At some point I realized the lens was not intended for still use...
What is it?
The 12-50mm f/3.5-6.3 is Olympus' "higher end" kit lens. Beyond extending the focal length range into truly wide angle and a decent telephoto, this lens has video cred: the zoom ring is a fly-by-wire option that produces no noise during zooming. Another addition is a serious 43mm (fixed focal length) macro ability. Minimum focus in that mode is about 8" (20cm), which nets you a bit better than 1:3 magnification ratio. Note that the working distance, due to the length of the lens, is only about 4" (10cm) at maximum magnification.
Which brings us to the size issue: Overall, the 12-50mm sticks out from the camera about 3" (80mm) and doesn't collapse. Because of that, it's not a great option as a kit lens for the more pocketable m4/3 cameras, such as the GF5 or E-PM2. On an OM-D or GH3, however, it looks a lot like the scaled 24-100mm lens it is and is much more appropriate in size.
Weight is a minuscule 7.4 ounces (211g), which is due to the extensive use of plastics in the design.
There are some other cost-cutting corners on the lens: no distance scale, DOF markings, or IR index, plastic 52mm filter threads at the front, and only a five-blade aperture (rounded).
On the side of the lens are two buttons, an L.FN programmable function button that some (but not all) m4/3 cameras allow you to program (e.g. OM-D does, Panasonic models do not); and a Macro button.
Inside are 10 elements in 9 groups, with one ED and two aspherical elements. The filter ring does not rotate during focus or zoom and has a 52mm filter thread, a bit unusual for Olympus m4/3 lenses. Stabilization Olympus style is done with the sensor, so there is no stabilization in the lens. The lens is an MSC design, meaning fast and silent autofocus on m4/3 cameras. Olympus describes the lens as having a "dust and splash proof" construction, and it is available in either all-black or silver finish.
The lens comes with nothing but a lens cap for US$500; Olympus offers a four-year extended warranty in the US for US$80. The LH-55B lens hood is a US$25 option. Olympus's page for the lens is here.
How's it Handle?
As noted above, this lens is fly-by-wire. You have two choices for zooming. With the ring moved towards the camera you get a rough approximation of a direct-couple (normal) zoom. I use the word "rough" because there's a tiny bit of lag, a bit of a gravely feel, and no hard stops at either end of the zooming. With the ring moved forward you get a smooth motorized zoom; move the ring clockwise (from camera back) and it zooms out, the other direction and it zooms in. There's a tiny bit of hysteresis in the motorized zoom (i.e. it starts and ends just slightly slower than the speed in the middle of the zoom). Again, there's a tiny bit of lag.
If you're coming from all direct-coupled zooms—which would be most of you—it will take some getting used to the fly-by-wire zooming. I found that early on I was always doing a second "tune" of my focal length, though this disappeared the more I used the lens. I can live with the action, but I'm not thrilled by it.
Likewise, manual focus is fly-by-wire with a narrow but easily found ring at the front of the lens. Ring action is smooth, but again there is a bit of lag and no hard stops to help you. But this is no different than most m4/3 lenses.
The programmable L.FN button can be assigned a wide range of options on the OM-D, including focus and lens functions as well as basic camera settings like ISO or white balance. If you're into customizing your camera, the 12-50mm and OM-D combo gives you seven customizable buttons, which has to be a near record. On the other hand, if you're using the 12-50mm on multiple cameras as I am, some of my cameras can't utilize the button, I found it to be less useful.
The fixed 43mm focal length macro setting is reached by pressing the Macro button on the lens and pushing the zoom ring to the front. The zoom ring is then locked in place and MACRO appears in a small window on the right side of the lens (from the back of the camera). It's nice to have this facility in the lens, but just be aware it's not a general purpose macro function that works at all focal lengths. Basically you have either a wide-angle-to-telephoto zoom lens, or a fixed telephoto macro lens.
Overall, this is a "modern" electronic lens, not an old-style mechanical lens. Some people will be put off by that, others won't. I wouldn't rate the handling as great or exceptional, but it suffices for me. I tend to only use the lens in macro mode or the direct-couple zoom mode, though.
How's it Perform?
I had high hopes for this lens. If I'm going to use a zoom on my camera, I like it to go beyond 28mm equivalent at the wide end. Unfortunately, other than perhaps the 24-70mm full frame lenses we pros use on our big DSLRs, I've yet to find a 24-xx equivalent zoom in a smaller mount that really shines in every respect. More often than not, you find that that there is a gotcha compromise involved in building a zoom with 24mm at one end at 100mm+ at the other. The 12-50mm is no exception to that.
Sharpness: This lens doesn't compete with the primes at any focal length. In the center and even at the edges, it's quite good wide open, but not exceptional. f/4 at 12mm seems to be the best the lens can do in the center. Any additional stopping down will start taking a little bit of sharpness away, especially towards the edges, which is an unusual result. At 50mm f/6.3, the lens is a little less sharp, but still on the good side. The corners are a little bit weaker than at 12mm, but still surprisingly decent. Unlike at 12mm, at 50mm there is a small benefit to stopping down (at least until diffraction begins robbing MTF).
