When possible I prefer to use incident metering, which I described in my post How (and why) to use a hand-held light meter. But sometimes you’ll want/need to take a spot meter reading. Maybe you’re trying to take a candid image from across the room and it isn’t possible to walk up to your subject for an incident reading. Maybe you don’t have an incident meter and want to use your camera’s built-in spot meter. Maybe the scene is so extremely backlit that you can’t get an accurate matrix/evaluative reading. In any case, here’s how you do it.
When you spot meter correctly you’re actually using Ansel Adams’ Zone System. I’m not going to get into the Zone System in detail because an in-depth understanding of the subject isn’t necessary for portraits. All you need to do is to be able to estimate skin color.
What’s a spot meter?
Your camera probably has a built-in spot meter. My Canon 5D has a 3.5 degree spot meter, which covers the area of the center viewfinder circle. My Sekonic L-758 has a much more accurate 1 degree spot. The spot meter is going to measure only the reading from inside the circle. So if your subject is, for example, heavily backlit and you put the spot on your subject’s skin, the backlit sun will not affect the reading.
Basic spot meter operation
(1) set your camera on manual metering mode, set to spot metering
(2) select your desired aperture
(3) put the imaging circle on your subject’s skin then take your reading. If using your camera’s meter, adjust your shutter speed until your meter is at the center zero point.
(4) take your shot!
Well, there’s a little bit more to it.
The above simple technique will only work perfectly if your subject’s skin tone approximately matches 18% gray, which is what your meter is calibrated for. * Here’s what 18% gray looks like:
* Note: there is some dispute about the 18% gray figure. Some claim that meters are calibrated closer to 12%. It’s a hotly debated topic on the internet for folks who didn’t have a date to the prom.
Your meter is going to assume that it is metering an 18% gray. So if your subject is lighter than 18% gray, it’s going to turn it gray by darkening (underexposing) it. If your subject is darker than 18% gray, it’s going to turn it 18% gray by lightening (overexposing) it.
Adjusting for skin tone
The trick to accurate spot metering is to adjust your exposure based on the brightness/darkness of your subject’s skin. If you take pictures outdoors in snow, you know you’ll have to add +2 exposure, right? Same principle here. Basically, light skin is going to require you to add exposure, dark skin will mean subtracting it. So, for example, a very light-skinned person may require you to meter at +1.5.
Here are some examples of how I would adjust exposure for different skin tones. I may be slightly off, but I’m probably within a half stop or so. Close enough.
Here’s a mother and son with different skin tones.
If you’re struggling to estimate skin tones, imagine Halle Berry. (shouldn’t be too difficult for you guys out there). She makes a perfect +0 neutral reading. Compare your subject’s skin to hers and adjust accordingly. It would be great to take her with me everywhere I go so I could use her to spot meter….
When using my hand-held spot meter, I use the exposure value to adjust my shutter speed. For example, suppose I’m metering a subject I estimate to be a +1 skin tone. I’m shooting at f2.8 and the spot meter says 1/500 sec. To adjust for the +1 skin I would simply set my camera’s shutter speed to give me one extra stop of light (1/250) and voila, perfect metering! If using your camera’s meter, there’s no need to run any calculations in your head, simply adjust your shutter speed until the meter reads +1.
The hand trick
Let’s say you’re in the exact same light as your subject. You’re both under the shade of a tree on an extremely sunny day. If you use your camera’s matrix/evaluative meter, you’re going to get a shutter speed like 1/2000 or something like that, which is going to severely underexpose your subject. In this situation, I would use my hand-held meter for an incident reading. But what if you don’t have a hand-held meter? Just set your camera’s meter to spot and use the procedure above.
But what if you’re not sure how to evaluate your subject’s skin? You can use the old hand trick. With experience and a little testing, you can pinpoint your own skin’s exposure value. Instead of taking a reading off of your subject, just take a spot meter off of your own hand and use that. Example: your hand is a +0.5. Set your camera to manual mode, spot metering. Take the meter reading off of your hand and adjust your shutter speed until the meter reads +5. Easy peasy.
For super accurate reading you can always carry a small 18% gray card with you and use that to meter off of instead of your palm. No need to make any adjustments here – just set your shutter speed until the camera’s meter is at zero and you’re good to go.
Do you like to people-watch for fun? Try this exercise: the next time you do, look at each person who walks by and assign them an exposure value: “she’s a +1, he’s a -0.5,” etc. Kinda geeky, but then I’m kinda geeky…
What about spot metering for landscapes, architecture, travel?
If you Google “zone system” you’ll find lots of resources here, but basically, here’s how I would meter non-portraits:
- medium green grass: +0
- the north blue sky: +0
- average tree bark: +1
- snow: +2
- sunlight reflected off sand: +2
- shadows that you want to have some detail: -2
- dark leaves: -1
That’s it. Add spot metering to your bag of tricks, it will prove useful in many situations.
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