Have you ever wondered how the camera is able to give you a proper setting and exposure in the first place? What standard or scale is it using to do that?
Our digital cameras have built-in light meters that measure the amount of light that comes through the lens that is reflects off the subject you’re photographing. It’s called a reflective metering system. When you focus on a subject, the camera is also measuring(metering) how much light is reflecting off that subject and calculates what it believes to be the proper exposure settings(Aperture, Shutter Speed and ISO, if you’re using auto ISO).
I won’t get into details on different meting modes, because they are well explained in your camera instruction manuals. Plus each camera companies have come up with different names for different meting modes, so trying to explain it here could get confusing.
The main difference in each metering modes is basically the difference in the amount of area on the subject the camera uses to calculate the amount of light reflecting off.
For example, 3D Matrix Metering(on Nikon cameras) or Evaluative Metering(for Canon cameras) are the default metering modes which meters based on a wide area of the scene in your frame. It works well in most situations.
Spot metering on the other hand, meters from a small area of wherever you set the focus point on the subject. So if you want a specific point of the subject properly exposed, spot metering may give you a better reading.
It’s helpful to know the different mode and for you to experiment. But you might be thinking, “Another thing to think about?” In all honesty, I don’t change my metering mode when I shoot and use Evaluative Metering most of the time. However, I understand the following information that I’m about to share with you, so I know when to dial up or down and adjust my exposure. That comes with studying and practice, so read the rest of this article and go practice!
I really don’t like getting too technical, but let me just share this information about neutral grey. I believe it’ll help you understand how camera meters.
Cameras can’t read color. Cameras set their exposure by reading the amount of light reflecting off subjects.
Camera makers are smarter than me, so after much research(I’m sure), they decided that the amount of light reflecting off neutral grey color is a good base and standard to calculate exposures. Some call it middle grey, it’s also know as 18% grey, but some say, it’s not 18%, it’s 12% grey…ahhhh!?! See why I don’t like to get too technical? Anyhow, it turns out the neutral grey works. We end up with proper exposures…most of the time.
There are some situations when the camera gets tricked and won’t give you an exposure you desire. Most commonly our cameras has trouble when we photograph dark or white subjects, or when the background is brighter than the subject.
PHOTOGRAPHING BLACK(DARK) OBJECTS
If you’re photographing a black or dark object, not much light reflects off of it because black absorbs light. Since the camera bases its exposure off of light reflecting off of neutral grey(which reflects more light than black), the only thing the camera is noticing is that not as much light is reflecting off as neutral grey would. So the camera adjusts its setting to brighten up the scene, which results in an over exposed image.
Here are set of photos of chocolate chips(mmm…chocolate…) in a bowl, taken in Aperture Priority mode. Notice how in each metering mode, the initial exposure set by the camera(represented as 0 EV) are all overexposed. Some more than others. I used exposure compensation to dial down toward -, for better exposure.
PHOTOGRAPHING WHITE(LIGHT) OBJECTS
And when photographing a white object, more light reflects off of white than it would on neutral grey. Again, the camera sets its exposure based on light reflecting off of neutral grey. So even when more light reflects off of white, it still sets the exposure as if light was reflecting off of neutral grey, hence giving you a darker exposure setting. The camera will make the white look like grey.
Here are sample photos that will help you understand this better. Here are set of pictures of white salt in a bowl. I used Aperture Priority mode, ISO at 800, and Aperture at f/2.0. I focused on the salt, which means the camera is also metering off the white salt. Notice how in each metering mode, the initial exposure set by the camera(represented as 0 EV) are all underexposed. Some more than others. I used the exposure compensation by dialing up the exposure toward +, for better exposure.
PHOTOGRAPHING WHEN THE BACKGROUND IS BRIGHTER THAN YOUR SUBJECT
If the subject or object you’re photographing is backlit, where the background is brighter than the subject, the camera will likely end up giving you a silhouette image. It’s fine if you wanted a silhouette image, but if you wanted the subject to be visible, you have to use exposure compensation to brighten up the subject.
The more accurate way to do this is to use spot metering mode. If you focus on the subject using spot metering, it’ll expose your image based on the subject.
Regardless of how the scene is lit, spot meter measures light from wherever your focus point is. So focus on your subject you want properly exposed, then take your photo. You would want to use this also for tip #3 on my article, “3 tips on taking pictures in the sun“.
This also works on your smart phone. Where you tap on your phone camera screen is where it will focus but also where it will meter from.
Here are some sample photos of a monster truck backlit by window light. I used different metering modes to show you the different results. Notice how the initial exposure of the monster truck set by the camera(represented as 0 EV) are underexposed. There’s just light coming from the window behind, so the camera can’t accurately meter. The spot metering mode, however, comes closes to a proper exposure.
That was a long post but here are my 3 main concepts I want you to walk away with:
1. If you’re taking pictures of a subject darker than middle grey, or brighter than middle grey, your camera won’t accurately meter the scene.
2. When photographing a black/dark subject, remember that you’d have to change the setting to darken the image(exposure indicator below -0).
3. When photographing a white/light subject, remember that you’d have to change the setting to brighten the image(exposure indicator above +0).
If you’re using Program(P), Aperture Priority(A or Av), Shutter Priority(S, Tv) mode on your camera, you can use exposure compensation to adjust the exposure.