Canon EOS 70D video AF test: how well does the new sensor-based AF perform

By Kevin Carter - Tuesday September 17 2013

Camera Article
Introduction | Autofocus for video | Canon EOS 70D VS Sony SLT A77: Video Comparison | Canon EOS 70D VS Sony SLT A77: Shooting Time Lag | Conclusion | Videos around the web

Most users expect to produce the image they see when pressing the shutter release button, however a delay between that and the actual capture can ruin a photograph. In order to capture an image, however, the camera has to perform many time-consuming operations, the slowest of which (in most cases) is focus acquisition and adjustment.

To reduce this time lag, most DSLR and DSC cameras perform some of these operations before the photographer triggers the shutter. Users can, for example, initiate autofocus by depressing the shutter button halfway, while some cameras like the Sony SLT-A77 have a proximity sensor (located just above the viewfinder eyepiece) that can activate autofocus when the camera is brought up to the eye.

These two scenarios primarily define the shooting time lag and the shutter release time lag that users experience directly. With the new time lag measurement available on DxO Analyzer v5.2 (see the presentation here) we measured the shooting time lag on both the SLT A77 and the Canon EOS 70D.

How we measure the time lag:

Time lag is measured by calculating the time interval between auto-focus activation (by depressing the trigger button to the half-way position) and the point at which the shutter is triggered.

The trigger point is measured by synchronizing the camera shutter release button with a specific timer, in this instance we used an LED timer, which, by simply taking a photo, records LED positions on each line at the trigger point. In essence, the first lit LED on the five horizontal LED bars represents the capture beginning, with the lag shown between each row as the shutter closes.

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The LED Universal Timer. Not only can we can measure shooting time lag but we can also gauge the electronic rolling shutter.

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The mechanical trigger has an electronic switch (initiated by the red button on top) that allows synchronization with the timer.

The electro-mechanical trigger shown in use with the Canon EOS 70D

The electro-mechanical trigger shown in use with the Canon EOS 70D

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The DxO Touchscreen Probe mounted on a smartphone

The DxO Touchscreen Probe mounted on a Canon EOS 70D touchscreen

The DxO Touchscreen Probe mounted on a Canon EOS 70D touchscreen

For the Canon EOS 70D, the more intriguing (and challenging) test was to compare the shooting time lag between the two focusing modes; the viewfinder based dedicated phase detection AF sensor module versus on sensor AF phase detection.

The results are somewhat surprising:

Canon EOS 70D
Stills mode
Canon EOS 70D
Live-view mode
Sony SLT A77
Shooting time lag (s)
0.2 0.8 0.2

The difference between the two modes is significant but while Dual Pixel CMOS AF takes longer there could be a couple of reasons for this.

First, shooting time lag is longer. Contrary to the viewfinder based AF system, the mirror has to be safely locked in the up –position and the shutter has to close before re-opening again, which should lead to an higher shutter lag time.

Second, we performed the live view test with the touch screen, which could have a slow reaction time.

With the shooting time lag close to one second with the live view mode, it’s worth noting that with some subjects the Canon may require a little preparation.