The RF50mm f/1.8 STM is the fast, affordable nifty fifty lens that many EOS R system users have been waiting for. How does it fare when compared with its EF mount counterpart, the EF50mm f/1.8 STM, the third generation of a popular lens? We examine the two lenses on six aspects. (Reported by: Kazuyuki Okajima, Digital Camera Magazine)

#1: Body
#2: Resolving power
#3: Bokeh
#4: Backlight performance
#5: Image stabilisation (with EOS R5/R6)
#6: Operability

 

#1: Body

While the EF50mm f/1.8 STM is around the same size and weight as the RF50mm f/1.8 STM, you will need a mount adapter to attach it to an EOS R series camera. This adds a little extra weight and a few centimetres to the overall length, but the balance with the camera is still decent.

The two have different filter diameters: the one on the RF50mm f/1.8 STM is 43mm, smaller than that on its EF counterpart (49mm). However, the maximum diameter of the lenses remains the same. The RF lens looks more compact and stylish, but those who like lots of accessories and attachments on their cameras might feel that they look more "cool" with the EF lens and mount adapter.

There are subtle differences in the colour and coating, though how much that matters will depend on your preferences.

A: Focusing ring	RF50mm f/1.8 STM Texture that feels good on the finger. Turns easily. Movement is seamless as the torque is just right. Easy to carry out precise focusing. EF50mm f/1.8 STM Slipperier for the finger. A little harder to turn, but no issues for focus adjustment.
B: Texture	RF50mm f/1.8 STM A dark grey, semi-matte lens barrel. The silver mount core ring and other details like the diamond-textured focusing ring give it a luxurious feel. EF50mm f/1.8 STM The lens barrel looks high quality but it doesn’t have the same luxurious look as the RF version. This is partially due to the texture of the focusing ring.

C: Lens mount

RF50mm f/1.8 STM The metal mount is highly durable and provides a luxurious feel. The frontmost lens element protrudes slightly more than on an EF lens, but the rearmost lens element recedes more. EF50mm f/1.8 STM Until the EF50mm f/1.8 II, the mount was plastic. However, this version has a metal mount like its RF mount counterpart.

 

#2: Resolving power

EOS R5/ Aperture-priority AE (f/8, 1/1000 sec, EV -0.7)/ ISO 100/ WB: Auto

A (image centre)

Comparing the image centre in the above two shots, the lenses have more or less the same resolving power. To me, the image by the RF lens seems a little sharper with slightly finer lines and better microcontrast. 

B (image peripheries)

At the image edges, the EF lens image seems to be slightly better resolved, whereas the details on the RF lens seem to be slightly softer.

I enabled the Digital Lens Optimizer for my shots. On both lenses, the image seems to be the sharpest at f/5.6 and f/8. Sharpness gradually declines after f/11, and at the f/22 minimum aperture, the image becomes significantly softer.

 

#3: Bokeh

EOS R5/ Aperture-priority AE (f/1.8, 1/1000 sec, EV +0.7)/ ISO 100/ WB: Auto

The images above were shot at the closest focusing distance of the EF lens (35cm). This achieves a maximum magnification of 0.21x, but the RF lens is capable of shooting closer than this, with its shorter closest focusing distance of 30cm providing a maximum magnification of 0.25x.

I fixed the camera on a tripod to keep the shooting distances the same for both lenses, but the images shot on the RF lens appear slightly wider. This makes the bokeh on the EF lens example look slightly more intense, but the bokeh on the RF lens feels smoother, with more gradual transitions from the in-focus areas to the out-of-focus areas (see the area in the red frame). In comparison, 

The edges of the bokeh circles created by the EF lens are sharp—so sharp that I found them somewhat distracting. Those on the RF lens are softer and more gradual, blending in with the rest of the bokeh.

 

#4: Backlight performance

Both images: EOS R5/ Aperture-priority AE (f/22, 1/320 sec, EV -2.0)/ ISO 100/ WB: Auto

On the EF50mm f/1.8 STM, there is slight ghosting around the sun, but other than that, I couldn’t see any significant difference in the optical performance between the two lenses in this scene. The image would probably look more immersive if shot on the RF50mm f/1.2L USM, but there was no visible deterioration in contrast, resolution, or other aspect of image quality on either lens.

Shooting street photographs, the EF lens seemed more prone to ghosting and flaring whereas with the RF lens, this was not an issue in similar scenes. However, it is hard to compare as the shooting conditions were not exactly the same.

 

#5: Image stabilisation

EOS R5/ Shutter-priority AE @ 1/50 to 2.5 sec/ ISO 100/ WB: Auto

When attached to the EOS R5 or EOS R6, the combined image stabilisation effect possible with the RF50mm f/1.8 STM is up to seven shutter speed stops’ equivalent. I tested this, shooting at various shutter speeds.

 

The results show that the image stabilisation effect available on the EF lens was approximately one stop below the RF lens.

 

#6: Operability

The RF lens was slightly quieter and acquired focus faster. There also seemed to be less focus hunting.

Both lenses have a switch on the left side of the lens, but the functions are different.

On the EF version, the switch toggles between autofocus and manual focus (AF and MF) modes.
On the RF lens, the switch changes the function of the control/focusing ring. To switch between the AF and MF modes, you either go to the camera menu and select the ‘Focus mode’ item (unavailable when the EF lens is attached), or assign a button to “Stop AF”.  The control ring becomes unavailable during manual focusing.

Note that if you use the EF lens with the Control Ring Mount Adapter, you will be able to use both the focus ring and the control ring during manual focus.

 

Summing up: RF50mm f/1.8 STM performs better overall, but the EF version isn’t too far behind

If you already own the EF50mm f/1.8 STM…

The control ring, shorter closer focusing distance and larger magnification of the RF50mm f/1.8 STM are its key attractions. The optical performance of the RF lens is also marginally better than that of the EF50mm f/1.8 STM.

However, if you already own the EF50mm f/1.8 STM, in my opinion, the differences are not enough to necessitate upgrading to its RF mount version. Depending on your uses, it might be more cost-effective to buy the EF-EOS R Mount Adapter to use the lens with your EOS R system camera. If you are not satisfied with the image quality of your existing EF50mm f/1.8 STM, consider upgrading to a higher class of RF lens instead. I personally am looking forward to an RF mount version of the EF50mm f/1.4 USM.

Also see:
Lens Impressions: RF50mm f/1.2L USM in Portraits & Street Photography

If you don’t already own the EF mount version…

There is no need to hesitate between the two: go ahead and get the RF50mm f/1.8 STM. After all, it was designed for the EOS R system! (See: 6 Significant Features of RF Lenses)

The RF50mm f/1.8 STM is a great choice if you want a fast prime lens to add to a kit that consists of mainly zoom lenses, or even if it is the first lens you are buying for your new EOS R system camera. It provides excellent performance for its price point, is very portable, and balances well with the EOS R5/R6. Not only so, a nifty fifty like this is much lighter to carry around for shoots than a zoom lens, making it effective for learning how to use your footwork to enrich your photographic expression.

Find out how to harness the unique characteristics of your standard lenses in:
Professional Composition Techniques (3): Making Good Use of Lenses
Standard Lens Techniques: Using the Point of View to Draw the Viewer In

Considering whether to buy the EOS R5 or EOS R6? This article might help you decide:
EOS R5 vs EOS R6: 5 Key Differences to Note

 

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