ALL ABOUT EXPOSURE CONTROL, PART 2

Following on from last issue’s article, here we delve deeper into the key elements of controlling exposure and also the visual effects that are directly linked to the aperture and shutter speed settings.

In response to requests from many readers, we’re revisiting some of the key technical elements of photography so you don’t have to always rely on the automatic systems. In this issue we look at more of the critical elements of exposure control, including how light metering actually works.

Determining exposure settings – apertures, shutter speeds and sensitivity – is not solely dictated by how much light is available, but also by the type of subject that you are photographing and what visual effects that you want to create in your picture.

Modern cameras are able to control exposure automatically based on the amount of available light, but these settings are entirely dictated by technical requirements with no scope for any creative considerations. In other words, leaving the camera in auto won’t give you the amount of control you want, particularly in terms reproducing movement – blurring or freezing – or obtaining the necessary depth-of-field. Consequently, you may wish to switch to either semi-auto exposure control or fully manual operation. Here’s how the exposure control modes work.

PROGRAM (P) – the camera sets both the aperture and shutter speeds automatically according to the available light level.

APERTURE-PRIORITY AUTO

(A) – you set the aperture manually and the camera sets the corresponding shutter speed automatically. You may also see this mode written as aperturepriority AE (for ‘auto exposure’). Canon uses the term ‘Aperture Value’ (Av for short).

SHUTTER-PRIORITY AUTO

(S) – you set the shutter speed manually and the camera sets the corresponding aperture automatically. Again, you may see this mode written as shutterpriority AE (for ‘auto exposure’). Canon uses the term ‘Time Value’ (Tv for short).

MANUAL (M) – you set both the apertures and shutter speeds manually, guided by the camera’s metering system.

– most consumer-level cameras offer a selection of exposure modes tuned to specific types of subject such as portraits, sports action, close-ups, landscapes and night scenes. These are fully automatic, but the apertures and/or shutter speeds are selected to best

match the pictorial requirements of these subjects. These modes mostly work well, but are no real substitute for learning how to control exposure manually.

Creative exposure Control

There are a number of ways that you can either control exposures totally manually or override the automatic controls to better suit the subject or situation.

Let’s look at the auto override functions first, because these can be very useful in quickly adjusting exposures to obtain a desired creative outcome. You can use any of these if the camera is being used in any of its automatic exposure control modes (i.e. program auto, aperture-priority auto or shutter-priority auto). The exposure compensation facility allow you to dial in a certain amount of under- or overexposure – over a range of up to plus or minus five stops, depending on the camera – and in either half-stop or one-third stop steps. You’ll see this feature written in a camera’s specifications as, for example, +/-5.0 EV (the ‘EV’ obviously standing for ‘exposure value’). Many photographers use a small amount of under- or overexposure as a matter of course because, with experience, they’ve found that their images look better this way. Often with digital capture, it’s advisable to use a small amount of underexposure (say two-thirds of a stop) in order to preserve detail in the brighter highlights. This is related to the dynamic range of imaging sensor and something we’ll cover in a future Back To Basics article.

If in doubt about the exposure settings to use, you can select the auto exposure bracketing (AEB) function. Bracketing is the term given to the procedure of taking a number of frames of the same subject, slightly adjusting the exposure each time, usually from under to over. You can do this manually, of course, but auto bracketing saves time by automatically capturing a sequence of three (or maybe more) frames, applying whatever under/over adjustment you’ve set. The bracketing range available varies according to the camera model, but is generally around plus/minus two stops per frame. AEB sequences can be up to nine frames on some models, but in reality you’re unlikely to need to go beyond either three or five frames to get one with the desired exposure.

Bracketing exposures is good insurance, but obviously it triples your usage of memory space. With experience, you should be able to tell what exposure correction is required so you might adjust the bracketing settings accordingly.

Another useful override is the auto exposure lock – or AE lock – which, as the name suggests, locks in a light reading (and the applied exposure settings) so it won’t change if you subsequently move the camera to reframe a scene. The AE lock is especially handy when using either spot or selective area metering patterns – more about these in a moment – to measure a small area of the scene which represents the tone (i.e. the brightness level) on which you want to base the exposure. Pressing the AE lock button holds the exposure settings while you recompose your shot… otherwise, the metering would simply keep changing depending on which bit of the scene it was measuring.

Measuring Light

All exposure control deliberations start with the camera’s light meter which measures the amount of light coming through the lens… hence it’s known as TTL metering. D-SLRs mostly employ a separate sensor for light metering while mirrorless cameras and fixed-lens models use the imaging sensor. of the subject and/or the lighting situation. All the meter can do is tell you how much light is available, but nothing about the brightness range within the scene (known as contrast) or where the areas of brightness and darkness occur and how much of the frame

What’s important to know about a light meter of any sort is that the information it gives you is often only a starting point for determining exposure settings, and you may also have to apply some subjective judgements based on your personal evaluation

“what’s important to know about a light meter of any sort is that the information it gives you is often only a starting point for determining exposure settings.”

they occupy. This is where you need to learn evaluate the scene to determine whether the meter’s recommendations will need modifying or overriding using one of the manual camera’s exposure controls described previously. This may well be necessary if you want to be creative with your exposure control (for example, deliberately under- or overexposing to give a particular visual effect).

