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By Andrew S. Gibson – the author of Understanding Exposure: Perfect Exposure on Your EOS Camera (currently 36% off at SnapnDeals).
Imagine that it is early 1959. Alaska has just been admitted as the 49th state in the USA. Fidel Castro has become premier of Cuba. Elvis Presley and Buddy Holly are in the charts. And Canon releases the Flex – its first SLR camera in a time when only eight other SLR camera models were in existence.
The Flex (pictured above) was so basic by today’s standards that it didn’t even have a built-in light meter. Instead, it used an external selenium photocell coupled to the shutter speed dial for metering. Most photographers back then used external light meters and dialled the exposure settings into their cameras, so I guess to some folk even this primitive system seemed like magic.
The first Canon camera with through-the-lens (TTL) metering came in 1965. The Pellix had just one way of reading light levels – a 12% spot meter in the centre of the viewfinder.
Centre-weighted averaging came along later and served photographers well until the evaluative metering system we are familiar with today was invented in the late eighties.
The first Canon camera to feature evaluative metering was the EOS 650, released in March 1987. It was also the first Canon camera to use the newly designed EOS mount.
The EOS 650 had a six zone evaluative metering mode. Evaluative metering has come a long way since then, and now most EOS cameras use a 63 zone metering system and advanced micro-processors to analyse the information gathered.
The auto exposure sensor from the EOS 60D. Note the grid of seven by nine black squares in the centre. These are the 63 metering zones of the EOS 60D’s iFCL evaluative metering system.
Evaluative metering is the most sophisticated metering mode on your EOS camera. As it is the one you are most likely to use at any time, it is useful to know how it works. Indeed, if you use your EOS camera in any fully automatic mode, it is the only metering mode the camera lets you use. You can only switch to one of the others in Program, Shutter Priority, Aperture Priority or Manual modes.
Most EOS cameras have four metering modes: evaluative, partial, spot and centre-weighted metering (One series cameras also have multi-spot metering).
Partial, spot and centre-weighted metering all have a single characteristic in common – they take an exposure reading from the centre of the viewfinder.
This is fine if, as in the photo above, that’s where your main subject is.
But if your subject is off-centre, like in this portrait, you have to point the centre of the viewfinder at your subject, press the shutter button half-way to lock in the exposure setting, then recompose. This is bit of a pain to say the least and wastes time.
Evaluative metering was developed as a way of accurately metering off-centre subjects. It works by dividing the viewfinder into zones, each giving a separate reading that the camera analyses to calculate exposure. It also co-operates with your camera’s autofocus system. The reading is weighted towards the active autofocus point on the basis that it is most likely to be covering the main subject.
This is the 35 zone evaluative metering pattern used on the EOS 300D, 350D, 400D, 450D, 500D, 1000D, 10D, 20D, 30D, 40D, 50D, 5D and 5D Mark II. Note how each AF point occupies a different square.
Canon introduced Intelligent Focus, Colour and Luminance (iFCL) evaluative metering on the EOS 7D. It has been included on every new EOS camera since then (excluding One series models).
The premise behind this new system is that exposure meters that react to the brightness of the subject only are more sensitive to the red light than the human eye. They may think that a red coloured subject is brighter than it really is and return a false reading that underexposes the subject.
To counter this, the exposure meter used by iFCL evaluative metering has two layers. The top one is sensitive to green and blue light and the bottom one to green and red. Each layer measures the colours that it is sensitive to and the camera combines the readings to calculate exposure settings.
In addition, iFCL metering uses information from multiple autofocus points. It knows which AF points have achieved focus, and which ones have nearly achieved focus, and weights the exposure reading towards the zones those AF points are in on the basis they are likely to be covering the subject.
The 63 zone evaluative metering pattern used by the EOS 100D, 550D, 600D, 650D, 700D, 750D, 1100D, 60D, 7D and 6D. All but the last two models use the 9 point AF pattern in the diagram.
Evaluative metering really comes into its own when used in conjunction with a portable Speedlite flash unit. It is important that the camera meters the light reflected from the subject in order to accurately calculate the exposure required from the flash. Centre-weighted, spot or partial metering can only do this if the subject is in the centre of the frame. Evaluative metering can cope regardless of which part of the frame the subject occupies.
That’s a brief overview of how evaluative metering works on your EOS camera. Hopefully it helps you understand how your camera’s metering system works and will help you take better exposed photos.
If you’d like to learn more about how exposure works on your EOS camera my ebook Understanding Exposure is available on special now over at Snapndeals.