”Embrace light. Admire it. Love it. But above all, know light. Know it for all you are worth, and you will know the key to photography.” – George Eastman.

Getting an accurate measure of light was a tough task in the initial days of photography. Photographers tried various methods. When everything failed, the concept of ‘bracketing the shots for exposure’ was born. For getting that perfect exposure, it is important to understand how the light meters work.

When the cameras started becoming popular, they did not have any kind of light meters. Photographers guessed the ambient light and set the camera settings accordingly. Mostly with experience, they got away with a reasonably well exposed frame. Some photographers carried a set of ND filters which they used to get an idea of available light by judging the amount of light that could pass through and then they got an approximate reading for exposure using a chart. This was a slow method and inaccurate since it depended on subjective perception of light.

Things started becoming easy when the light meters stepped in. Initial light-meters were simple photovoltaic meters that generated electricity when light fell on them. The higher the amount of light falling on them, the greater the power generated. These meters worked for most of the normal light conditions and never required a battery. I have an old Zenith camera that has this meter and it still works!

With advancement light-meters also changed and they became integrated within the camera itself. In almost all the SLRs, the readings were taken through the lens. Engineers (who unfortunately were not photographers) felt that in most of the photographs, the subject occupies only some part of the frame and which is usually in center. This gave an idea to improve the metering by focusing more on center and thus was born the center-weighted metering and spot-metering. Both these metering modes work in the same way but the total area metered varies.

Center-Weighted Metering, while calculating, gives about 80% weightage to the central 15-20% of the area and about 20% weightage to the remaining part of the frame. It is thought that in most of the scenes, when all the colors and brightness levels are combined, it comes to about something called as 18% grey (this is a mid-level grey). In most scenes, the dark portions balance out the bright portions and the average comes to about this medium grey. With this concept, as the driving principle, the center weighted meters try to set the exposure as close as possible to this grey.

Very effective method when compared to the earlier methods, but it has its own problems. As you would have guessed, how can everything equate to mid grey? Imagine a late evening highway with a couple of passing vehicles. You want to capture the light streaks. Everything is set and then you meter with center-weighted metering and shoot as suggested. The photograph is sure to turn out over-exposed and grey in color. What went wrong? The light meter tried to calculate the exposure as close as possible to the middle grey. Another scene – after a fresh snow storm, trying to photograph the streets in daylight, with everything covered in snow. The center-weighted metering will turn the photograph into a dirty grey by underexposing the scene to the 18% grey.

Come to think of it, nothing is 18% grey apart from the 18% grey card available in photography stores. Professional photographers stopped using this light meter altogether and shifted to something called as incident light meter. The light meters in the camera measure the amount of light that gets reflected from the subjects. It is therefore affected by the color, darkness and even the reflections. Incident light meters measure the amount of light falling on the subject and so they are not affected by the factors affecting the light reaching the camera. Problems with incident light meters come when there are filters in use and the light reduction due to filters is required to be manually factored in (using filter factors) or when the subject can not be tested for incident light. Incident light meters are not useful for distant landscapes and for wild animals (or for that matter wild humans too). Coming back to camera’s metering modes. The next ones are spot meters.

Center-weighted metering is very useful for landscapes with mixed elements, especially while using dark filters like polarizers and ND filters. The camera takes a reading through the filter and so calculations related to darkening caused by filters (filter factors) are not required.

Spot Metering Mode is similar to the center-weighted metering but with a small change. They give importance to a small spot in the center which is about 3-8% of the frame. This roughly corresponds to the split-focusing aid found in older cameras or the focusing point highlights found in some of the DSLRs. It works on the same principle of approximating everything to 18% grey but due to the small spot, the accuracy is increased. Sometimes when I try to replicate Ansel Adam’s Zone System in my photographs, I use spot-metering.

The spot-meter used to be in the center of the frame but now it can be changed to meter the area where focus point is set in new DSLRs. No more metering and recomposing!

Spot-metering is useful for carefully evaluating the exposure values of various elements in a photograph (Basics of Exposure). It also works well when the main element of the composition is very different in overall light condition compared to the rest of the scene.

Partial Metering Mode (found in Canon) is a small variation of the spot metering mode. The surrounding area is given slightly more consideration while evaluating exposure in comparison to the spot metering mode.

Matrix Metering or Evaluative Metering was the next major leap in metering modes. This is an algorithm that camera uses to calculate the exposure. The light meter remains the same. In fact, all the cameras have a single light meter which calculates in the various manners described above.  Matrix metering tries to address most of the common problems seen with center-weighted metering and spot-metering. The algorithm uses various inputs like focus point used, lens settings, color of the subject, kind of subject being photographed. It also ignores things which usually throw off the meter readings like bright point sources of lights in the composition or extremely dark areas with no details visible. This therefore leads to an acceptable exposure in most cases. The night scenes look as they should and the snow remains bright white.

Did I say ‘most’ cases? Yes, even the matrix metering fails at times. One common failure that happens is when dark filters are used. The matrix metering underexposes in such cases. So, if you are using dark ND filters, switch to center-weighted or spot metering.

What metering modes do I use?

In the above example with flowers, I used the raw file from the spot-metered exposure to create my final image. Center-weighted was too dark and the matrix metered had significant loss of details in the white petals due to overexposure.

I use all of them, except for the partial metering mode, which is absent in my Nikons. For most of my pictures, I use matrix metering. Sometimes I use spot-metering when I feel the need for incident meter or when I want to tweak the exposure very accurately. Center-weighted metering helps me out when I fool around with different filters. With the preview screens and the histograms, the best option is to check the photograph for exposure instantly and change the settings accordingly. Do not fall prey to chimping though (To Chimp or Not To). Another pointer – stick to one metering mode most of the time. This helps in understanding the limitations and predicting the exposure errors that might happen.

(Stairs – Nikon Df with Zeiss 25mm using 3D Matrix Metering mode, f/8, 1/40 sec at ISO 100)