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Metering
We talked in some detail about exposure in three earlier sections so by now you'll understand how shutter speeds, apertures and ISO all combine to affect the brightness of an image. Naturally, different conditions will lead to differing amounts of light being available and therefore a decision must be made regarding which shutter speed and aperture combination to use - for this an exposure meter is used. This device, be it a dedicated device or one built into your camera, simply measures the amount of light available which is then converted into aperture and shutter speed.
Different Types of Light Meter Light meters may be in-camera light meters or handheld light meters. All modern cameras have built in light meters but there can be advantages to using handheld light meters on occasion. We'll cover in-camera light meters and their various options in a moment but first we'll touch upon the different types of handheld light meters. Handheld light meters typically fulfil one or more of the following functions:
- reflected light metering
- incident light metering
- flash metering
Reflected Light Metering
The meter that is performing a reflected light reading is measuring the light reflected off your subject. You point the light meter at your subject and it tells you what the exposure would need to be in order to render that subject at a mid grey (typically referred to as 18% grey). This type of meter will give you different exposure results for a black subject or a white subject as it only tries to make an exposure to make things grey. It has no way of knowing if your subject is black, white or somewhere in between so it just measures the light coming off the subject and assumes the elements of the image will average out to be grey.
At its most basic level this is how an in-camera meter operates.
A special type of reflective light metering device is called a spot meter. This measures the light reflecting off a very small area of your subject, a small 'spot' if you will. To use a spot meter you aim it at a part of your subject that you wish to appear to be mid-grey and set your exposure as it tells you. Alternatively you can point it at a darker or lighter portion of your scene and adjust the exposure accordingly (more on this later).
Incident Light Metering With an incident light meter you measure the strength of the light source. To do this you would hold the meter in front of your subject and point the meter at the source of light to take a measurement. This type of metering avoids the complication caused by your subject being of a reflective nature other than mid-grey as it is measuring the light totally independent of the shade or colour your subject is.
Flash Metering A flash meter is a special type of incident light meter. A flash meter works in just the same way as described above except that it will automatically trigger your studio flash, take a reading while the flash is illuminated and tell you what aperture is needed for your currently selected ISO (shutter speed does not matter as it will be described in the 'Flash Photography' section of this book).
In-Camera Light Metering As mentioned, an in-camera light meter is taking reflective light readings through the lens from the scene the camera is pointed at. If your camera is in an automatic mode the light meter will enable your camera to automatically select the exposure based on the scene it is measuring. If it is in manual mode the light meter will tell you if it thinks the picture will be under-exposed or over-exposed. Modern cameras come with at least one of the following metering modes (not to be confused with exposure modes - metering modes usually reside somewhere in the menu system):
- centre weighted average metering
- intelligent metering (somtimes called evaluative, matrix or multi metering)
- partial metering
- spot metering
Your camera may come with all of these modes. This is what they do.
Centre weighted average metering measures the light across the whole scene but pays more attention to the brightness of the scene in the centre. This was the most common type of metering on film SLR cameras and many people continue to use it as it is easy to understand and predict when it may be fooled into providing an incorrect meter reading.
Intelligent metering is called something different by different manufacturers. Canon call it evaluative, Nikon call it matrix. It works by evaluating the whole scene and using an algorithm to try to identify what the scene is and therefore what the exposure should be. Whilst this mode is generally quite successful, some users choose to avoid it as the manufacturers don't publish how it makes its decisions and therefore it can be a little harder to predict.
With Canon's implementation they bias the exposure reading to the currently selected focus point as they assume that the focus point will be placed over the main subject of the scene.
Partial metering measures a smaller part of the scene than the previous two but a greater part of the scene than spot metering. Partial metering takes its reading from a circle covering approx the centre 9% of the frame.
Partial metering can be useful where there is a wide dynamic range in your scene and you want to ensure a certain part is exposed correctly, for example a spot lit actor on a dark stage.
Spot Metering measures a small part of the scene typically 1% to 5% and is useful for being precise about where exactly in the scene you want your light meter reading to come from. This is always taken from the centre of the frame except on some professional level cameras when there may be an option to have it taken from the selected focus point.
How to read your in-camera light meter. Whilst most of the following discussion will apply to evaluative/matrix type metering there may be times where that type of metering may behave differently than other metering modes. As mentioned above, this is because this type of light metering uses unpublished and probably fairly complex algorithms to decide on a correct exposure. For the most part the following information will be fine for that type of metering mode but if your camera meters in a way that isn't explained here while in that type of mode it's simply because it is trying to be clever!