Any camera exposure meter ultimately arrives at a light reading that represents an average of all the brightness levels contained within the viewfinder frame. Some camera meters are cleverer than others, but the output is still the same… a value that represents a mid-tone equal to an 18 percent grey tone. Even advanced meters that have some colour sensitivity – such as in the higher-end D-SLRs – still have to arrive at a single value which determines the aperture and/or shutter speed setting. Depending on the contrast characteristics of the scene, this may or may not be ‘correct’. And here’s the most important point to make about exposure control… it’s rarely about being technically correct, it’s about either what looks right in terms of the image’s brightness or the particular visual effect that you wanted to achieve (which could either be underexposed or overexposed compared to the so-called ‘correct exposure’).

To illustrate, if you pointed the meter at a white cat, the exposure settings it recommends would actually give you an 18 percent grey cat. Likewise, if you point it at a black cat, and then used the exposure settings recommended by the camera, you’d also end up with an 18 percent grey cat. So, if you want the white cat to be reproduced as white, you’d need to deliberately overexpose, and if you want the black cat to be black in your picture, you’d need to underexpose.

Here then, is a basic rule for metering; pick a tone in the scene that either is 18 percent grey (or close to it) or, more importantly, that you want to be 18 percent grey… and use this measurement on which to base the exposure. In the first option all the tones lighter and darker than 18 percent grey will then be reproduced pretty accurately. In the second scenario – where you pick a tone to ‘act’ as 18 percent grey, but in reality it’s actually brighter or darker – all the other tones will ‘shift’ accordingly thereby producing under- or overexposure respectively. You would pick this option if you want to expose for either the highlights or the shadows… which, of course, if you didn’t apply some correction would result in, respectively again, underexposure or overexposure.

Even if you simply understand that the camera’s meter is always averaging the contrast range in a scene to a mid-tone, then you’ll appreciate why some under- or overexposure adjustment will usually be needed… either just so the image brightness looks right or it looks the way you want it to.

Of course, digital imaging allows for brightness to be adjusted after the picture has been taken, but there are still compelling reasons for getting the exposure right incamera, including – as noted earlier – the dynamic range limitations of the camera’s sensor.

Light Metering Patterns

Once you know how the camera’s light meter is working, you can then start to appreciate when you’ll need to make corrections to get what you want in terms of the exposure. Another good example of this is a bright snow scene – all that reflected light will result in massive underexposure if you don’t do anything about it and you’d end with snow that’s blue (in colour) or grey (in B&W).

To ensure white snow in your picture, you’ll need to give more exposure than the camera is

“The camera manufacturers provide a choice of metering patterns to give you more options when dealing with specific subjects or lighting situations.”

recommending, perhaps as much as one or two stops more such is the high level of reflectance off ice crystals. As we’ve now seen, you can apply this overexposure in one of a number of ways – dial in some plus exposure compensation if using an auto control mode, switch to manual control and increase the exposure by using either a larger aperture or slower shutter speed (or a combination both) than is being recommended by the camera’s meter, use auto exposure bracketing, or use the spot metering pattern and find something in the scene that’s more representative of a mid-tone than snow (maybe a rock or the trunk of a tree) then use the AE lock to hold this measurement while you recompose your picture. Other circumstances may dictate which method is the most effective or efficient, and when shooting snow (or sand) there needs to be a balance with the exposure settings to avoid ending up with overly bright highlights… but you get the idea.

The camera manufacturers provide a choice of metering patterns precisely to give you more options when dealing with specific subjects or lighting situations, or using exposure more creatively.

Multi-zone – Also called multi-pattern metering or by a proprietary name such as Matrix metering in the case of Nikon D-SLRs. The image area is divided into a pattern zones or segments and the brightness level of each is measured separately. This information is then compared with a database of scenes stored in the camera to determine which pattern fits best, and which zone(s) should be used as the basis for the exposure recommendations.

Multi-zone metering works well with scenes with a wide brightness range, but where there is roughly equal amounts of bright and dark tones. Remember, though, it still arrives at an average reading, and large areas of brightness or shadow will cause problems. Centre-Weighted AverAge – This type of metering was very common in 35mm SLR cameras from the 1970s and ’80s, and is still on most D-SLRs and mirrorless cameras. It averages all the light and dark areas in the frame, but then biases (or ‘weights’) this reading more towards the brightness level predominant at the centre of the frame. If your subject isn’t located centrally, or the background is either very bright or quite dark, this meter will give an inaccurate reading that would result in either under- or overexposure respectively.

Spot – This method of metering concentrates all its attention (or ‘weighting’) on a very small area represented as a spot in the centre of the viewfinder frame. Typically spot meters only cover between two to five percent of the total frame area. However, they’re quite useful if you understand the principles of exposure control and prefer to meter off a selected tone to obtain bright highlights or deep shadows. Selective area metering – or partial area metering as Canon calls it – works along the same lines, but bases the exposure measurement on a slightly larger area, which some photographers prefer.

Some cameras allow for the spot metering to be linked to the autofocusing point so the exposure measurement will be based on the subject, rather than being influenced by any other part of the scene, such as a bright background.