If you could improve autofocusing on a traditional DSLR, what would you do? First up, most people would enlarge the area over which the autofocus system works, and second, it would be great for the autofocus to work better during movie capture.
Some of these improvement are already possible if you’re happy to use an electronic viewfinder (such as some Sony, Olympus and Panasonic models), but if you like using an optical viewfinder, you’ll have to work within the limitations of a reflex mirror system.
There are two basic approaches to autofocus: active and passive. DSLRs use passive systems and there are two types of passive systems available, phase detection and contrast detection.
Most DSLRs use the faster phase detection autofocus system when capturing stills. Simplistically speaking, phase detection works by comparing the image recorded from opposite sides of the lens separately. The light is focused onto separate sensors and compared. If the values are ‘out of phase’, it means the lens is out of focus, so the lens is moved until the values are ‘in phase’ and the lens focused.
Phase detection is considered superior to contrast detection because it can determine more accurately how far and in which direction the lens has to be moved to achieve focus with a single measurement. Contrast detection works on a similar premise, but it relies on differences in contrast values as measured by the separate AF sensors and determines the correct focus by making lots of measurements – a process of iteration. Eventually it determines the correct focus, but it can hunt around a bit in the meantime.
So, why use contrast detection at all if phase detection is better?
With the traditional DSLR design, a separate autofocus sensor using phase detection is positioned in the camera, but it can only be used when the reflex mirror is in the viewing position. This is because the reflex mirror has a hole in it through which light is diverted, using another mirror or two, onto the autofocus sensor. As soon as the mirror flips up for movie recording or using live view, the autofocus sensor is in the dark and stops working. Another system is needed.
So, when the mirror flips up, the camera uses a contrast detection system, employing pixels on the image sensor instead. The problem with this approach is that the image sensor can’t do two jobs. It can’t measure light for autofocusing and record an image at the same time. So as not to degrade image quality, only a small number of pixels are used for contrast detection autofocus and software is used to fill in the missing pixels on your photographs.
In practice, the dual system works reasonably well, but it also explains why autofocus isn’t as quick when you’re using your camera in live view or movie recording modes.
Canon claims to have solved this problem by redesigning the image sensor completely. Instead of each pixel site comprising a single photodiode for image capture, the new Dual Pixel CMOS employs two photodiodes for every single pixel site. Even better, these pixels use the superior phase detection system, so focus is quicker and more accurate in live view and movie recording modes.
With the new sensor, Canon claims it can use 80 per cent of the viewfinder area to focus on which is a useful improvement, but this is 80 per cent of an APS-C size sensor. It will be interesting to see if the same area can be covered with a full frame sensor in the future. And of course this only applies if you are using live view or movie recording modes – for optical viewfinder shooting, you’re still using the same very effective 19-point AF phase detection system found on other Canon DSLRs.
The first camera to have the new focusing system is an enthusiast model, the Canon EOS 70D. Scheduled for sale in September, it features a 20-megapixel APS-C CMOS sensor, uses a 14-bit DIG!C 5+ processor and shoots at up to seven frames per second. Additionally, a native ISO range of 100-12,800 (expandable to 25,600) enables shooting in lower light conditions, something the new autofocus system will revel in.