Metering & Exposure Guide

Gaining correct exposure is critical to shooting inspirational photography. Get it wrong and it’ll ruin the shot; get it right and you can capture the essence of a moment, or even add your own mood to that moment.

So, how do you gain correct exposure? What contributes towards it? Does the camera get it right every time? And, what can you do to better control an exposure?

Let’s explore each of these questions and hopefully give you a better insight into metering and gaining that all important “correct exposure”.

Well, “correct exposure” in itself needs to be clarified first.

 

What is “correct exposure”?

A correctly exposed image means that the right amount of light has reached the image sensor. However, what is “the right amount of light”? Well, you haven’t underexposed or overexposed the image, but also, you have captured either the scene as you saw it, or as you desired it to be captured. The latter is not necessarily the same thing as the former.  For example, you may for example be trying to achieve a silhouette, in which case the correct exposure is probably based on the sky or a bright light source. Or maybe you want to capture a couple having a candlelit romantic meal with some real atmosphere, in which case you’ll meter on the candle, and probably underexpose the surroundings to bring focus to the subject.

The amount of exposure depends on what you want to emphasize in the scene. So, “correct exposure” is correct in the eye of the beholder – i.e. you. Does the exposure match what you are aiming to achieve with this shot? From the outset you need to decide what it is that you are trying to achieve. How are you composing the shot? What’s the subject? Which area (if any) of the scene needs to be correctly metered to achieve your desired result?

It’s important to remember that there isn’t necessarily a bad exposure. It’s a matter of taste.

 

Exposure basics

If you need some help with exposure basics & implications/complications of making changes to shutter speed and/or aperture, have a read of this exposure tutorial on photoxels or this exposure article on Cambridge in Colour.

 

The meter

Every photographer needs to learn three photography basics; aperture, shutter speed and ISO, as each is a building block of exposure – combining together to make the “exposure triangle”. Your DSLR’s exposure meter brings these elements together. The meter measures the light reflected from the scene you’re pointing your lens at, and selects a combination of aperture, shutter speed and ISO that it determines will create a balanced exposure.

It sounds fool-proof doesn’t it? But actually it’s far from it since cameras are very easily fooled. That’s why a sound knowledge of photography basics will help you take control of your DSLR, stop wasting pictures and get correct exposure more often.

Most meters work on the assumption that a scene should be exposed as if it’s 18% midtone grey. The reason being that it is generally considered that most scenes will reflect 18% of the light that falls on them.

However, you’re in control of that meter and you can determine how it takes it’s measurements.

 

Does the camera get metering right every time & give you a correct exposure?

Some scenes you shoot will be metered perfectly fine, such as a church under a clear blue sky. However, the brightness of a scene can vary enormously across the picture. For example, the sky will usually be much lighter than the foreground, and typically an average reading is needed for a scene like this.

There are many scenes that will confuse the camera and trick it into an inaccurate exposure reading. In reality the world is not full of mid-tones. For example, a bright white winter landscape will cause the camera to underexpose; it’ll see all the white brightness and try to make that 18% grey. The snow therefore will not be white – but grey. Also, let’s dark a dark interior within a building. Instead of the shot being dark, with furniture/surroundings being black or dark brown or whatever, the camera will overexpose and try to lighten things up to 18% grey; therefore things will come out grey when they should be darker. You need to be aware of what your shooting and take control by assessing the scene, checking the calculated exposure, checking the histogram on your first few shots and then making changes to the camera’s settings. So you need to learn how to read a histogram. Learn what to expect in a histogram for a shot that is over or under-exposed and also learn what histogram you should be seeing for various scenes. This is covered below. We’ve also mentioned changing camera settings. So what settings can you change that will impact metering? Well, you could change metering mode, change where you meter from within the scene, and also set something called exposure compensation. These are all covered below too. Arriving at the perfect exposure isn’t made easier by the fact that the key area of your image will vary according to your composition and what you want to highlight.

The default mode on most DSLRs will do a great job in most situations. However, as soon as you shoot high-contrast or mostly dark or mostly light scenes then many DSLRs struggle and need help in understanding what within the scene needs to be exposed correctly. In these types of scenes the DSLR will typically try to create an exposure whose primary histogram peak is in the mid-tones, not to the left (for dark scenes) or right (for light scenes) – where it should be.

 

Measuring the brightness – the basics

A critical part of getting the right exposure is measuring the brightness of the scene you’re shooting. That’s the job of the meter. As light is reflected from a scene or subject through the lens, it hits the mirror in front of the imaging sensor and is reflected up to the camera’s focusing screen and metering sensor.

The shutter/aperture/ISO values that the camera sets depends entirely on your chosen exposure mode. So if your shooting in Aperture mode, then the DSLR can only set shutter speed and maybe ISO. If your shooting in Shutter Priority then the DSLR can only set aperture and maybe ISO. If your in Program mode then the DSLR will choose a combination of both shutter speed and aperture, and maybe ISO too.  I say “maybe ISO” since that depends on whether you’ve chosen to manually control ISO or have set the DSLR to “Auto-ISO” mode (if your DSLR has this mode of course).

