Photography News Issue 38

Photography News | Issue 38 | absolutephoto.com

Technique 51

Camera School NEWSERIES Here we lift the lid on all things camera related, showing how to get better results from your DSLR and providing all the info you don’t find in the manual. So, stick with us and you’ll soon wield your camera like a pro. This month, how different metering modes affect your camera’s reading of the light...

A histogram is nothingmore complicated than a bar chart representing the light in the scene. It can therefore be very useful when judging accurate exposure. The chart reads from black on the far left to white on the far right, and with all the other tones from shadows, throughmidtones to highlights covered through the middle. You’ll find a histogram available for pictures you’ve shot, and in live viewmode, or via an electronic viewfinder, they can also give you a live reading of the scene, a bit like watching the camera’s metering systemworking away as you shoot. They are influenced by the meteringmode you’ve set though, as well as the exposure settings that are currently being used. Change either of those, and the histogram should change, too. Howto read an exposurehistogram

Words & pictures by Kingsley Singleton

Last month we looked at the basics of exposure and how aperture and shutter speed work with ISO, governing the amount of light recorded and the brightness of the exposure. The analogy usedwas filling a glass of water, which represents filling the sensor with light. You can decide how the glass is filled by making the shutter speed faster or slower, the aperture larger or smaller and the ISO level higher or lower; but how do you know how much light you’ll need in total? That comes from the camera’s metering system, which reads the reflected light in the scene. Reading reflected light It’s on the amount of reflected light that the camera decides what shutter speed or aperture is required (and what ISO to use if that’s set to Auto). So, say you’re shooting in aperture-priority (A or Av) with the lens at f/8 and pointing at a subject that’s reflecting a lot of light, which is giving you a shutter speed of 1/250sec. If you then point the camera, with the same aperture setting, at a subject that is reflecting less light, the shutter speed will be slower. This is because the camera thinks it needs more light to achieve a correct exposure, hence the longer shutter speed. The same would happen in shutter-priority mode, wherein the second subject would need a wider aperture to get what, according to the metering system, is the correct amount of light for a good exposure. What is a ‘good’ exposure? What the camera thinks is a good exposure is based on the idea that the amount of light reflected is consistent across a range of subjects. Most cameras metering systems are calibrated in this way and assess the scene based on around 18% reflectance, ie that 18% of the light falling on the scene will be reflected. Most of the time this works fine, because, although some objects reflect more light and some less, across the whole of a scene, it usually averages out as about 18%. Where the system fails is if you’re shooting very dark or light scenes. So, if you shoot a very light subject, like a white flower in a white room, the extra reflectance means the camera thinks less light is needed to create a good exposure, and will underexpose. Conversely, if you shoot a very dark scene like a black cat sitting on a black chair, the camera thinks very little light is being reflected, and will probably also meter incorrectly, overexposing the shot by trying to record too much light. How to improve metering You can improve metering in several ways, and these include picking a mode that’s suitable for the subject and also biasing the metered setting using exposure compensation. The main metering modes are Multi- segment (also called multi-zone, wide and by manufacturers’ own names like Matrix or Evaluative), centre-weighted, partial (on Canon bodies) and spot. These modes use smaller and smaller areas on which to base the metering, and the advantage of this is that you should be able to get a truer-to-life reading and therefore a better exposure.

More highlight tones

Which should you use? While multi-segment and wide modes use the whole of the frame on which to base an exposure, modern cameras use these settings intelligently, reading the type of scene, where you’ve focused, and setting the exposure accordingly. This means they can often deal with tricky situations like backlit subjects that would fool less intelligent systems. For that reason, multi-segment modes tend to be used by default, while the others are for special purposes. Centre-weighted metering still uses a wide area, but bases its setting on the middle part of the frame, while partial and spot use very small areas; typically 5-10% for the former and less than 5% for the latter. These small areas can be aimed over something in the scene you know to be a midtone (something with 18% reflectance), giving very accurate results, but used on the wrong part, results will be way off. We’ll cover spot metering in particular detail in a later instalment. Shooting in manual mode you control shutter speed, aperture and ISO independently, so youmight think metering is of less use. But, take a look on the camera’s screen, or viewfinder and you’ll find there’s still a small exposure bar, telling you whether, in the current metering mode, the camera thinks that you’re under or overexposing. You can use this bar to base your exposure on, making sure to set it over or under the metered value if shooting a predominantly dark or light subject. Meteringmodes On DSLRs and CSCs you’ll usually find metering modes via a button on the body, and the icon used – a circle within a frame – tends to look similar no matter what the model.

Moremidtones

Multi-zone

Centre-weighted

Spot

More shadow tones

Nextmonth How to use Exposure Compensation to improve the camera’s metering and get the look you want.

Differentmetering areas These three shots were taken using multi-zone, centre-weighted, and spot modes. Multi-zone has read the whole scene, trying to balance the sky and subject, so the latter is underexposed. Centre-weighted is about right, as it’s concentrated in the middle of the frame where the subject is. Spot has metered a shadow area, judging that to be a midtone, so has overexposed.