GREY CARD - INSTRUCTIONS FOR USE
When a reflected light meter (such as the TTL meter in an SLR camera) measures light prior to exposure, it mixes the light values from the whole scene and then makes an "average" reading.
Centre weighted and more advanced selective metering systems bias this average in many ways but the result is still an average - one exposure value. In order to make a good exposure it must then use this value to place the exposure in the centre of the light levels acceptable to the film according to its ISO sensitivity rating. Making the exposure central ensures that light objects burn out the film producing white and black objects have insufficient reflectance to register producing black. The mid tones fall in between these extremes giving a full picture.
in order to place the exposure in the centre of the film latitude the camera has presumed that all scenes, once averaged, will produce an average tone, a mid point between black and white. Since most scenes fall quite close to this assumption the majority of exposures will be correct, or nearly correct. Modern film has more exposure latitude than is necessary simply to record a full range of tones, so slightly non average scenes are still reproduced acceptably.
The problems start when a scene is more than a little non average. Snow scenes are averagely white but presumed to be averagely grey by the camera and are underexposed. This makes the whites look grey, detail in darker areas is lost and colours are weak.
Darker than average scenes are likewise presumed average and thus over-exposed. This causes colours to bleed, destroying definition, more noticeably blacks are grey and highlights are burned out.
You cannot compensate for your camera's inaccuracies to match the scene unless you know how far from the average the scene is. This is very difficult thing to judge so the answer is to give the camera what it expects to see, the prevailing light falling on your scene bounced off a surface which reflects 18 % of light in all colours, ie; an average reflectance, average grey. A Grey Card!
a) Place or hold your Grey Card in front of your subject.
b) Angle it to reflect the light falling on your subject towards the camera.
c) Adjust the angle to eliminate any specular reflections on the card, ie; if the card looks shiny from the measuring point adjust it slightly to eliminate the shine.
d) Measure the light reflected from the card. Make sure that you read only the card. If you are using your cameras TTL meter, ensure that the card fills the frame in your view-finder when you measure. If you are using a hand held light meter hold the meter close to the card. Most meters have a viewing angle of about 300 so you should measure at 300mm (12") from the Grey Card.
Take care not to cast a shadow onto the Grey Card as this will make the reading meaningless.
3. Remote Light Readings
It is not always necessary to make your light measurement next to your subject. If you are shooting from a distance and the light is the same where you are standing as the light falling on the subject, take a reading from the Grey Card where you are standing.4. Reflectance Compensation
A Grey Card reading will be completely accurate for 95% of subjects. However because this system of measurement presumes average reflectance, subjects of very high or very low reflectance may be incorrectly read.
If your subject is very dark and largely non reflective (eg black velvet) increase exposure by ' - 1 stop.
If your subject is very light and highly reflective eg polished metallic surfaces, decrease your exposure by ' - 1 stop.
5. White side readings
The reverse side of the Grey Card is white. This side reflects 90% of all light and gives a reading 2 stops above that of the grey side (ie; 2 stops over-exposure). This can be useful in low light situations where your light meter may not be so accurate. The method is the same as for the grey side except you must take 2 stops off the reading (ie; give the photo 2 stops more exposure than the reading given).
6. Setting the white balance (Video)
The white side of the Grey Card can also be used for colour calibration. Setting the white balance on a video camera (where this feature is adjustable) can be done thus;
a) Set the indoor/outdoor switch to the appropriate position for the lighting conditions.
b) Point the camera at the white side of the card using the same method and precautions as for the Grey Card reading (see steps 1 - 4).
c) Press the white balance button whilst the camera is pointed at the white card. The camera will usually confirm that the white balance is set in the view-finder.
7. The Grey Card as a Colour Printing Aid
The Jessop Grey Card is an invaluable constant, that can be used in many ways for colour printing. If you have a colour analyser the Grey Card can be used to create an average negative to be used to program the analyser. Simply photograph the Grey Card (full frame) under the same lighting conditions as the subject of your film. This is simplest in the studio as only one frame is necessary per lighting set up. Your average negative (once processed) is then used to program your colour analyser.
If you are printing 'by eye' then the average negative can still be useful but a picture of your subject with the Grey Card included is often better. Because the Grey Card reflects all colours equally it effectively illustrates the colour bias of the light and the film. Comparing the Grey Card (in the photo) with the subject should give you a very effective 'fix' on the colour bias making corrective filtration much easier.