Canon EF 50mm f/1.8 STM

SLRgear Review
July 30, 2015
by Andrew Alexander

The Canon EF 50mm ƒ/1.8 STM is the third lens in the 50mm line for Canon, the original having been released in 1987 and updated in version II in 1990. Twenty-five years later, the lens has been overhauled with the new STM focusing system and a new look, but otherwise the optical design is unchanged.

The lens was designed to fit the EF (35mm) mount, and will also work on the EF-S mount for 1.6x cropped sensor bodies, as well as Canon's existing APS-H (1.3x crop) bodies. On these bodies the equivalent field of view is 80mm and 65mm respectively.

The lens is available now for approximately $125.

Sharpness
Used wide open at ƒ/1.8 or ƒ/2, the lens exhibits substantial corner softness. On a sub-frame camera like the 7D it's not quite as noticeable as on a full-frame camera like the 1Ds mkIII, because the sensor doesn't capture the entire field of view offered by the lens. Either way, at ƒ/1.8 there's only a small area of sharpness in the center of the frame, which quickly gives way to softness going out towards the corners.

Stopping down to ƒ/2.8 improves sharpness dramatically: on the sub-frame 7D, images produced are almost tack-sharp corner to corner, with a light amount of softness in the extreme corners; however, on the full-frame 1Ds mkIII there is still some corner softness to contend with. It's not until ƒ/4 that we see true corner-to-corner sharpness, which improves negligibly as the lens is stopped down to ƒ/8.

Diffraction limiting sets in at ƒ/11, but you won't see any practical impact on sharpness until ƒ/16 and ƒ/22.

Chromatic Aberration
The Canon EF 50mm ƒ/1.8 STM is very resistant to chromatic aberration: there is a slight amount present when the lens is used at its widest apertures, but as it is stopped down it is less prominent.

Shading (''Vignetting'')
With the Canon EF 50mm ƒ/1.8 STM mounted on the sub-frame 7D, there is a slight amount (1/3EV) of corner shading that disappears as the lens is stopped down past ƒ/2.

When mounted on the full-frame 1Ds mkIII, the lens produces some impressive corner shading when used at ƒ/1.8 or ƒ/2 - the extreme corners are one and a quarter stops darker than the center. Stopped down to ƒ/2.8, this corner shading becomes just a half-stop differential, and at ƒ/4 and smaller, corner shading becomes negligible.

Distortion
The Canon EF 50mm ƒ/1.8 STM produces images with a small amount of barrel distortion, less noticeable on the 7D than on the 1Ds mkIII: on the latter camera, images show +0.5% distortion in the corners. This is easily correctable in post-processing.

Autofocus Operation
The Canon EF 50mm ƒ/1.8 STM employs an autofocus system called a "Stepping Motor", which allows the lens to provide smooth and quiet autofocus operation -- particularly beneficial during video capture. The lens took around a second to focus from infinity to close-focus. It offers full-time manual operation by just turning the focus ring after autofocusing. Attached 49mm filters will not rotate during focus operations, making life easier for polarizer users.

Macro
The lens' minimum close-focusing distance is just over thirteen inches, and at this minimum distance it provides a magnification of 0.21x - not great for macro work, but not terrible, either.

Build Quality and Handling
At just over five ounces, the new 50mm ƒ/1.8 STM is actually slightly heavier than the 50mm ƒ/1.8 II, which weighed an ounce less. The new lens sports a satin black finish, and uses a metal mount to attach it to the camera, however it uses 49mm filter threads instead of the more common 52mm size. The lens is devoid of any ornamentation other than its identification badge - there is no distance scale or depth-of-field markings. There's only one control switch on the lens, to enable or disable autofocus. The diaphragm is made up of seven curved aperture blades, compared to the previous version's five, which should make for more pleasing bokeh results.

The focus ring is a scant 1/4" wide, and turns forever in either direction, owing to the fly-by-wire STM focusing system. You can do full time manual focus by being in one shot mode and half-pressing the shutter button, allowing you to override the autofocus results. The lens extends by 3/8" when it focuses to its minimum focusing distance.

The lens cap is now a 49mm center-pinch design. No lens hood is included with the lens, but the front lens element is quite recessed, so a lens hood isn't as mandatory as you'd think. If you really want one, the optional ES-68 round hood is available for approximately $27.

Alternatives

Canon EF 50mm ƒ/1.8 II ~$110
At the time of writing, it's still on store shelves, but probably only until it sells out - however, at the price there's no reason to rush out and buy this lens instead of the new STM, unless you're not a fan of the fly-by-wire focusing system. Optically, the new lens is much sharper - the results at ƒ/1.8 were pretty poor on the version II ƒ/1.8 lens.

Canon EF 50mm ƒ/1.4 USM ~$350
We'll look forward to an STM update to the ƒ/1.4 lens, as its performance wide-open isn't great, either; if you'd like the extra 2/3 of a stop in light-gathering performance, this is the lens for you.

Canon EF 50mm ƒ/1.2L USM ~$1,450
For an even smaller amount of extra light-gathering ability, you can pay a lot of money, and that's where the ƒ/1.2L comes in - it's a sharper version of the ƒ/1.4 lens, and provides sharper results at ƒ/2 than the 50mm ƒ/1.8 STM. After that, they're about on par with each other.

Sigma 50mm ƒ/1.4 EX DG HSM ~$500
An economical ƒ/1.4 lens, it provides sharp images even at its wider apertures, though on the full-frame sensor corner softness is present and interesting. It's about as sharp as the 50mm ƒ/1.8 STM at the same apertures, however corner shading is a bit more prominent through the entire range of apertures.

Sigma 50mm ƒ/1.4 DG HSM "A" ~$1,000
Sigma's "Art" lineup of lenses features the updated 50mm ƒ/1.4, which produces excellent performance at its wider apertures.

Conclusion
It's probably not as sharp as some people would like when used wide open, but stopped down to ƒ/2.8 or ƒ/4, it's as sharp as sharp gets. For the price point, you really can't go wrong with this lens, and it's a welcome improvement in sharpness over the previous version II of the 50mm ƒ/1.8.

Product Photos

Sample Photos

Click here for Real-world Gallery Images on our Flickr page!

The VFA target should give you a good idea of sharpness in the center and corners, as well as some idea of the extent of barrel or pincushion distortion and chromatic aberration, while the Still Life subject may help in judging contrast and color. We shoot both images using the default JPEG settings and manual white balance of our test bodies, so the images should be quite consistent from lens to lens.

Still Life shot

VFA target

As appropriate, we shoot these with both full-frame and sub-frame bodies, at a range of focal lengths, and at both maximum aperture and ƒ/8. For the ''VFA'' target (the viewfinder accuracy target from Imaging Resource), we also provide sample crops from the center and upper-left corner of each shot, so you can quickly get a sense of relative sharpness, without having to download and inspect the full-res images. To avoid space limitations with the layout of our review pages, indexes to the test shots launch in separate windows.