Yesterday’s Topic: ISO
It’s time for Metering Modes!
What are Metering Modes? Your eye and brain and awesome at figuring out what is the correct exposure for any scene and adjusting to it. You squint without thinking and in dark situations your eyes 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.
The only 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.
For most DSLRs 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 stop they can be captured in a single image, is maybe 7 stops. They are improving all the time. The dynamic range of the human eye in a single instance is estimated to be around 14 stops, with an overall dynamic range of around 25 stops.
As the camera is limited in range, 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.
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 settings depending on your preference. 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’s 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 exposure.
Now lets take a look at how these metering modes can affect a scene. 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’s chest (don’t worry, he’s a guy bison, he won’t mind)?
Zoiks! While that one patch of bison fur is exposed well, the 10 second shutter speed is making most 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.
So which mode is right for which situations? Without being by your side for every shoot you go on, I can make some generalizations:
- Good general walk around setting
- The beach
- Midday sun
- Mountain ranges
- Grand vistas
- Sunsets/Sunrise (to meter and then recompose)
- Group shots
- The Moon
- Sports action from afar (where the subject is pretty much filling the center of the lens)
- Back-lit subjects
Play around with the modes and get to know them better. You will find times when maybe the main subject is still too bright no mater how much you adjust the exposure compensation and 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 exact 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!!)
31 Days To Better Photography is a series written by professional photographer Peter West Carey on The Carey Adventures.Com. The series is designed to unravel the mysteries of photography so you can take better pictures. Subscribe here to receive all the updates and bonus material. Your comments are always welcome.