Olympus OM-D E-M1 Review
The E-M1 uses an autofocus system Olympus calls 'Dual Fast AF'. This means that it can use either contrast detection (as used by previous Micro Four Thirds cameras) or on-chip phase-detection to autofocus, with the method used depending upon the focus mode and the type of lens attached.
|With a Micro Four Thirds lens attached, the EM1 presents a 9x9 auto focus grid and uses contrast detection AF. The focus points are smaller, and cover more of the frame, compared to previous Olympus Micro Four Thirds cameras.
You can either leave the camera to choose the focus point automatically, or manually select your preferred point using the 4-way controller.
|The EM1 also offers a grid of smaller AF points when using a Micro Four Thirds lens, that can be individually selected. This allows very precise selection of the AF area.
It's also possible to expand the AF area to cover a 3x3 grid of the larger areas shown above.
|With a Four Thirds lens, the camera switches to phase detect focus and uses a 37-point grid in the centre of the frame.|
With a Micro Four Thirds lens, the camera uses contrast detection AF - providing 81 AF points in a 9x9 grid covering most of the frame. It's this system that is used for the camera's fast single AF acquisition. It's also used when shooting movies with these lenses. Continuous AF and tracking use a combination of phase detection and contrast detection.
|Four Thirds Lens||Micro Four Thirds Lens|
|Continuous-AF||PDAF||PDAF + CDAF|
|AF during movie recording||Manual Focus||CDAF|
Having used a selection of Four Thirds lenses on the E-M1, our assumption is that Olympus has disabled autofocus in video because the motor sounds are so loud and the focus movement is too abrupt/disruptive for use while recording.
On-sensor Phase Detection autofocus
Key to the E-M1's promise of bringing together the Four Thirds and Micro Four Thirds systems is its ability to perform phase-detection autofocus, using dedicated pixels on the imaging sensor itself.
The key difference between contrast-detection autofocus (as generally used in compacts and mirrorless cameras), and phase detection (as traditionally used in DSLRs) is that phase detection is able to assess how out-of-focus the image is, and determine directly how far and in what direction the lens needs to move its focus group to achieve a sharp image. Contrast detection has to scan through at least part of its focus range to find the point of optimal focus.
This difference totally changes to the way lenses need to be designed - those optimised for phase detection need to be able to race to a specified location very quickly, whereas contrast detection lenses need to be able to scan back and forth very quickly. Traditionally, very few lenses designed for phase detection have coped very well with the subtle, scanning motion required for contrast detection. Those designed for Four Thirds SLRs could autofocus on previous Micro Four Thirds cameras, but only slowly and hesitantly.
The E-M1 devotes every 16th pixel to autofocus duties, arranged in offset stripes of left and right-looking pixels. The focusing pixels sit behind clear sections of the color filter array to increase their capability in low light and do not contribute to the final image. The result is a 37-point AF array that is used when a Four Thirds lens is mounted.
Any focus inaccuracy (or variance in a particular copy of a lens) can be corrected using the camera's AF fine-adjust system, which is accessed from the menu (Custom K: AF Focus Adj.). This allows the user to input correction values either globally for all 37 focus points at once, or individually for each AF point on its own. For zoom lenses, it's also possible to specify different corrections for the wide and telephoto ends. Alternatively it's possible to apply a single adjustment value that will apply to every lens, but we can't really see why you'd use this on the E-M1.
|The E-M1 allows you to fine-tune the autofocus for either all of the 37 phase-detection points at once, or each point individually.
For a zoom lens, the camera allows different corrections for each end of the lens range, and allows different values for each lens.
What's clever about the E-M1 is how you set the focus adjustment. To start with can you select either all the focus points together, or any single point on its own. Press OK and the camera will take you to a live preview, which allows you to see directly the effects of any adjustments you make. Half-press the shutter button and the camera will autofocus; you can zoom in to check critical focus using the front dial. If the AF looks inaccurate, set a microadjust value using the rear dial and try again, repeating as necessary until it's focusing correctly.
Overall, this process lets you work out and set AF microadjustments much more easily than any current SLR, which require multiple test shots to be made and analyzed. This is ironic, really, as the E-M1's on-chip phase detection should be far less prone to needing microadjust corrections compared to the conventional PDAF systems found in SLRs, which employ an entirely separate light path and complex optics.
Watch the video below for an illustration of the process, using the 50-200mm F2.8-3.5 SWD. Here we're not attempting to set a microadjust value, just show how the lens's autofocus bias can be changed and the effect previewed on the same display. Initially we're focusing with the adjustment set to 0, then showing how the focus position moves when microadjust is set to +20, -20, then back to 0.
This shows that the lens is focusing pretty accurately to start with, but setting microadjust to +20 shifts the AF visibly backwards, whereas -20 shifts it forwards. So if you have a lens which focuses persistently behind or in front of the subject, this process lets you fix it relatively easily.
While it's positive to see this level of control, we have to wonder how many people will have the time, patience and ability to accurately assess the 74 corrections per lens that this system allows. We'd love to see a process that could compare the focus achieved by PDAF with that ascertained using CDAF, so that the calibration could be conducted automatically, but no such system currently exists.
As with the E-P5, the E-M1 offers both magnified live view and focus peaking to aid manual focus. The focus peaking option highlights high-contrast edges with your choice of black or white fringes, to precisely position the point of focus.
|When it's turned on, peaking engages when the focus ring is rotated. You can also direct the E-M1 to provide a magnified preview when the focus ring is turned.
If you want to use peaking with a manual focus lens, you have to assign it to one of the function buttons, and then switch it on and off manually.
Whichever of these 'MF Assist' features you choose in the setup menu is automatically engaged in manual focus mode, when you turn the focus ring on a Micro Four Thirds or Four Thirds lens. If you're using a non-native lens, via an adapter, you'll have to assign Peaking or Magnify to one of the main six customizable buttons on the camera.
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