Camera Basics #7: Metering
The metering function measures the brightness of a subject and decides how much exposure is best for the photo. Let’s take a look at each metering mode available, and get a better idea of which of them to use is best to use under which conditions/scene. (Reported by Tomoko Suzuki)
The metering function serves to measure the brightness of a subject
- Evaluative metering can be used for almost all scenes.
- Spot metering is most effective when there is a particular part of the subject that you want correctly exposed.
- Each metering mode has a set of scenes/shooting conditions that it works best in. Consider this when selecting the mode.
Digital cameras have an automatic exposure (AE) feature that automatically decides how much to expose the image (i.e., how bright the shot will be).
In an AE mode, when you half-press the shutter button, the camera will automatically determine the aperture and/or shutter speed values, and therefore provide the (camera-determined) correct exposure. The feature that helps the camera to determine which aperture and shutter speed to set does so by measuring the brightness of the subject, and this action is called “metering”. There are usually 3 metering modes in the camera: evaluative metering, spot metering and centre-weighted metering.
On most cameras, the default mode is evaluative metering, as it carries out metering across the entire image and therefore suits all sorts of scenes and subject conditions. Spot metering, on the other hand, is effective when you just want to make sure that a specific area of your image is exposed properly. Meanwhile, centre-weighted metering measures the light in the centre area of the image frame, and therefore works best when your main subject or interest is in the centre of the frame.
Let’s take a closer look at each metering mode.
Benefits: Versatile; seldom gives the wrong exposure.
Drawbacks: Doesn’t work well for scenes where one area differs greatly in brightness level from the rest.
EOS 5D Mark III/ EF24-105mm f/4L IS USM/ FL: 88mm/ Shutter-priority AE (f/11, 1/4 sec, EV+0.3)/ ISO 100/ WB: Auto
In evaluative metering, the camera breaks the image frame down into a number of different zones, measures the amount of light in each zone, and then analyses the findings to determine the optimal exposure.
Benefits: Handles scenes with stark differences in brightness level well.
Drawbacks: Metering is carried out only in a very small area, so any mistakes in selecting this area could easily lead to incorrect exposure for the entire image.
EOS 60D/ EF-S18-135mm f/3.5-5.6 IS/ FL: 135mm (216mm equivalent)/ Aperture-priority AE (f/5.6, 1/250 sec, EV-0.3)/ ISO 100/ WB: Daylight
Spot metering can only measure light in an extremely limited area in the centre of the image frame. However, it’s the best mode to use for scenes with areas that differ greatly in brightness level, such as backlit scenes.
Benefits: Prioritizes the centre area, but in the process ensures that the rest of the image is properly exposed.
Drawbacks: Not effective for small subjects.
EOS 5D Mark III/ EF50mm f/1.4 USM/ FL: 50mm/ Aperture-priority AE (f/2, 1/60 sec, EV±0)/ ISO 100/ WB: Daylight
Centre-weighted metering measures the light across the entire image, but focuses mainly on the centre area. The exposure of the entire image depends on the subjects in and around the centre of the frame.
Link this to the idea of “metering”
Keyword: AE Lock
When you press the AE lock button, it “locks” the exposure settings so that the aperture and/or shutter speed settings won’t change even if you were to shift or adjust your image composition, refocus and shoot. You can use it if there is a large difference in brightness level between major elements in your picture, or if you are unable to obtain the exposure you want.
AE Lock is a very convenient function to use especially in combination with spot metering, and particularly for backlit scenes. For example, if you have a backlit scene where the main subject appears dark, all you need to do is to align the centre AF frame with the subject, half-press the shutter button, and then press the AE lock button, and the exposure settings will be locked to the correct exposure for the area that you want to capture properly.
EOS 5D Mark III/ EF24-105mm f/3.5-5.6 IS STM/ FL: 32mm/ Aperture-priority AE (f/6.3, 1/40 sec, EV+0.3)/ ISO 160/ WB: Manual
A scene with very bright and very dark areas. Depending on which area you want to achieve correct exposure for, carry out spot metering on either Position A or B.
Position A: For correct exposure in the bright areas, use AE lock on the scenery outdoors
As spot metering was carried out on the bright scenery outside the window, the fan in front has become black.
EOS 5D Mark III/ EF24-105mm f/3.5-5.6 IS STM/ FL: 105mm/ Aperture-priority AE (f/6.3, 1/125 sec, EV+0.3)/ ISO 250/ WB: Daylight
Position B: For correct exposure for the dark areas, use AE lock on the fan
As spot metering was carried out using the fan, the scenery outdoors is overexposed and blown out.
EOS 5D Mark III/ EF24-105mm f/3.5-5.6 IS STM/ FL: 105mm/ Aperture-priority AE (f/8, 1/125 sec, EV+0.3)/ ISO 2500/ WB: Daylight
How to use AE Lock
After you have half-pressed the shutter button and established focus, press the AE lock button (circled in red). When you want to resume metering, press the AE lock button again.
For more about AE Lock, check out:
Camera FAQ #9: What Kind of Scenes Should I Use AE Lock For?
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Digital Camera Magazine
A monthly magazine that believes that enjoyment of photography will increase the more one learns about camera functions. It delivers news on the latest cameras and features and regularly introduces various photography techniques.
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After graduating from the Tokyo Polytechnic University Junior College, Suzuki joined an advertisement production firm. She has also worked as an assistant to photographers including Kirito Yanase, and specializes in commercial shoots for apparels and cosmetic products. She now works as a studio photographer for an apparel manufacturer.