You need to decide what’s important. What is the subject? Where within the scene does the exposure need to be totally accurate? What are you trying to accentuate?

 

Metering modes

The light metering systems on modern DSLRs are sophisticated and complex. Your camera has three or perhaps four metering modes to choose from which will help you measure the brightness of your scene. You need to decide which of those metering modes to use, according to that scene. Don’t simply leave the camera in default mode, take control! This is where you really can make a difference, enhance the exposure and get a truly great shot. You can decide to take a reading from a very small “spot” in the scene, a slightly wider spot (partial), a reading biased towards the centre of the shot (centre-weighted), or an averaged reading from across the whole scene (matrix, evaluative, average or pattern). Each mode takes an exposure reading from a progressively larger area of the frame.

Firstly, let’s explore the metering modes across the different manufacturers and how they function:

  • Average; This mode doesn’t exist on all DSLRs. In fact you’ll only find this on some of the high end Nikons. The light across the whole frame is averaged into a single reading. It is very susceptible to small bright areas in the scene. It’s advantage is that it’s easy for experienced photographers to interpret the reading and adjust settings;
  • Matrix (Nikon), 3D Colour Matrix,  Evaluative (Canon) or Pattern; These are all the same thing but they are called different names by different camera manufacturers. Almost all DSLRs use this mode of metering as the default. This mode splits the scene up into different zones which are measured individually. The camera then builds up a picture of the distribution of light in the scene and checks this against an internal ‘database’ to try to work out what kind of subject you’re shooting and the exposure that will give the best result. The scene may be split into hundreds or possible thousands of zones; dependant on model. The central area may not necessarily get priority. The mode will often give priority to darker areas to prevent under exposure.
    • So this is an “intelligent” metering mode. Although it takes readings from all over the frame, it will be biased towards the active focus point and will try to determine the type of scene you are shooting.
  • Centre-weighted or Centre-weighted Average; This mode takes readings from across the whole frame, but weights it’s average towards the readings at the centre. A centre circle takes precedence of the readings compared to the outer frame. On more advanced DSLRs you can change the size of the central area. It’s easier to predict where it’ll go wrong compared to matrix/evaluative metering.
  • Partial; Found on some more modern Canon camera;
  • Spot; Called the same thing by all manufacturers, this measures a very small area of the frame. It hence needs to be used with care and only in specific circumstances. On amateur models the “spot” is slightly larger to allow more leeway for errors. Keep in mind that the spot is not always in the centre; it is linked to your manually selected AF point, allowing you to meter from off-centre subjects. The area can be anything from 1.5% – 10% of the frame; determined by the manufacturer/model and who the DSLR is aimed at – amateur, enthusiast or pro.

 

NB: Graphics shown are produced by and copyright of Digital Camera Magazine:

 

When should you use these different metering modes?

  • Matrix, Evaluative or Pattern; This mode is considered good for evenly backlit subjects like portraiture and landscapes. It’s also the way to go when you’re not sure which metering mode to use. This is the reason why it’s the default setting for fully automatic camera settings.
  • Centre-weighted ; Centre-weighted metering assigns the greatest weight for exposure from the middle area of the frame. Therefore, it’s good for times when your main subject is in the middle of the frame and you want to take a quick exposure. Centre weighted metering would be effective when you have a bright background or backlit subject. For example, if you were taking a photograph of a persons face on a sunny day at the beach. You wouldn’t want the strong background light, or the white sand, to effect the exposure on their face. As long as the persons face was correctly exposed, that’s all that matters. It’s times like this, you would choose centre weighted metering.
  • Spot; when your subject is either very dark or very light against it’s background you should consider using spot metering. For example, a white bird in it’s natural surroundings, say water and embankment or trees/shrubs. The bird will only take a small area of the frame, so using matrix metering the bird will end up losing all detail and be overexposed.  If we spot meter on the white bird, the DSLR will expose the bird correctly, but now the background will be underexposed, but that simply adds to the effect and the viewers eyes are drawn to the bird, which enhances the photo anyway, like bokeh does.
    Let’s take a black bird in the same surroundings; in this case the bird will be underexposed and we won’t have detail in the feathers, beak, eyes etc. and we’ll lose colour too. If we spot meter on the black bird then the background will overexposed, and therefore lighten the surroundings, but this has the same effect in that it enhances the subject. However, if your DSLR has partial metering I would recommend using that mode in this scenario since partial metering is better here than spot metering.
    Spot metering is also great for shooting the moon. Typically the moon will take up a very small percentage of a dark scenery, but be very bright.
    And, spot metering is good for shooting people sometimes too. If lighting is tricky then switch to spot metering. For example a person on a stage or a candlelit meal. Spot metering on the persons face will ensure the camera exposes the skin colour correctly and not take a reading from the stage or restaurant lights.
  • Partial; while spot metering typically meters around 2.5% of the viewfinder, partial meters typically 6.5%. These figures vary by DSLR but you get the idea. So if your shooting a dark subject against a light background, use partial if your DSLR has it, otherwise use spot metering and if you can increase the size of the spot (again DSLR dependent).

