Photographing scenes with no midtone regions is tricker than usual as it is prone to incorrect exposure. By the way, midtone is defined as the region of the scene that displays average tonal values.
A scene with no midtone will have extreme light and dark areas. Under such a condition, direct meter reading will either lead to under or over exposure.
To overcome this problem, we can take a meter reading from the main subject and then apply a level of exposure compensation depending on the subject’s tonal condition.
For example, you can take a meter reading of an area that is lighter than the typical midtone. After taking the reading, we will need to apply positive (eg +1.5) stops exposure to compensate for the light tone.
Another method is to use a grey card. To do that, position the grey card in the same spot where it is in the same light condition as the subject. Take the meter reading from the grey card, instead.
About Using Grey Card
Using Grey cards for light metering is a photography technique.
Typically, light meters are calibrated for mid-tone meter readings. This mid-tone reading is actually 18 % grey. A grey card is simply a piece of card that is 18% grey in colour.
In the absence of midtone in the scene, a grey card can be used to assess and capture the meter reading.
You can do so by simply placing the grey card in front of the subject for it to be used as a midtone.
In the absense of a grey card, the palm can be used as a midtone substitute. However, the palm is usually 1 stop brighter than 18% grey or midtone. Because of that, you will need to adjust (open) the camera’s meter reading by 1 stop.