ACE Camera Photography Magazine

The Tool for Better Exposures
More Uses for a Gray Card, cont.

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Copying flat art and old photos
A gray card is an invaluable tool for determining proper exposure when photographing paintings, artwork, and old photographs. You can see from the example (forgive us Leonardo) how light and dark areas can easily fool the light meter. The answer is to evenly light the artwork, place the gray card against the artwork and take your meter reading from the gray target. Test for even lighting by moving the card around, comparing light meter readings taken from the corners and the center. Adjust the lights until all readings are within 1/3 stop or less.
  gray card helps determine proper exposure for copying paintings and artwork
Zone System photography
The gray card is an essential tool for Zone System photographers. The Zone System was developed by Ansel Adams and is practiced by many black and white photographers who appreciate the exposure control it brings to their craft.

The Zone System divides light into ten steps from black to white, and they are numbered from Zone 0 (absolute black) to Zone IX (absolute white) with Zone V representing medium gray, the same value found on an 18% gray card.

There are books written about the Zone System, photo workshops which teach Zone System techniques and entire web sites devoted to the topic. If you want to learn more, we suggest you start with Cicada's photography resource on the Zone System.

Determine Lighting Ratios
Lighting ratios are very important in studio portraiture and product photography because they reflect how much contrast there will be between important light and dark areas in a picture.

Some contrast is essential to give a three-dimensional quality to a photograph, however too great a difference will exceed the ability of the film to capture both the highlights and shadow detail. For best quality prints, the lighting ratio should not exceed 3:1 for color film and 5:1 for black and white film.

In calculating a lighting ratio, the combined illumination from the main light and the fill light are compared against the illumination provided by the fill light alone. This ratio is expressed as two numbers, as in 2:1. The first number represents the combined strength of the main light and fill light, while the second number represents the fill light alone.

Lighting Ratio Technique
You can use an incident light meter to determine the lighting ratio, or a gray card and your camera light meter can do the same. Let's see how this is done.

determining lighting ratios diagram 1. Read main light
plus fill light

2. Turn off main
light and read
fill light

3. Compare the
difference in
f. stops

Placing the gray card close to the subject, first read the main light plus the fill light, positioning the card for the highest reading on your light meter. Usually this means aiming the gray card at the main light. (Position 1 in the diagram.) Note the reading. Turn off the main light, aim the gray card toward the camera lens (position 2) and read the fill light illumination. Note the reading and calculate the difference in f. stops.

Stops Difference Lighting Ratio Stops Difference Lighting Ratio
2/3 1.5:1 2 4:1
1 2:1 2-1/3 5:1
1-1/3 2.5:1 2-2/3 6:1
1-2/3 3:1 3 8:1

There are many ways to adjust a lighting ratio, including moving one light further or closer to the subject, changing to a higher or lower wattage bulb in one light, using a polished reflector for the main light and a wider matte reflector for the fill, or using a light diffuser over the fill light.

A reference value for comparing light meters
Rarely will two light meters give precisely the same reading at all light levels. This is normal as long as the difference doesn't exceed 1/2 stop. With a gray card and a constant light source, you can compare different cameras and handheld light meters. This may require shooting test rolls of color slides and keeping exposure notes to determine which meter is slightly out of calibration. Mark any exposure compensation needed and tape it on the equipment.

Color slide film is the best tool for checking meter accuracy because it doesn't tolerate exposure errors. If you shoot only color print film, you may never realize your light meter is inaccurate because color print film has a wider latitude for exposure mistakes and the photo lab compensates for over and underexposed negatives.

You need to be aware light meters can be accurate in bright light but out of calibration at low light levels, or vice-versa. When you check light meters against a gray card, do it both outdoors and indoors.

Gray card sources
Several companies make gray cards. Kodak packages two 8x10 and one 5x7 gray card in a clear plastic envelope ($15 street price.) Until recently every gray card I've ever used has been made from coated cardboard. They just don't last very long with normal use, especially when carried around in a camera bag.

Last fall I came across the highly durable, washable polystyrene gray cards made by Fotowand Technic in Germany. Fotowand's white bordered cards have been used to photo-illustrate this article. If you'd like to read more about the distinctive light measuring products available from Fotowand go here.

- Doug Clifford

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Published February 17, 2000