Most, if not all, of the time you will be using your camera's in-built light meter. As we have mentioned this type of light meter is carrying out reflected light metering and is trying to render the scene so that it averages out to a mid-grey tone. It is surprising how often this works out just fine but this can mean scenes containing very bright or very dark elements can fool a camera meter into under or over exposing.
If the scene you are photographing contains very bright elements it may cause your camera to think it needs less exposure than it really needs for example:
- a snow scene
- a sunny day on a beach
- a bride in a white wedding dress
- a back-lit subject (i.e. where the light is behind the subject)
In these examples the camera will be trying to render your subject grey whereas they really need to be white or close to white. This will cause your meter to suggest the wrong exposure and if you listen to it you will end up with an underexposed picture. To rectify this you will need to tell your camera to overexpose by one or two stops. In manual mode to simply select an exposure where your camera meter is telling you it will be overexposed by one or two stops (depending on how much whiter than mid-grey your subject is), in an automatic mode you will need to use exposure compensation (see exposure part 4).
If the scene contains very dark elements your camera may over expose as it thinks there is less light than there is. Remember, it is trying to make your subject grey. If your subject is darker than that it will turn out too bright in your photograph as the camera tries to compensate. For example:
- a close-up of a person wearing dark clothes
- a person against a dark background such as a person lit by spotlights on stage.
In these circumstances you can adjust for your light meter's misreading of the situation in the same way as above. The only difference is this time you need to tell the camera to underexpose.
Grey Cards Because light meters can be fooled by dark or light subjects, photographers sometimes use grey cards. These are, like the name suggests, cards that are grey. To be more specific they are cards that are designed to be the shade of grey that your light meter is looking for. Therefore if you take a light meter reading by pointing your camera at the grey card you should get the correct exposure. To make this work, the grey card needs to be in the same lighting conditions as the subject so you may ask your model (if you are taking a portrait) to hold the grey card close to his or her face while you take a spot meter reading from it to set your exposure.
If you don't want to carry your grey card around with you, you can use your hand. Take a reading off your grey card outside your home then take a reading of your hand. Your hand and grey card are likely to give different exposure results but as long as you know this then you can compensate in the field. For example, a white person may find that their hand and their grey card give a one stop different exposure with the hand apparently requiring one stop less exposure. This means that in the field someone like this can take a meter reading off their hand and adjust the exposure so the camera appears to overexpose by one stop.
Some metering tips
White skin is often one stop above mid grey. You can therefore take a spot meter reading off a white person's face and then overexpose by one stop to get a good exposure.
Snow is whiter than caucasian skin by about one stop. You can therefore take a spot meter reading off snow and over expose by approximately two stops to get a good overall exposure.
A blue sky at midday is often just about perfect mid-grey (not in colour but at least in luminosity) and so you can take a spot meter reading off the sky to get a decent exposure.
A wedding dress will often be one or two stops above mid grey.
A black suit (tuxedo, for example) will be one or two stops below mid grey.
Do some experiments. Take a photograph of a white piece of paper filling the frame (in even lighting, outdoors in the shade for example) using a semi-automatic mode like aperture priority, or using manual and exposing so the meter tells you the exposure is correct. Notice how it turns out grey in the end result. Do the same with a grey card and a black card and notice how they look the same as the white paper because the camera will choose different exposures.
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| White paper |
Grey card |
Black material |
The above images are genuine photographs taken in aperture priority mode: they are out of focus to hide texture differences. They were taken with spot metering mode selected so that slight differences in lighting from centre to the edges did not affect the exposure. They were also desaturated so that slight hue differences do not distract from the main point being illustrated.
You could also try photographing white paper at one and two stops under and over exposed to see how it affects the brightness of the resulting images. Compare those shots to a grey card and this will give you a visual indication of what a 'stop' increase or decrease in exposure will look like.
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Photographs
This is a site about photography so I'm sure you are expecting to see plenty of pictures.
For now, why not take a peek at the flickr galleries belonging to the two authors of this site.
Colin's Flickr Page
Phil's Flickr Page
"The best light occurs when I am stuck in the office with the tax man or have left my camera at home."
- Bob Croxford
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