Nikon metering patterns: what are matrix, centre-weighted and spot modes (and when should you use them)?

The histogram

When it comes to determining correct exposure, the histogram is your friend. If your not sure about histograms, read “How to read and use histograms” by DPS.

A well balanced shot will show in the histogram as having the peaks in the midtones, with no slopes up to the left or right. This is known as a “normal curve” or “normal distribution”. However, many and perhaps most shots don’t show this. When lighting is difficult, you need to determine what your shot shows and what you should expect in the histogram.

For example, let’s take the white rabbit in a snow landscape example; everything is mostly bright, with maybe a few trees and shadows. Here nearly everything in the shot is white, with a few dark areas. The histogram will not show a normal bell curve, but show a small slope up to the left (representing the dark pixels for the shadows), then almost certainly few midtones, but lots of highlights. So the curve will be low in the centre, and high on the right, with the majority of pixels on the right hand side. If you shoot this scene in Matrix/Evaluative, the DSLR will shift the highlights peak to the centre (since the DSLR will try to achieve a scene of 18% grey) and the snow will come out grey.

Let’s take another example, a dark interior with some natural light coming in through a door or a large window. If you use Matrix/Evaluative metering here the DSLR will almost certainly get this wrong. You want the focus of attention on the interior, not the exterior through the door/window. Your probably not really interested in preserving detail in the door/window and don’t mind if these highlights blow out. In fact that will add to the atmosphere of the shot and place the viewers attention more on the interior – so it’s a desired result. However, we need detail in the interior, so we need to use negative exposure compensation, or spot/partial meter on something in the interior. If you just use Matrix/Evaluative metering and no EC, then the histogram here will be shifted to the right from where it should truly be. This will occur since the DSLR will again try to achieve 18% grey and the dark colours will not be dark but grey. So a dark interior should show it’s majority peak on the left hand side and a small slope up on the far right for the door/window highlights.

 

Exposure compensation

Read this Digital Camera Magazine article for a background on exposure compensation.

In the “white rabbit in snow scene” you need to use positive exposure compensation to shift the curve to the right. For simple improvements you can adjust by +1 or +2 EC. So set +1 EC and take another shot. Check the histogram, has the curve moved nicely to the right? Is it far enough to the right without being too far to blow all the highlights? (i.e. the curve hits the RHS very high up). If the snow is still grey, then set +2 EC, take another shot and check the histogram again. There are some nice grey and then white snow shots in this article as examples. For a better exposure thought the best thing to do would be to switch to spot metering and take a reading direct from the snow, perhaps the brightest area where sun falls on the snow.

In the “dark interior” situation you need to either spot meter from the dark interior, or apply negative EC (e.g. -1EC or -2EC) if using Matrix/evaluative. Best to spot meter.

 

Exposure bracketing

Occasionally, within one picture, you might have a few areas of underexposure (typically in the shadows) and perhaps a few areas of overexposure (typically the brightest parts of the sky for example i.e. “highlights”).

If you meter for the shadows, the highlights will be ‘blown’ (overexposed). Contrariwise, if you meter for the highlights, the shadows will be underexposed, losing details in the darkest areas. One potential solution to this dilemma is to use exposure bracketing, but use of a tripod is mandatory for this solution. Shoot one photo metering for the shadows. Then (without moving the camera even a fraction), take a second photo metering for the highlights. Lastly, take a third shot with an average metering. Load all 3 images into your favourite image editing software and overlay one on top of another. You can do this in many editors and align all the shots using the software. By carefully removing the portions of the image that are not properly exposed (or, if you prefer, by carefully combining the portions that are correctly exposed), you end up with one image with correct exposure throughout.

This Digital Camera Magazine article will help achieve the DSLR aspects. And these articles will help with exposure blending in Photoshop, and Photoshop Elements.

 

How to expose for difficult lighting situations – a flowchart

Digital Camera Magazine have produced a truly excellent photography cheat sheet to help you along in your photographic endeavours. It draws on the key photography basics of exposure to help you bag a well-lit shot no matter where you may be shooting. It offers step-by-step instructions in the form of a flow chart for calculating the best camera exposure and metering settings in four of the trickiest conditions in which photographers can shoot. Drag and drop the JPG to your desktop and follow their steps for exposing your camera correctly for Dark subjects, Light subjects, Highlights and High contrast. If there’s one cheat sheet that you need, it’s this ultimate guide to understanding the photography basics of exposure.

 Tip: Why not print the below flowchart & place in your DSLR bag? This was provided free on charge in a print back issue of the camera magazine.

 

Additional reading/graphics: