Knowledge Base

Light Metering

Light measurement is one of the most elementary requirements when capturing successful images and, without this or an external means of calculation, it fundamentally becomes a guessing game. It is how a camera determines the correct shutter speed, ISO and lens aperture to be used.

The way cameras measure light has evolved considerably over the years; from models that had no on-board method (requiring an external light meter, or application of the ‘sunny 16’ rule) to todays multi program marvels that can make full assessment of the environment and expose an image perfectly with little or no input from the photographer.

As all objects reflect varying amounts of light, cameras are calibrated with a standardised amount of light in mind – 18% grey (also known as mid grey) being the norm. With this in mind it is easy to see that if a camera is pointed at objects reflecting light that is brighter or darker than this, under or over exposure will occur. In other words, it is easy for the meter to be ‘fooled’ into giving the wrong exposure value.

A good example of this is when photographing a snow scene. The light reflected from snow is so intense, the camera interprets it as 18% grey which severely under exposes the scene, leaving you with disappointingly murky grey results. Similarly, a scene with mysterious large deep black areas will be overexposed as the camera attempts to render the scene mid grey, causing the richness of the black tone to be lost in a sea of dark grey.

Glancing at today’s crop of cameras, it is not difficult to notice that nearly all boast more than one method of metering. So, if program metering is so accurate, why are alternative, more archaic methods still available? To a large degree, the answer lies in a photographer’s personal preferences. We each have our own ideas as to how a final image should look and, composition aside, exposure is of huge significance; slight under exposure can give an image a more saturated look – but that look is not to everyone’s tastes (myself included). Conversely, over exposing a scene a little can lighten the colour (but run the risk of blown out highlights). Sometimes our intention is not for a technically perfect exposure as this can detract from the atmosphere or mood we try so hard to create.

Taking this into consideration, it is easy to see that there are still valid reasons for a cameras specification to include differing metering options, and the purpose of this article looks at a few of the most enduring and useful methods. It is by no means exhaustive and serves as a basic guide for the novice to gain an understanding as to how their use affects results.

  • Average Metering

    Average metering is one of the more basic metering methods and has been around for a long time. While it lacks the sophistication of more advanced solutions, it still retains a place in modern camera designs. As the name suggests, a reading is taken from multiple points of the image area, which are averaged out to deliver a final calculation. It is a simple and very effective method that can be used in many situations requiring a straight forward light measurement technique.

    In my early photography days, I used a camera with a Selenium cell powered light meter that was uncoupled from the viewfinder and positioned on the camera top plate. It provided basic average metering only and care had to be taken to ensure that when taking a light reading, the camera was pointing at the subject.

    While average metering is good for general use, it is easy fooled by highlight and shadow areas which leads to severe under/overexposed results for the uninitiated. After some trial and error it becomes easy to work around the short fallings, but requires the photographer to think carefully in advance of releasing the shutter. More obvious areas of weakness are around images that may contain bright lights or the sun; without manually over riding the meter’s recommendations, subject matter is often reduced to silhouette and lacking detail. Creatively known as ‘contre-jour’ lighting, it can look very atmospheric under the right circumstances.

    Average metering is completely ‘blind’ to situations of under and over exposure (such as snow or darkly lit scenes mentioned previously). Therefore, it becomes necessary to manually compensate for its shortcomings to produce accurate results.

  • Centre Weighted Average Metering

    This has been used for decades and is one I have been familiar with since the days of my youth when my now ageing Olympus OM1 was fresh from its box. It works in a very similar way to Average Metering, inasmuch as readings are gathered from various points of the picture area. However, the readings are ‘weighted’ towards the central area as this is where the main subject is more likely to be found.

    Generally, I found this metering method fairly accurate and, as it was my sole method of metering for almost a decade, soon became familiar with any associated caveats (which are very similar to Average Metering). For snow scenes I found I had to over expose by at least one full stop (with an additional two thirds of a stop added in really bright conditions). For general purposes, I underexposed most images by a third of stop in an effort to retain highlight details. This was probably due to the way the camera’s light meter was calibrated rather than an observation that can be applied to the actual metering method.

    It is noteworthy that this form of metering has survived down through the decades and has been included in every camera I have purchased since the early 1980’s. What is interesting is how I gravitated back to using it for around 75% of my shots taken with my latest cameras.