As a convenience lens, the optical sharpness stays in a pretty tight range that never really fails at anything. But if were shooting with this lens all the time, I'd tend to stick to the widest apertures except for at 50mm or the 43mm macro setting, where I'd probably stop down one stop.
Aberrations: At 12mm we get a highly visible lateral chromatic aberration presence, and at most apertures you'd ever use. That's with the in-camera correction active, so raw files can be laden with CA. I did note variable correction at 12mm between different camera bodies, even with the latest lens firmware installed. The 12mm chromatic aberration in the corners may exceed what some of the cameras can deal with. At longer focal lengths, the chromatic aberration mitigates to the point where it can be mostly ignored in JPEGs, though it is still highly present. More so than any other m4/3 lens I've used recently, I find myself doing post processing chromatic aberration correction with images shot with the 12-50mm.
Vignetting: 12mm is once again the problem focal length: there's a fair amount of vignetting at 12mm, enough that in many situations you'll want to apply post-processing correction. It peaks at about 1.5 stops in the corners at 12mm in raw files and is pretty much gone (<1/3 stop) by the time you double the focal length (24mm). On JPEGs, most cameras correct the 12mm vignetting down to about a half stop in the corners. Curiously, at 12mm there's a pretty large area where there's less than a half stop of vignetting—it's really the extreme corners that get hit badly. Curious about that, I applied a movie crop to the files (16:9) to see what happened. Surprise, surprise, you never hit even 1 stop of vignetting at 12mm. Indeed, virtually all of the frame would be at about 2/3 stop or less.
Distortion: Are you getting tired of hearing that 12mm is weak? Sorry, but here's another example: in raw files you'll find extreme vignetting (>5% barrel) at 12mm. Once again double the focal length and the problem is gone (literally; we hit 0% at about 26mm). At the telephoto lens there's very little pincushion, perhaps 1% in raw files. JPEG files get corrected reasonably well in camera, but there's still visible barrel distortion at 12mm (typically a bit less than 1%).
Flare: Surprisingly bad. Sun at the edge of frame tends to cause clear flare issues, both in terms of small colored spots as well as concentric flare rings. Once again Olympus doesn't provide a lens hood with the lens, but the flare characteristics are such that you'd be well advised to buy one (hint: don't pay Olympus' ridiculous hood prices. Do a search on eBay for a knock-off).
Macro: Old style macro! Put the lens into a different mode (via button push and lens ring movement), and the elements are realigned to offer a bit more close-up capability at a fixed 43mm focal length. This kind of approach doesn't net 1:1 macro, but ~1:3 is certainly handy for less demanding types of macro work and gets you down to an 8" (0.25) focus distance. Overall, the macro capabilities are adequately sharp and useful.
Somewhere in the midst of testing this lens I released that it simply wasn't designed for still use. Virtually every flaw of this lens gets mitigated in some way when you're using it for video. The silent zoom/focus capability via wire is one giveaway to the design decision making, but apply a 16:9 crop on this lens and a number of the optical issues start to go away, especially with the in-camera corrections applied. Compressed and sub-sampled video will sometimes cause visual artifacts if a lens is too sharp. The extreme edge performance problems actually aren't hit with the 16:9 crop. So what we have is a very good video performance. The vignetting at 12mm mostly goes away with the video crop. The distortion moderates at 12mm with the video crop and is mostly negligible at other focal lengths. Heck, the flare looks a lot like the old Hollywood style diagonal echo, used to great effect in the 60's by a number of filmmakers.
Thus, performance-wise, it's a mixed bag for this zoom. For still work, the addition of 12mm at the wide end and the extra 8mm at the telephoto end aren't truly compelling over the standard kit lens, as the lens struggles with chromatic aberration at the 12mm end where you'd really want the extension. If I need 12mm I'll put the prime on, or maybe one of the wide angle zooms (7-14mm, 9-18mm). The one redeeming factor over the kit lens for stills would be the quasi-macro capability. I'm not sure that's enough to justify using it over the kit lens, though. The kit lens does more than well enough, indeed, often better than this lens, on most things for stills.
For video work, though, the lens starts to become more interesting. Much more interesting (though I grew out of using flare as a visual element in my work back in the late 70's ;~).
I consider the 12-50mm a "kit lens for videographers." Most of the weaknesses of this lens hurt still users, but not video users, plus a video user likely feels more restrained by the kit lens than a still user. The 4x zoom range gives them more flexibility than the ~3x zoom range of the regular kit lens, the silent zoom/focus is necessary for close work, and even the quasi-macro range opens up abilities that a video user would seek.
While Olympus and others have suggested this lens with the E-M5 as the perfect kit lens combination, I have reservations about that. The E-M5's 16mp is restricted by the performance of the 12-50mm, especially at 12mm and in the corners of the image. Why you'd want to build a camera that excels and then show it off with a lens that doesn't, I don't know, but the marketing departments of the camera companies have done worse things over the years. Just don't fall for the "perfect combination" line you're likely to hear at the sales counter. It isn't. Unless you're buying an E-M5 primarily for video.
(falls between my Not Recommended and Recommended ratings)
review source: purchased lens
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