The E-PM1 is overall an impressively fast and responsive camera. Autofocus is very fast, at least with static subjects, and thanks to some intelligent buffering its continuous shooting performance is pretty good as well. Meanwhile all other operations - browsing through images in playback, or navigating through the various setting screens and menus - are performed quickly and with no fuss. Fundamentally, the PEN Mini is a camera that hardly ever feels like it's getting in the way while you're using it, and that's exactly what you want.
Continuous Shooting and Buffering
The E-PM1 offers High and Low-speed continuous shooting modes and you can define the speed for each option in the Custom C menu. As with previous PENs (and in contrast to current Panasonics at their lower frame rates) it's somewhat hamstrung for serious action work by the fact that live view isn't maintained in continuous shooting mode. Instead you get shown the picture two shots before the one you've just taken, which isn't very helpful when trying to track moving subjects (and results in a blank screen for the first two frames in a burst). Live view does however return a few seconds after you take your finger off the shutter button.
The E-PM1 will shoot a series of frames at full speed in an initial burst, then once the buffer is full, slow down and shoot at a reduced frame rate. As you'd expect, larger file sizes take longer to clear from the buffer, so that shooting in raw + JPEG produces the longest write times. On the positive side, the camera does let you continue shooting (at a reduced frame rate) while image data is being written to the card. And while you cannot review images during this period, you can access and change the vast majority of the camera's menu settings.
The table below summarizes the maximum frame rate, the number of frames that can be shot at maximum speed before the buffer is full, the reduced frame rate that follows, and the write time for all images to clear the buffer, using our fastest SD card. In all timings below, the camera was set to H drive mode.
Burst of JPEG 12MP/Super Fine images
Burst of Raw images
Burst of Raw plus JPEG 12MP/Super Fine images
Taken as a whole, the results above are quite impressive for a camera of this class. While it would be nice to be able to capture more images at its maximum framerate, we feel the ability to continue shooting while data is being written to the card is perhaps even more beneficial to users in most real-world situations.
Autofocus speed / accuracy
Olympus makes great claims about the E-PM1's autofocus speed, and it is indeed very fast when paired with suitable lenses (including the 14-42mm F3.5-5.6 II MSC kit lens), for static subjects at least. Its performance is on par with the best of its competitors in day-to-day shooting. And, like other cameras which employ contrast-detection autofocus using the main imaging sensor, it's essentially immune from the front- and back-focus problems which can affect DSLRs. The following sample is taken from the E-PL3.
As seen on the E-P3, the E-PM1 includes an autofocus illuminator that addresses one of the big criticisms of previous PENs, which struggled to focus in low light. It's not a panacea - as usual it won't cover off-center subjects, its effective range is limited, and its proximity to the lens means it can be blocked by those with larger barrels - but it's a big improvement, and makes the camera much more usable in dimly-lit conditions.
Once your subject starts moving, though, things are a little less clear-cut. Contrast-detect AF systems have historically been less capable at tracking moving subjects than the phase-detection systems used in SLRs, so the E-PM1 has its work cut out to match this more-established technology. In continuous AF mode the camera's continuous frame rate drops dramatically while the camera checks focus between shots. And the lack of live view means you can't follow your subject between frames in continuous drive mode, so there's no way to ensure it stays covered by your selected AF point.
To combat this problem, the E-PM1 also has a subject tracking mode (C-AF+TR). Once you've designated a subject by focusing on it initially, then the camera will attempt to track it around the frame and keep it in focus, just as long as you keep the shutter half-pressed. But again, this has problems once you start shooting in continuous drive mode; naturally it only works if your subject stays within the frame (and there's no way of knowing this), and as soon as you release the shutter the camera stops tracking and resets the AF point.
All-in-all, while Olympus is certainly making some progress in this area, the E-PM1 still isn't the most practical camera for shooting moving subjects in the real world, even compared to other mirrorless cameras. For example Panasonic's implementation of continuous drive mode mode in the GF3 maintains live view between shots taken at up to 3fps, with AF tracking that stays locked onto your subject until you reset it. Pretty well any SLR will do better too.