Far and away, the most important accessory in the studio (even in available light situations) is your handheld light meter. Even more important is the ability to read it properly and know how the numbers affect your images.

F-stops are symmetrical and mathematical measurements of light. If we begin with an aperture of f8, for example, “opening up” the lens by one stop, to f5.6, will double the amount of light reaching the sensor. Conversely, “stopping down” the lens one stop, to f 11, will cut the amount of light reaching the sensor by half.

When we power the light to a “whole” stop – f2.8, 4, 5.6, 8, 11, 16 or 22 – we’ll only see a zero next to the f-stop number and we’ll know that if we set the camera’s aperture to that number, f5.6 in this case, the exposure will be right on the money.

Digital camera apertures can be set in thirds of stops, which gives you the opportunity to be extremely accurate when powering your lights for a particular effect. They are shown on your light meter as an extra number just to the right of the primary f-stop number, such as this reading, f5.6.6. This reading means that the light falling on the subject is 6/10s of a stop stronger (brighter) than f5.6 itself. If you ignore that extra .6 you will overexpose your images, something you definitely don’t want to do.

So, to be more accurate, you should set your camera to f7.1 (which is actually f5.6 +7/10s of a stop more). To be completely accurate, and get the most correct exposure, either move the key light slightly closer to the subject, move the subject slightly closer to the key, or power the key light up, slightly, to a light meter reading of f5.6.7 (f7.1 on the camera). If your meter is calibrated your exposures will be right on the money.

While you are probably aware of the whole stop numbers, you may not understand the new, 1/3 stop numbers and how to translate those easily from your light meter to your camera. Here’s a chart that shows the correlation:

If the meter says: f2.8 f2.8.3 f2.8.7 f4 f4.3 f4.7 f5.6 f5.6.3 f5.6.7 f8

Set the camera to: f2.8 f3.2 f3.5 f4 f4.5 f5 f5.6 f6.3 f7.1 f8

f8.3 f8.7 f11 f11.3 f11.7 f16 f16.3 f16.7 f22

f9 f10 f11 f13 f14 f16 f18 f20 f22

When working outdoors, you won’t have the control over tenths of f-stops as you would in the studio. When that happens, if the reading is +1/10th over a target f-stop, set the aperture to the lower value. In other words, if the meter reads f8.1 set the camera’s aperture to f8. If the reading is, say, f8.2, set the aperture to the next highest value, f9. In either case, the files as they are will be close enough to go straight to proofs and easy to tweak to a more perfect exposure with Photoshop.