So imagine you are shooting a portrait of a couple with the sun behind them. You adjust your DLSR to meter the exposure to zero. But when you take your shot, you noticed that while the background is exposed correctly, your subjects are underexposed. What’s going on here?
In another scene, if she is now standing under a shade with the background in the sunlight. You want to have the subject properly exposed, but instead your camera exposed for the background again and left your subject in the dark.
Finally, in the last scenario, you are shooting a wide shot of a girl at night with the city skyline behind her. She is with just an ambient light. When you meter for the scene, the dark city night turns into a lighter, murky gray color and now your subject is overexposed. Is something wrong with your camera? Well no.
Ok, so what has been going in all of these instances is that your camera doesn’t know what the most important part of the scene is to meter. For any scene that has high contrast from either backlighting, indoor and outdoor lighting mix, a dark subject against a very light background, or a very light subject against a very dark background, just metering for the entire scene can result in the wrong exposure.
Luckily, there are two ways that you can properly meter your subject without having to break out the lightmeter (which is still a good idea if you have the luxury or the time to do so). Both solutions have their advantages and disadvantages.
Solution: Fill Your Frame with Your Subject or use Spot Metering
Fill Your Frame with Your Subject
The first solution is to simply fill your image frame with your subject by either walking closer to your subject or zooming into your subject. The reason you do this is so your camera can get a proper metering off of your subject only. This way, none of the background will affect the metering of the subject.
At this point, if you are shooting in manual, all you do is set your exposure to zero. Once you got the exposure set, recompose your image again. Your metering will start to tell you that you are now underexposed or overexposed, but you want to ignore because your current exposure setting is based on the subject’s exposure only.
If you are shooting in Aperture or Shutter Priority, you want to lock in the exposure by pressing your camera’s Auto Exposure Lock (AEL) button. The AEL will hold the exposure that is calculated from the subject so that when you go back to the original composition, the exposure will not automatically change.
The disadvantage of this technique is that you have to fill your frame up first. This may not work if your subject is too far away from you and you cannot completely fill up the frame, or if you only have a split second to capture the moment.
Use Spot Metering
The more popular way of exposing for the subject in a high contrast scene is to use your camera’s spot metering. Unlike evaluative/matrix metering ( which meters off the entire scene) or center-weighted metering (which meters of a portion of the center, spot metering only takes a reading from a small portion of the scene, and you get to choose which spot to meter. This is can be a quicker and easier way to meter a high contrast scene if your camera allows you to quickly switch to it.
On my Panasonic GH2, I have a button dedicated to spot metering.
The disadvantage of this method is that because the spot metering is based on one point of the image, your metering can vary quite a bit if your subject is not evenly toned in brightness and if you move your metering from one part of your subject to another. So for the girl or the guy, different parts of their face may meter differently.
In this example below of the monkey, spot metering on the brown fabric versus the red and white lips may yield different metering result, whereas filling in my frame with the monkey and using evaluative metering to get an overall average reading of the monkey may be more accurate.
The great thing about both methods is that you are not limited to only one or the other. When faced with a high-contrast scenario, all you have to do is assess your subject and decide which of the two methods will get you the more accurate metering. You can either fill up the frame with your subject or just use spot metering by itself. With either method, you are telling your camera what is the important element in the scene that needs to be properly exposed.