonal shades to black in photography. That is white plus 254 shades of grey plus pure black. This is more than the human eye can resolve.
However it is the subject of an image that requires the correct exposure and your automated metering process has no idea of what the subject is. If the subject takes up most of the frame, or at least the metered area of it and is a quite evenly lit mid-tone, then your meter might do a good job. Just how much of the frame it has to take up before the meter is no longer influenced by the surrounds will vary from one camera make and model to another…they all have their own algorithms to which their programs as based on! What you need to bare in mind here is that using a zoom lens will alter this ratio as well.Where most meters have a problem is when your subject is very white, or black. If your subject occupies a large portion of the frame, or its’ metered area and is very white, then the meter program is going to try to obtain an average across the whole frame, or its’ metered area. If white occupies the larger proportion of the frame it will be reduced to a shade of grey of up to 12% (18% reflected perhaps) down from pure white depending on how much of the metered area it occupies. So if you want white to remain white and not come out grey, you need to do your own exposure compensation. You might have to add up to a stop of extra exposure (+1 Ev) to do this. The larger the portion of the metered area is occupied by your white subject, the more compensation you need to apply. Experimentation is required!
The same applies to a black subject in reverse. The meter program will attempt to lighten the black to 18% grey (reflected) if it takes up most of the metered area and at any portion in between. You then need to compensate with some under exposure to get them back to the correct black. We can only assume that if there is an equal portion of black and white over the metered area that the meter will then be accurate — assuming most algorithms follow this logic.
If your subject is white take a spot meter reading and add +1 EV of compensation as a starting point.
If your subject is black take a spot meter reading and add -1 EV of compensation as a starting point.
If your subject has both black and white components, determine which occupies the larger area and dial in less compensation on the larger mass, to no compensation at all when the two tonal masses are equal.
Remember a zoom lens can change the ratio of subject to background tonal mass as the focal length is changed.