Canon RF 50mm F1.8 STM
Michael Topham tests an inexpensive standard prime in RF mount for Canon’s EOS R cameras
An affordable standard prime has been missing from Canon’s RF-mount lineup since the launch of the EOS R system. Those who’ve bought into Canon’s full-frame mirrorless system have had the RF 35mm F1.8 IS Macro STM (£529) available to them since 2018, and now, nally, there are compact and affordable alternatives to the RF 85mm F1.2L (£2,869) and RF 50mm F1.2L USM (£2,389) in the shape of the RF 85mm F2 Macro IS STM (£649) and RF 50mm F1.8 STM (£219). In this review we’re looking at the latter; we’ll begin by studying how it differs from Canon’s existing and affordable standard primes in EF-mount.
Features
Although it’s the cheapest native lens you can currently buy in RF mount, the RF 50mm F1.8 STM costs £100 more than the EF 50mm f/1.8 STM for Canon DSLRs. In the same way the EF 50mm f/1.8 STM (£119) offered several improvements over the older EF 50mm f/1.8 II, the RF 50mm F1.8 STM advances where the EF 50mm f/1.8 STM left off.
It shares a similar arrangement of six elements in ve groups with the EF 50mm f/1.8 STM, but adds a new aspherical lens element. The inclusion of this element is important as it’s alleged to reduce aberrations across the frame and contribute to superior sharpness from the centre to the edge. Canon’s Super Spectra coating is also claimed to play its part in improving image quality and colour rendering. Again, just like its EF-mount predecessor, it’s equipped with seven diaphragm blades and has a
minimum aperture of f/22.
The STM abbreviation reveals it’s equipped with a stepping motor (STM) to drive autofocus and ensure operation is kept quiet. Canon has used this motor technology across many of its lenses since 2012, including the EF 50mm f/1.8 STM, and I’ll touch on this in more detail shortly in the autofocus section of this review.
Optical image stabilisation isn’t featured and as things stand, only the EOS R5 and EOS R6 provide in-body image stabilisation (IBIS). Users of Canon’s EOS R and EOS RP models who intend to use the lens handheld but don’t have the luxury of IBIS to fall back on will want to be careful of camera shake when dropping the shutter speed to the vicinity of 1/50sec.
Its minimum focusing distance (30cm) is a small improvement over the EF 50mm f/1.8 STM that focuses to 35cm, and lters and adapters can be mounted via a 43mm thread at the front. If you like to backlight subjects or frequently shoot towards the light and would like to reduce the amount of lens are and glare in your shots, you’ll want to budget an additional £22 for Canon’s ES-65B hood, which rather annoyingly isn’t supplied as part of the boxed contents.
Build quality
Older nifty fties like the EF 50mm f/1.8 II are known for their fairly cheap build quality and plastic nishes. Plastic lens mounts are quite rare on mirrorless lenses today and the RF 50mm F1.8 STM boasts a durable metal lens mount and re ned build quality that looks the part coupled to Canon’s enthusiast and professional EOS R models. The nish is marginally better than the EF 50mm f/1.8 STM, with the knurled focus ring offering improved grip and the silver ring around the lens mount complementing the attractive mount on EOS R camera bodies.
Canon’s decision to exclude an AF/MF switch and add a focus/ control switch forces users to head into the camera’s main menu to change focus mode. With the focus/control switch set to focus, the focus ring, which is fairly thin, behaves like a normal manual focus ring and rotates
uidly with no hard stops. Flick the switch to control and it turns into a customisable control ring that can be used to change exposure variables, the AF mode, picture style or white balance. This can be done directly or by
holding the exposure lock button to avoid unintentional changes.
In terms of size, it’s a lovely compact lens and won’t weigh you down. It’s ideal for times when you’d like to get creative with depth of eld on your travels, or when you’d simply like a portable, lightweight lens to walk about with. It’s smaller and lighter than using the EF version with one of Canon’s EF-EOS R mount adapters too, but it should be noted that focusing isn’t accomplished internally, so the front element does extend and increases in length the closer you shoot to subjects.
‘Focusing isn’t accomplished internally, so the front element does extend’
Autofocus
Given its compact size, the lens employs Canon’s gear type of STM that uses helical gears to drive the focus rather than the physically larger lead-screw type of STM. As I discovered during testing, the lens produces some residual low-level noise as it autofocuses, however it’s only really noticeable when you’re shooting in quiet surroundings like an empty church and it couldn’t be traced in audio recorded outdoors where ambient sound is usually loud enough to cancel it out.
Entering manual focus and twisting the focus ring emits a hum that gets louder the faster it’s rotated. Manual focus is the
y-by-wire type and I found there to be only a small amount of lag between twisting the focus ring and the lens responding. Although it’s not as fast to focus as Canon lenses that use the lead-screw type of STM, it’s not what I’d describe as slow when acquiring focus. I only observed signs of hunting when I asked it to focus close to its minimum focusing distance in extremely dark lighting conditions.
Image quality
With the lens yet to be supported by a lens pro le in Adobe Camera Raw or Lightroom Classic, my analysis of its optical performance was based on lens aberration corrections being turned on in-camera and no pro le being applied in software. Centre sharpness at f/1.8 exceeds expectations for a lens that costs £219, however sharpness at the periphery of the frame drops off considerably. Stopping down to f/2 or f/2.8 has no major impact on centre sharpness, but edge sharpness is perceptibly better. Shoot at f/4 and you’ll notice the corners get even sharper, with the sweet spot of edge-to-edge sharpness observed between f/5.6 and f/8. Diffraction softens ne detail a touch at f/16 and f/22. The level of sharpness the lens manages to resolve between f/4 and f/11 can’t be grumbled at for the price.
Vignetting and chromatic aberration were identi ed when I inspected raw les closely on my computer. Reduced brightness towards the periphery was obvious in real-world images at f/1.8, however it alleviates as you stop down, vanishing completely by f/4. Shooting towards the sun without the optional lens hood highlighted that it’s susceptible to quite severe lens are and prominent green and purple fringes of colour along high-contrast edges at wide apertures. The defringe eyedropper tool in Camera Raw was used to mitigate the chromatic aberration during processing, but nothing could be done to rectify the lens are in a few of my sunny portrait images.
Standard primes with a focal length of 50mm usually handle distortion well and that’s exactly the case here. No barrel or pincushion distortion was evident in my sample images, with horizontal and vertical lines appearing straight and true.