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Understanding The Light Meter and How To Use It For Creative Photography
So, we have seen how aperture, shutter speed and ISO (the exposure triangle) can be used to determine the brightness/exposure of a photo. This is a really simple and effective system, but how does it actually work? How do we know what values to apply for the exposure triangle? Well, the light meter lies behind the whole process, and using it well is essential for getting good pictures. Let's find out more...
What is the light meter?
The light meter on DSLR cameras measures the intensity of the light that comes through the lens. This information is then transferred to the little display that you see in the viewfinder - the exposure meter display - which tells you whether a photo will be over or underexposed with the current settings.
The exposure meter (above) is our guide for setting aperture, shutter speed and ISO. In the middle is '0', which represents a 'correct' exposure (according to the intensity of the light judged by the light meter). To the right is '+1', '+2' etc. and to the left is '-1', '-2' etc. Each number represents 1 stop of exposure, and is separated by increments of 1/3 of a stop.
So, without the light meter the camera would not know how bright the scene is, and without the exposure meter display we would not know how this brightness should translate into settings on the exposure triangle. Every time we change the settings for aperture, shutter speed, or ISO to alter the exposure value of a photo, the exposure meter shows the new reading.
Metering in different shooting modes
How we deal with the light meter's reading depends on the shooting mode we are in:
Program (fully automatic): So, you point your camera at the scene in question, the light meter takes its reading, Program mode selects the appropriate aperture and ISO settings to create a good exposure and the exposure meter display points to '0', i.e. a correct exposure. This leaves you having to do nothing whatsoever but press the shutter release button.
Aperture Priority: Let's say we've chosen a wide aperture of f2.8 for a narrow depth of field. The light meter takes a reading and an appropriately fast shutter speed is set to balance the aperture of f2.8 for a correct exposure. The exposure meter display we be at 0 and you can fire away.
Shutter Priority: Shooting in shutter priority mode is just the same in reverse. The light meter judges the brightness of the scene and an aperture setting is selected to balance the shutter speed you have chosen. Most of the time this will work fine, with the exposure meter display showing 0.
But sometimes, if you are working with a really quick shutter speed to freeze action, your camera might not actually have a sufficiently wide aperture setting to create a bright enough photo. In this event, the exposure meter display will simply read (-1 or whatever it happens to be), and you will have to adjust the ISO to increase brightness and nudge the display up to 0.
Manual: When shooting in manual mode, the exposure has to be entirely balanced by you. You can play with both the shutter speed and aperture until the exposure meter display is correct, altering the ISO too if needs be.
Getting an accurate light meter reading
Not all light meters are the same, but they do all have a few common flaws. For example, dark tones are often mistaken for shadows and low brightness.
In other words, if you pointed your camera at a black dog sitting in a well lit room, you would probably end up with a poorly exposed photo.
Conversely, if you then pointed the camera at a white cat in exactly the same well lit room, you would end up with an entirely different, although equally faulty, exposure.
Snowy landscapes are notoriously tricky to meter.
The solution to this is to search for something with a medium tonality to meter off. In other words, avoid dark black and bright white surfaces. Stone walls, grass, sand, blue sky, clothing - anything that would look a medium grey tone in a black and white picture.
Alternatively, if you find that you are shooting somewhere that your camera's light meter seems to be routinely struggling with, simply dial in a stop or two (negative or positive) of exposure compensation to rectify it.
In fact, you may find that your light meter needs a little helping hand from exposure compensation quite a lot of the time. Not all cameras' light meters are entirely accurate.
Actually, I don't know many photographers who don't choose to 'overexpose' (according to the light meter/exposure meter display's suggestion) by at least 1/3 stop as a default setting.
Get to know the light meter in your camera and compare results of shots on the 'correct' reading and those that are 'overexposed'. A lot of it is personal taste too.
Just as we change the points on the exposure triangle to get the right exposure, we can change the parts of the scene from which a light meter reading is taken to get the right suggested settings.
Most cameras have a few different metering modes, each distinguished by the size and location of the area from which they assess brightness. Let's take a look at the three basic categories (the names vary between manufacturers).
Although the technology is slightly different between matrix and evaluative metering, they work in broadly the same way. Basically, this mode uses all the sensors that a light meter has to gather information from right across the scene.
So no parts of the image you see in the viewfinder are neglected. The brightness from each area is averaged out, and an exposure meter reading is given based on this average.
When it's useful
Matrix/evaluative metering is great when there is basically even light throughout a scene. So the difference between foreground and background brightness, for example, is not very much. You know that there aren't going to be any areas that end up badly overexposed or underexposed. It's also really useful when you don't have time to take a more careful/specific reading using one of the modes below, like when capturing a moment at a sports game or on the street.
When it's not so useful
This mode is less good in scenes with a lot of contrast, where you need one area to be perfectly exposed. For example, a sunset on the beach. The sky is ablaze with incredble bright colours, and the foreground of sand and waves needs to be included in the shot. But because the foreground is so much darker, the average reading taken by evaluative/matrix metering ends up burning out the sky in order to brighten the foreground.
Centre-weighted metering takes a reading from the whole scene (just like matrix/evaluative) giving extra weight to the central area. Meanwhile partial metering only measures from the central area, totally ignoring the rest of the scene. So these two modes are very similar and come in really handy when the only part of your photo that matters can fit within the centre of the viewfinder.
When it's useful
A lot of the time, especially when taking portrait pictures, the backlighting is much brighter or dimmer than that falling on the subject. Using evaluative/matrix metering gives equal weight to the part of the scene that doesn't matter, but centre/partial metering let's us get the perfect reading for the subject only.
What do you do if you want to use this metering mode whilst in program, aperture priority or shutter priority mode, and re-position the subject off-centre (which would cause the light meter to take new reading from the wrong area)? Simply use the exposure lock button on your camera ("AE-LOCK") to lock in the settings until you have taken the shot.
Image by: Bui Linh Ngan
When it's not so useful
Centre-weighted/partial metering is not worth using when you have to be really quick to take every shot and the light is generally consistent throughout the scene. For example, when taking wildlife pictures you rarely have time to square up the subject in the centre of the viewfinder to get a precise reading. Matrix/evaluative is generally good enough, and if you find it going wrong too often simply use some exposure compensation.
Spot metering takes a reading from a small area in the very centre of the scene that only covers about 3% of the total area. So this is the most precise and accurate metering mode, but as a result it is also the most time consuming and risky.
When it's useful
Spot metering is ideal in situations where there is a small area that you need to be perfectly exposed for the photo to work, that's much more important than any other parts of the image. For example, a flower in a dark room catching light coming through the window. You do not want the camera to measure the light from the room, as this might cause the flower to be overexposed. It can also be useful for close-up portraits, when you have the time and opportunity to get the perfect exposure for the eyes.
When it's not so useful
Spot metering is really specific, so when a subject is quite evenly lit, with a few patches of shadows and highlights, there's a danger that you'll point at an area that doesn't represent the average for the scene. The need to find an area of neutral grey is exaggerated when using spot metering. It takes time, so action shots and candid shots are no good with spot metering.
So the light meter lies behind the entire process of exposing photos. The reading that it sends through to the exposure meter display determines our choice of settings. This reading depends on the metering mode we have chosen, each of which is suited to certain kinds of situations. On the whole matrix/evaluative metering does a good job, but when a touch more accuracy is needed (and you have the time available) centre-weighted and spot metering are great options to use.