Dual Pixel AF

In addition to an updated phase-detection AF sensor for viewfinder shooting, the 5D Mark IV also receives Canon's 'Dual Pixel AF' system in Live View, first seen in the EOS 70D and then in most subsequent, higher-end Canon DSLRs. However, this is the first time Dual Pixel AF has made its way to a full-frame sensor while offering Servo AF for stills, now promising to bring significantly improved - and accurate - autofocus in both live view video and stills shooting.

Canon's schematic of its Dual Pixel CMOS AF sensor structure. The top layer illustrates the light-gathering micro-lenses and conventional Bayer-type color filter array. The lower layer shows how each pixel is split into two photo-diodes, left and right, which are colored blue and red respectively. (Note that this does not indicate different color sensitivity.)

The Dual Pixel AF sensor essentially splits all of its 30 million imaging pixels in half, one side taking in light from the right hand side of the lens, with the other half taking it in from the left. For the final images, pairs of pixels are combined to give 30 megapixel files sampling all of the light entering the lens. However, the ability to compare the light from the left and right of the lens allows the camera to offer phase detection autofocus even the mirror is up. A more in-depth explanation can be found in our EOS 70D review.

The improvements to the Dual Pixel AF system in live view mean it now outclasses the camera’s viewfinder AF in several respects. It offers more extensive coverage and a high level of accuracy for shallow depth-of-field photography. You can select your subject with a tap of the screen or jump instantly between detected faces with a touch of the joystick. The camera will then decisively track and refocus on that subject wherever it moves in the frame: freeing you up to focus on framing and capturing the moment.

This combination of powerful hardware and processing with ergonomic simplicity will be an asset to stills and video shooters alike: from photojournalist and candid portrait shooters to documentary and wedding cinematographers.

Dual Pixel Raw

For the first time, the 5D IV will be able to save double-size Raw files that store data from the left- and right-facing separately. This information can then be used in a variety of ways.

Image microadjustment

Because the left and right-facing pixels see a slightly different perspective of the scene, it's possible to take advantage of this slight separation. Just as Lytro's light field camera split light across multiple pixels to capture where the light had come from, the Canon gains a tiny bit of insight into the direction that the light has arrived from. And, like the Lytro, this then allows you to render the image as if focused on a slightly different point, but with a very different trade-off being struck between resolution and degree of refocusability.

This means you can render an image with a very slightly different point of focus, as you process in Canon's Digital Photo Professional software. With the Lytro, the amount of refocusability depended on a number of factors, such as focal length and subject distance, and we expect a similar correlation will hold true for Canon's 'image microadjustment'.

Canon's 'image microadjustment' will allow for very slight shifts in focus to compensate for focus accuracy errors common with shallow depth-of-field photography. It can do so because split pixels underneath one microlens essentially constitutes - albeit very rudimentary - light field technology. Canon 5D Mark III | EF 24mm F1.4L. Photo: Rishi Sanyal

We've seen that the degree of correction is extremely small (as one of our editor's put it: 'Canon should've called it image nanoadjustment'). So small that it's likely to be of little practical use, and it certainly won't be a replacement for actual lens calibration using AF microadjustment.

Still, it's an interesting concept, and we're curious to see if Canon develops it further. It would certainly be useful in future iteration with a slightly larger degree of refocusability: for example, perhaps the range covered my AF microadjustment's -20 to +20 (which has generally been enough to correct even my most miscalibrated primes).

Bokeh shift

Another use of the two slightly different views is the ability to view the out-of-focus regions of the image from either perspective. You can shift the position of the bokeh slightly, from one position to the other. We're not entirely sure why, but you can.

Ghosting reduction

It's actually the least exciting sounding use of the Dual Pixel Raw that may be the most useful. Because flare tends to come from light coming in from one side of the lens, it means that it will only be seen by either the left or right-looking pixels. This means that information from the other half of the sensor can be used selectively to cancel-out the effect of this flare (on those occasions where it wasn't an intentional part of your capture.)

Wi-Fi + NFC

The Mark IV works with the Canon Connect app and can be used in the same fashion as recent Canon DSLRs like the Canon 80D. NFC makes pairing the camera with a smart device very easy (and only a little more involved with Apple devices, despite not being able to use NFC.) Control over the camera, outside of exposure parameters, is pretty limited, but the view/transmit option is pretty straightforward.

Canon's Camera Connect app gives a good level of control over the camera, allowing remote working. It's also pretty effective for just receiving images from the camera.

In addition to pushing images to a smart device, the Mark IV can also transmit images via Wi-Fi directly to a computer, between cameras, to DLNA device or to a printer. The camera also works with EOS Utility for remote capture from a computer.