  • Spot Metering

    Spot metering became my favourite method of metering for many years, thanks to its inclusion in Olympus’ OM3Ti and 4Ti bodies. Modern cameras tend to use the focus point as the sensitive ‘spot’ from where light measurement is made. In the case of my OM3 and 4Ti’s the central 2% of the microprism is used. A reading is taken of the spot area only, effectively ignoring the area outside of it.

    This provides an incredible amount of flexibility when it comes to creating the final exposure which comes at the cost of trial and error to fully master. Obvious uses are in portraiture, when skin needs to be reproduced as accurately as possible. Or when photographing sunrises and sunsets – careful metering of the area just outside of the suns disc will give results that retain colour detail throughout the image without ‘blowing out’ the sun. As light falls away the further from the sun the metering point is taken, more shadow detail is revealed (at the expense of losing the highlights). It is great fun to experiment with this and it helps to perfect spot metering techniques.

    Using a snowy scene as an example again, spot metering can be used with great success. Simply take the metering value from an area darker than the snow to ensure that snow appears white instead of dirty grey. For me, metering off dark coloured stone usually works.

    Stained glass windows are also a good subject for spot metering as they are notoriously difficult to expose correctly due to the huge variation in light intensity that passed through the individual panes. And even a spot meter will give poor results if not used correctly. In a situation like this, I would look for a mid-tone area and use this as a basis for exposure. Alternatively, spot meter off areas that are the brightest and darkest and use a value between the two extremes.

  • Multi-Spot Metering

    Multi-spot metering is a versatile force to be reckoned with and builds on the already very capable single spot metering method. It is a system rarely adopted in most cameras and was often thought of as over complicated. Used in the right hands, it is deadly accurate and provides the photographer with a tool that allows them to ‘paint with light’.

    I used it for many years when I shot with film but have struggled to find a suitable digital camera that includes it, which is strange given how versatile it is.

    The spot metering system I used allowed up to eight individual spot readings to be taken, from which the camera used nifty algorithms to calculate an average exposure based on those readings. In real use, I found eight measurements were excessive as it gave me results that were similar to those from average metering. Where it gets interesting though is when an exposure bias is required, i.e. when exposing to preserve highlight details. The same area can be spot metered two or three times (or areas that were very similar in light intensity) which means that the overall calculation will be ‘loaded’ to take the same (or similar) readings into consideration.

    In actual use, I found three to four spot measurements enough. Typically, I would meter off a person’s face, clothes and background. Or when composing a landscape image, I would meter off the sky (in one or two places depending on cloud presence) along with a couple from the land (possibly one from the shadows if I wanted them to be preserved).

  • Evaluative (Matrix, Multi-zone) Metering

    Evaluative metering is probably the most widely used method of light metering today and works best for scenes which are evenly lit. Referred to as Evaluative by Canon and Sony, Matrix by Nikon and Multi-zone by Leica, they are all fundamentally the same. Precursors to the latest terminology include ESP (Electro Selective Pattern), Honeycomb and Segment.

    Evaluative metering works by dividing a scene into zones and taking into consideration the overall brightness and colour, along with front and rear lighting conditions of each zone. A complex algorithm is then used which analyses the results before producing an average, based on the initial reading of the zones.

    Depending on the camera used, emphasis may also be put on the focus points used. Add to this the fact that different brands and models deploy different amounts and patterns of zones and we can begin to understand the sophisticated technical effort that goes into making Evaluative metering as accurate as possible.

    Generally it can be used in most situations but, like other metering methods, it is not entirely fool proof. For example, strongly backlit subjects will still require some manual adjustment or even changing to spot metering.

  • Which Metering Method Should I Use?

    Used correctly, all metering methods will provide satisfactory results. But some work better than other under difficult lighting conditions. There is no substitute for gaining a full understanding of each methods strengths and weaknesses though – relying solely on the cameras ability to provide 100% satisfactory results under all conditions at all times is simply not going to work. As a general rule of thumb:

    • Average and Evaluative Metering are best for capturing scenes that are evenly lit and will provide a high level of accuracy when used in these situations.
    • Centre Weighted Average Metering is great when used for subjects where emphasis needs to be on your primary subject matter. Like average and Evaluative methods, it can be used for a wide variety of scenes.
    • Spot Metering is best used in situations where lighting is a bit trickier, or you wish to get creative (e.g. creating mysterious shadow areas). It is excellent for back lit subject matter or, conversely, when the subject is much brighter than the background.
    • Multi Spot Metering gives the most accurate control over light measurement. Emphasis can be placed on multiple points of the subject which are calculated to give the desired result. Arguably the most difficulty technique to learn, but at the same time the most rewarding.