I've owned the Canon EF 50mm f/1.8 MkII (which I sold to my brother), and the Canon 50/1.4 with its peculiar clutch (which I part-ex'd for a Sigma 35/1.4 Art, but that's a whole 'nother story). I was missing the simple pleasures of 50mm primery. Then the other day I saw this, the Canon EF 50mm f/1.8 STM, going for GBP 97 -- and I couldn't help myself. So here we have the latest incarnation of the 'Nifty Fifty'. Now I make no apologies for comparing it to the MkII, and (spoiler alert) it is an improvement. But the improvements are in the details.
Construction and handling
From a distance, it doesn't look much different from the MkII. It's made of mostly black plastic, but the first significant change is the metal mount -- yes, metal -- so bye-bye plastique fantastique. The barrel is made of a higher-quality matte plastic (specifically which I don't know, possibly polycarbonate) and it doesn't flex when squeezed. Being an STM lens, the manual-focus ring is just a rotary encoder and feels pretty solid and sufficiently damped, although it is only 7mm wide which can lead to a bit of fumbling -- 11 or 12mm wouldn't have hurt here. The AF/MF switch is purely electrical and shifts cleanly. Overall, while it may not sport the near-indestructible feel of the 70-200 Ls, it feels much less flimsy than the MkII.
As far as I understand, the optical formula is unchanged from the MkII (why re-write a hit?). I see no significant resolution improvement here; at its widest apertures there's a bit of fringing in the centre and haziness at the borders, but the results are still useable. The central fringing is gone before f/2.5, and from f/4 onwards it's just clean and sharp everywhere. I observed no centering problems in this copy, but I wouldn't ever expect that in this lens design anyway.
There are two main changes to the optical pipeline. First, the aperture diaphragm: 7 blades up from 5, meaning no more annoying pentagonal OoF highlights. On bokeh in general, the picture below is an unfair, unscientific and ultimately meaningless comparison. To the left, the result from Sigma 50mm f/1.4 Art at f/1.4; and on the right, an attempt to re-shoot the same scene one year on with the 50/1.8 STM at f/1.8.
The bokeh is nice enough, but doesn't have quite so much of the smooth je-ne-sais-quoi produced by the higher-spec 50mm lens. There are a few sharp-ish lines present on close inspection. Then again what did I expect? The Sigma costs 6 times as much and has probably a comparable factor more glass in it. Like I said, a pointless comparison -- but interesting to know nevertheless.
Second, they've shaved 10cm off the MFD, so it's down to 35cm. I see a tiny bit of residual spherical aberration at closer focus, so the in-focus region shifts away from the camera slightly as the lens is stopped down. It's unlikely that this would ever be a problem in real-life, as (1) I'd use Live View and DoF Preview in such circumstances; and (2) my hands shake back and forth by more than the depth-of-field at that distance anyway. The example below is very close, about 40cm.
The lens has an insignificant amount of focus shift when stopped down at close range. Taken on a tripod with the lens's focus set manually. The red line shows where I think focus lies.
This lens produces very little transverse chromatic aberration (on APS-C at least). At faster apertures longitudinal CA (pink/green fringing) is noticeable on close inspection. Note that DPP 3.15 or later is required for this lens's aberration correction profile.
As with its predecessor, the entire optical assembly moves back and forth to achieve focus, extending about a 11mm from the barrel at closest-focus. The drive makes a normal robotic stepper motor noise, like the Canon EF-S 24mm f/2.8 STM but slightly louder. The whole mechanism feels more solid than the MkII's; there's no discernible lost motion, or the sloppy scraping action of its predecessor. However it's not one of the 'fast' STMs like those found in the EF-S 55-250mm or 10-18mm lenses.
The focus-by-wire manual ring is no better or worse than on any other STM I've tried. The absence of tactile feedback means it takes some getting used to, even with Live View. Manual in AF mode is full-time, provided AF is active (e.g. with the shutter button held halfway). Manual focusing does not work with the DoF preview button held down (at least on the EOS 7D).
As to phase-detect AF accuracy: don't make me laugh. In good light with an uncomplicated subject it hits the target most of the time, but can be jittery. My copy does better when the current focus setting is far away from the target; when already nearly there it tends to move about a little bit and then land back where it started. This isn't anything that can be AFMA'd away, and I see no overall bias towards the front or back. Personally I'll stick to Live View focus for anything faster than f/4.
It's undeniably good fun, in the same way a small car can be.
Now if I had a broken MkII and were on a tight budget, the STM is essentially no-brainer. But if money were less of an object, or I wished to upgrade, I'd probably be looking at the Canon 50/1.4, Sigma 50/1.4 Art, or Tamron 45/1.8 VC. They all have the advantage in terms of image quality and speed or stabilisation, but these things come at a cost: around GBP 250, 600 and 500, respectively. But taken on its own, the Canon EF 50mm f/1.8 STM is still a very worthy lens, and at under GBP 100, outstanding value.