Using A Gray Card For Exposure

There are many people out there that believe in the metering systems in today’s cameras so much that they claim there is no need for gray cards or light meters, and this simply is not true.

A reflected light reading will never be as accurate as an incident light reading. There are more than likely quite a few people that will immediately question the accuracy of the camera used for this article, or if a consumer level camera was used. The sample photos were taken with a Canon 1Ds Mark III that has been calibrated by Canon Professional Services and the images were only altered for white balance. I created this exercise exactly as the procedure should be done. This is the reality of in camera meters, and this is the cheapest, and easiest way to overcome exposure variances (when your intention is proper exposure).

There are many gray cards available on the market today ranging from only a few dollars to prices in the hundreds. The gray card used for this article is made of cardboard, manufactured by Delta and usually costs between two and ten dollars depending on where you buy it.

Note: The viewfinder image used is not the viewfinder of the Canon 1Ds Mark III , but is a digitally modified generic image for the sake of simplicity.

With the camera in manual mode, and the meter set for proper exposure the image is clearly underexposed. There are times when this will work to your advantage for mood and ambiance but there are also times when poor exposure will ruin an otherwise incredible image. (For the record, I do not claim this is an incredible image.)

In the case of this image there are two factors playing against you as a photographer. First off, color will always reflect different amounts of light as color changes. Color is after all, the result of light that was not absorbed by an object being reflected from that object. Secondly, the light reflecting in the glass has a very dramatic effect on the cameras exposure reading. (This is why automotive images will be underexposed very often.)

There is a right and wrong way to do this, in the photo above we are clearly illustrating the wrong way. The idea is to have nothing but gray card in the frame while you are metering, as previously stated, any extra color will reflect light other than the 18% gray. (This includes fingers.) As advanced as today’s cameras are, the metering systems are still referring to the traditional 18% gray. Gray cards are based on 18% gray, that’s why they work.

When properly placed, the gray card should fill the entire frame. In our example you see the meter is now measuring underexposed. (Don’t forget, this only works in manual mode.) While the gray card is filling the viewfinder correct your exposure settings to properly expose the gray card according to the cameras light meter. This will change again when you remove the gray card, but make sure you leave the settings as they are or the process will have to be started all over again.

Take note that the image is properly exposed but the meter is reading overexposure. By using the gray card, you are now taking an incident light reading (the light falling on your subject) as opposed to a reflected light reading (the light coming off of your subject). Keep in mind, proper exposure will change slightly as you move around your subject and it would be wise to meter again if you wander far from your original angle.

One last important factor to remember, as the sun moves throughout the day your exposure will also change, remember to meter every half hour or so if you are shooting for long periods of time.

Have you ever used a gray card? What were your experiences? Please share with us in the comments, and Happy Shooting!

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