Controlling exposure: how exposure metering works
In the second part of our series on controlling exposure we’re looking at how exposure metering works. Understanding your camera’s exposure meter is essential to understanding how your camera works and then moving on to shoot in manual mode.
But let’s not jump in at the deep end yet. Let’s first understand how exposure metering works and why you need to know it.
If you missed part 1, you can find it here – Understanding the exposure indicator
What is meant by exposure metering?
When you measure the exposure of a scene, you are measuring the available light. If the scene is not measured accurately it will lead to an under or over exposed photograph.
There are two ways of measuring light:
- a handheld light meter
- your camera’s inbuilt metering system
In this article we’re specifically looking at how a camera meters light, which is the light reflected off the subject.
You can use the camera’s metering system when in program mode, aperture priority shutter priority and of course manual mode.
Exposure metering modes
There are three exposure metering modes for Nikon and four for Canon. The symbols for the different metering modes vary between brands, as do the names. I’ve listed the metering modes for Canon and Nikon,
- Spot metering
- Partial metering (Canon only)
- Centre-weighted metering
- Matrix or evaluative metering
What’s the difference between the metering modes?
Meters the brightness of a specific area that you’ve selected, which is about 2 – 4% of the scene.
Use this when the light is coming from behind your subject, like when you’re indoors photographing a subject in front of a window. Your background may be overexposed, but your subject will be correctly exposed.
Partial metering (Canon only)
Meters 8 – 13% of the center of the scene.
This, like spot metering, is really useful when your subject is darker than the background, such as when your subject is backlit.
Meters the area around the center of the scene, which is about 60 – 80% of the scene, regardless of your focus point.
Matrix metering (Nikon) or evaluative metering (Canon)
Meters the overall brightness of the entire scene, so is ideal when the scene is lit from the front.
Learning when to use each of these types of metering will make a big difference to your photography.
What’s the point of different metering modes?
If the camera knows what part of the scene you want correctly exposed, it will be more able to do what you want it to do.
Things to watch out for
Because your camera measures reflected light and is designed to average the exposure out to an average midtone (that’s medium gray if you’re thinking in black and white), it will struggle when the scene is either dark or light.
Easy to meter scenarios are scenes without much contrast between light areas and dark areas, such as a brick wall, your back garden on an overcast day, a green landscape. Anything that is evenly lit and doesn’t have huge variations between lights and darks.
Because your camera measures the scene to expose it at an average tone, when faced with a dark scene, it might overexpose the scene as it tries to lighten the dark tones.
On the other hand, if the scene is very light, such as a field of snow or a white building, your camera will read this as very bright and will underexpose the scene.
Knowing this allows you to take control when faced with challenging situations for your camera’s exposure meter.
You have three choices to help your camera measure a scene correctly:
- Meter off a midtone within the scene
- Meter off a gray card
- Use exposure compensation
We’ll look at the first two below, but will look at exposure compensation in part 3 of this series on controlling exposure.
Where to find midtones in a scene
Grass, gray wall, skin (as long as it’s not too fair).
How to meter off a midtone in a scene:
- Select spot metering on your camera
- Point it at the midtone area to assess the exposure
- Adjust your ISO, aperture and shutter speed as needed
- Start shooting
What is a gray card and how to use it?
Gray cards are exactly that – medium gray non reflective material.
They don’t cost much and can be bought from from a camera dealer, or online. I use a Lastolite fold away gray card that looks like a really small pop up gray reflector (it’s the Lastolite EzyBalance 12 inch – 18% Grey / White).
How to use a gray card if you’re photographing a person:
- ask them to hold up the gray card,
- aim your camera at the grey card.
- Put your camera into manual mode,
- meter the exposure,
- adjust ISO, aperture and shutter speed,
- focus and start shooting.
If the light changes, or you change your position, meter again, but otherwise you’re good to go.
How to use a gray card if you’re photographing a scene without a person in it:
- position the gray card in the scene facing you,
- meter off it as above,
- remove it from the scene and start shooting.
Further reading: How to use a gray card to meter exposure
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If you have any questions about exposure metering, let us know in the comments.
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Missed Part 1? Here it is…
Read Part 1 of Controlling Exposure – Understanding the exposure indicator
Part 3 – How and when to use exposure compensation
The exposure metering system on your DSLR camera can be fooled, such as when shooting in snow.
We look at the different situations that will confuse your camera and what you can do about it. This is exposure compensation.
Part 4 – How to control exposure in all shooting modes
Maybe you want a faster shutter speed or shallower depth of field?
The camera doesn’t know how to achieve creative results, it only knows how to give an accurate average exposure reading for the area that’s being metered.
We look at when and how to use program mode, aperture priority, shutter priority and manual mode for creative results and controlled exposure.
Part 5 – Pros, cons and how to use auto ISO
Sometimes there’s so much going on that you need a little auto help. We look at the benefits and limitations of using auto ISO – even in manual mode!