Understanding Metering and Exposure

Recently, I published a 3-part series on the exposure triangle. Today, I’d like to expand on that with an article on understanding metering and exposure. You might say that this is part four of a three-part series. It is intended as an overview. You should dig deeper, Googling more extensive articles, or some of the many YouTube videos that are more in-depth.

Understanding how to expose a picture correctly is at the core of the art of photography. Shutter speed, aperture size and sensitivity (ISO) are the building blocks of exposure, and the meter inside a camera brings them all together. A creative photographer must control all of these elements to both expose a picture correctly* and influence the look of the final picture.

Happy Vendor in Grand Central Market

Happy Vendor in Grand Central Market

The meter inside a camera measures the levels of reflected light from a subject it’s pointed at, and selects the appropriate combination of shutter speed, aperture and ISO. Modern cameras work very well in fully automatic mode, but the serious photographer should always have some influence over the settings the camera selects. Using shutter priority mode allows the photographer to select an appropriate shutter speed for the shot, and is ideal for sports and action shots. Operating a camera in aperture priority mode means that the photographer can select an aperture for each shot, and influence depth of field. For full creative control, experienced photographers often switch their camera to manual mode.

Most camera metering systems work on the assumption that a scene should be exposed as if it’s an even mid-tone grey. This works for the majority of lighting conditions, but there are situations when the meter will be fooled. Bright white snowcaps and washed-out skies are examples of scenes where a camera’s meter will not expose correctly. The photographer can either override the camera settings in these unusual lighting conditions, or switch to manual mode and take complete control.

The human eye is able to deal with high-contrast scenes much better than most cameras can. What may appear to be a fairly balanced scene could have a range of tones which a camera won’t be able to deal with. The latest digital SLR and mirrorless cameras have systems which can effectively boost the dynamic range and help in these situations, but there’s no substitute for understanding how to deal with them. Most modern digital cameras are able to display histograms of the shots they capture. A quick look at the histogram will tell you if the range of tones has gone beyond what the camera can record.

Having an understanding of the metering modes your camera offers is a major step forward in getting exposure right every time. The majority of digital cameras offer three metering modes: Multi-zone (sometimes called Matrix or Evaluative), Center-weighted and Spot. Multi-zone is the default setting, and takes light readings from a number of segments across the frame before setting an exposure level. Multi-zone metering is fine for most situations, but can be fooled by scenes with high contrast levels. Center-weighted metering assumes that the main subject for a picture is at the centre of the frame, and exposes accordingly. For the most precise measurement of exposure, spot metering is the most accurate, but may not reflect the photographer’s vision across the entire frame.

Interestingly (to me), I’ve used a variation of the word “reflect”, a few times, in this article. The are two kinds of metering: Incidence and Reflective. The built-in metering system, in cameras, is reflective. It measures the light being reflected off the subject, onto the surface or the film or sensor. You may have seen the term “TTL” or through-the-lens metering. This also take into account any filters that you may have on your lens. Incidence light is the light falling on the subject. Incidence metering is available in hand-held meters, and is not the subject of this article. If you are interested in the subject, Google Sekonic or Gossen light meters.

Experienced photographers understand the need to expose every picture accurately to ensure detail is fully captured, or to make a creative image that may violate the ‘rules’. This means using a variety of techniques and tools offered by the camera. Understanding how cameras deal with contrast, selecting an appropriate metering mode and reviewing histograms are essential for getting every picture exposed to match your vision.

*When I say “correctly”, I am not making a value judgement. I mean achieving an exposure that reflects the vision of the photographer.

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