Let’s jump right into the guts of your camera and how it makes exposure decisions.
Metering Modes
What are Metering Modes? Your eye and brain are fantastic at figuring out what is the correct exposure for any scene and adjusting to it. You squint without thinking and in dark situations your pupils dilate to bring in more light (they do that at the eye doctor as well but that’s just because of those wicked drops).
Your camera has to try to do the same thing: judge the amount of light in a situation and set the ISO, Shutter Speed and Aperture appropriately to make a proper exposure.
NOTE: Metering Modes come into play when you are shooting in any Camera Mode other than manual.
The problem is, a camera is not nearly half as cool or as complex as your brain and eyes. So the camera manufacturers have come up with some methods for metering light and trying to figure out the best settings.
Different Modes for Different Situations
For most DSLRs and mirrorless cameras there are three different metering modes in common: Matrix or Evaluative, Center Weighted and Spot. In all of these modes (and others, depending on manufacturer) the camera uses a special sensor to evaluate light hitting it, as if it were hitting the main sensor.
As mentioned before, light is measured in a camera in stops. From one stop to the next is either half as much light or twice as much light. The dynamic range of most cameras, that is, the range of stops captured in a single image, is maybe 8 stops. Cameras improve all the time and now top 13 stops.
The dynamic range of the human eye in a single instance is estimated at 14 stops, with an overall dynamic range of around 25 stops.
As the camera is limited in range (we’ll ignore HDR for now), it has to pick what gets favored for exposure. Maybe a scene has a dynamic range of about 15 stops and your camera is limited to 7 stops. What to do? Metering will pick what’s important and select an exposure based on that. Let’s look at the modes.
The Three Main Metering Modes
Matrix/Evaluative Mode is going to attempt to meter 80% of the scene, give or take a bit. This mode works well for a wide range of subjects especially if there is a lot going on and you aren’t sure where you want to meter. It has its limits as it doesn’t cover the corners (usually not a big deal) and may try to balance a scene with a wide range in a manner not to your liking. This mode is the king of compromise.
Center Weighted (sometimes shown without the spot in the very center) starts to narrow things down a bit. In a lot of cameras the light reading is taken from the center 13% of the frame. A number of cameras even have the ability to change this percent to three different values. Anything not in the general center area will be ignored as far a light metering is concerned.
Spot Metering is handy when you know exactly what you want exposed properly and it is relatively small. Spot metering is a circle of about 2-3% of the frame (cameras will often show an actual center spot in the viewfinder to aid in selecting the area to meter). This can be handy when scenes are back-lit or there is a lot of brightness, contrast and other action going on, but maybe only one element is important to the exposur
Metering Modes in Real Life
Now lets take a look at how these metering modes can affect your image. I have set up a likely scene on a chair using my daughter’s friends Lamoin and Hopster.
In all these scenes I set the ISO to 100 and the aperture to f/8 to hold them constant. Only the shutter speed will change and it will only change because the camera is sensing differences in the light.
It will sense differences in the light because it will use different modes and we’ll start at the top with Matrix or Evaluative.
The shutter was open for two seconds. The camera metered for the entire scene and faced with a tough choice (lots of lights and darks) it picked something it thought was middle ground. If it could think. It can’t. Ha ha ha ha. Only Hopster can think. Ok, enough nervous laughter, next is Center Weighted.
That superimposed oval is about the area the sensor metered (and a good mathematician will tell me it’s far greater than 13%!). You will notice Hopster is much better exposed with only .6 seconds of a shutter speed, but we’re losing Lamoin’s hooves and the chair is getting dark. Still, I like this better than the first. Spot Metering time.
As the spot is only on Hopster’s white ‘fur’, the camera is attempting to exposure properly for it. The camera did a decent job, but you can notice what happened to the rest of the scene. Dark, dark, dark. What happens, though, if we move the spot to the darkest part of the scene, to the area just below Lamoin?
Zoiks! While that one patch of bison fur is exposed well, the 10 second shutter speed makes everything else overexposed. Can we mitigate this a bit?
That’s a bit better as far as Lamoin is concerned. Hopster is still blown out and not the least bit happy.
It is important to note DLSRs are often equipped to favor metering at times. Such as in Matrix Mode. Often the focus points that are locked on a subject will get a slight bump in their overall importance for light metering as the camera knows this is likely the main subject. Others take it a step forward and use face detection software to know where a face is and expose for that, instead of an overall evaluation of the scene.
What About My Phone?
Your phone uses Matrix or Evaluative metering most of the time. It looks at the whole scene and tries to balance the exposure.
Using an iPhone as an example (although most phones work the same), a large rectangle appears to show where the metering and focus are locked, when locked. In this example, the box is closer to center-weighted than true evaluative.
But that makes the leafs a little over exposed. All you have to do to change the area where your phone meters and focus is to tap that spot on the screen. In this case, I want the leafs better exposed, thus I tap them.
That’s better. What happens if I tap the ground to make it better exposed?
Whoa now! The ground is brighter but now the leafs are totally blown out and my Shutter Speed is so slow that things got blurry.
You might also notice how your phone will put a focus and exposure box around someone’s face when it finds one in the scene? That’s because the camera assumes you want the face exposed correctly and 99 times out of 100 that is true.
The point here is you can pick your exposure area in a phone just like you can in a DSLR or mirrorless camera. This “tap on the screen” method is also true for most drones.
The Right Mode for the Right Scene
So which mode is right for which situations? Without being by your side for every shoot you go on, I can make some generalizations:
Matrix/Evaluative Mode
- Good general walk around setting
- The beach
- Midday sun
- Cityscapes
- Lightning
- Bison
- Mountain ranges
- Grand vistas
Center Weighted
- Concert/Band
- Sunsets/Sunrise (to meter and then recompose)
- Group shots
- Waterfalls
- Portraits
- Birds
Spot
- The Moon
- Sports action from afar (where the subject is pretty much filling the center of the lens)
- Back-lit subjects
- Yeti
Wrapping Up Metering Modes
Play around with the modes and get to know them better. You will find times when the main subject is still too bright no mater how much you adjust the exposure compensation (more on that next week).
That is likely because your camera is in Matrix Mode and trying to balance everything. Switch to Center Weighted and chances are you will find things starting to come around. Get more precise with Spot and the control becomes even more finite.
To access the different metering modes, look for a dial or knob with the icons above on it. On some Canons it is a button press then a wheel turn. On Nikons it can be a rotatory dial on the side of the eyepiece.
Next Up: Camera Modes(and how to stop using the green rectangle!!)
Questions? Pop ’em like Pez in the comments section below. or email me at peter@peterwestcarey.com.
Photography Basics – A 43 Day Adventure, and its companion 40 Photography Experiments, are series written by professional photographer Peter West Carey. The series are designed to unravel the mysteries of photography, helping you can take better pictures. Subscribe here to receive all the updates and bonus material. Your comments are always welcome.
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