Olympus OM-D E-M5 part 3: autofocus on a mirrorless camera

A commenter suggested that the mirrorless Olympus OM-D E-M5 can’t be as good as a DSLR because it can’t possibly focus as fast.

Actually, Olympus claims that they have achieved the world’s fastest autofocus speed with the E-M5. Maybe there’s an asterisk in there somewhere, but the camera’s “Single AF” speed is probably faster than any DSLR in the same price range. (You should look at real camera review sites for actual focus-speed measurements.)

For some background here, autofocus DSLRs use phase detection autofocus (PDAF), which uses different areas within the lens to create a rangefinder which can determine how much out of phase the high-contrast areas in the focusing area are off, and move the lens exactly the correct amount to put it into focus.

Contrast detection autofocus (CDAF) is a more iterative process which focuses the lens the way a human would, which is to move the lens around until the focus area is as sharp (highest contrast) as possible. This is the focus method used on all non-DSLR autofocus digital cameras, and yes, traditionally it has been a lot slower. But thanks to Moore’s law, the computing power now exists to read out the sensor, determine contrast in multiple areas of the image, move the lens, and then do it over and over again, all in just a fraction of a second.

The advantage of CDAF is that it’s more accurate because it doesn’t depend on the precise calibration of the lens and the PDAF mechanism. CDAF uses the same sensor that records the image to determine sharpest focus. Online photography forums are full of complaints from people having focusing problems with their DSLR systems, especially when they use wide-aperture lenses. This generally doesn’t happen with CDAF-based focusing systems.

CDAF also allows other neat tricks. For example, there’s a face-detect mode in which the camera is able to identify a face and focus on the eyes. And the E-M5 has a touchscreen display; just touch the display where you want to focus, and the camera will focus there and take a picture.

But PDAF still retains an advantage with 3D focus tracking, because PDAF is an inherently three-dimensional measurement. However, most inexpensive DSLRs lack enough focus points to have quality 3D focus tracking. I see PDAF becoming a niche thing for people photographing sports, with CDAF being becoming the preferred choice for regular photography because of its better accuracy and other features.

Comments

[tim]

OT: does liberal Kevin Drum believe in HBD:http://www.motherjones.com/kevin-drum/2012/06/todays-high-school-grads-are-just-good-yesterdays

[mikeca]

"Olympus claims that they have achieved the world’s fastest autofocus speed."

I thought they claimed the worlds fastest mirror-less AF speed. Mirror-less cameras are not known for fast AF, so Olympus's claim may well be true.

AF speed depends on the lens as well as the camera. Small mirror-less cameras like the Olympus OM-D E-M5 have an advantage that they use shorter focal length lens because they have a smaller sensor. The lenses are also smaller because the camera does not have a mirror and the lens can be closer to the sensor. These lens should focus faster. There is less glass to move.

There is a hybrid approach from Sony called Single-Lens Tranlucent (SLT). This has a fixed semi-transparent mirror that deflects 10-20% of the light to a PDAF system located in the pentaprism housing. The viewfinder is a EVF rather than optical.

These camera are bigger than mirror-less cameras, more like a DSLR, but they have a PDAF system that can do AF while taking pictures at high frame rates or shooting video.

Unfortunately, from reading reviews of these cameras, it looks like Sony has not perfected this AF. Nikon and Canon have many years of engineering invested in auto tracking AF systems in their top end cameras (Nikon D4, D800, D7000, Canon 1Dx, 5D3, 7D). Tests indicate the Sony system just isn't competitive with those top end cameras, at least not yet.

Olympus OM-D E-M5 has a continuous AF feature, but DPReview was not very impressed with it. They indicated it sometimes lost the subject.

Canon has just announced a new hybrid AF system in the T4i (650D). This camera has a main sensor that contains pixels dedicated to PDAF. The T4i is a DSLR and also has a conventional PDAF system. The hybrid AF is suppose to be used in liveview or video. The PDAF is used for subject tracking and CDAF is used for final focus. Canon claims PDAF is better for subject tracking. I have not seen any reviews of this yet, so no idea how well it works. There are rumors that Canon is going to enter the mirror-less segment in the next few months. If they do, they will probably use this hybrid AF technology there too.

[Firepower]

THAT
must be
one helluva
- new camera

[daveg]

What is the delay between the initial click and the photo being taken?

With an DSLR there is virtually no delay.

Every non DSLR camera I have used has a delay on the order of 1/2 a second.