Canon RF 35mm f/1.8 STM IS macro - Review / Test Report - Analysis
Lens Reviews - Canon EOS (Full Format)

Distortion

Most users prefer to have image auto-correction activated and images are free of distortions in this case. However, the native distortion characteristic is actually pretty decent with a barrel distortion of just ~1%. Unless you do repro-photography, this is hardly noticeable.

Vignetting

Activated image auto-correction is advisable when it comes to vignetting. The native vignetting is rather massive at f/1.8 where it is beyond our usual scale for full format cameras at a whopping ~3.8 EV (f-stops). Given the very small front element, it seems likely that the lens is a bit underdesigned in this respect. You have to stop down to f/4 in order to tame the issue down to acceptable levels here. With auto-correction, you can still spot some light-falloff at f/1.8 (1.3EV). Or in other words - the image signal is boosted by 2.5 f-stops in the image corners with a corresponding increase in sensor noise. The issue is pretty much resolved from f/2.2 onward already albeit a strong amplification is, of course, still required till f/4.

MTF (resolution)

The resolution characteristic of the Canon RF 35mm f/1.8 STM IS macro is impressive - although it is worth mentioning that the 30-megapixel sensor in the EOS R (used for testing) isn't overly ambitious. At maximum aperture, most of the image field is very sharp and the corners are at least on a good level. The broader center zone has excellent quality at f/2.8 and there's a slight boost in the outer image field as well. The peak performance is reached between f/4 and f/5.6 with an excellent center and very good borders/corners. Diffraction is the limiting factor beyond but that's not really noticeable at f/8 yet and f/11 is also just fine by most standards. f/16 sees a more pronounced drop in resolution and f/22 should be avoided (as usual).

The field curvature is fairly low. The centering quality of the tested sample was good.

Below is a simplified summary of the formal findings. The chart shows line widths per picture height (LW/PH) which can be taken as a measure for sharpness. If you want to know more about the MTF50 figures you may check out the corresponding Imatest Explanations

Chromatic Aberrations (CAs)

Lateral CA (color shadows at the image borders) stay just below an average pixel width of 1px at the image borders. This is usually neglible and the remaining traces can be easily auto-corrected without any harmful side effects.

Bokeh

If you decide to buy a fast prime lens, you also do so for superior bokeh compared to the zoom lenses. And the Canon RF 35mm f/1.8 STM IS macro doesn't disappoint us here.

Out-of-focus highlights are nicely rendered with no outlining and a very smooth inner zone. Stopping down to f/2.2 to f/2.8 maintains the circular disc shape - at least in the image center.

The center isn't everything though. The images below illustrate the deterioration of the out-of-focus highlights towards the image corners. At f/1.8 the discs are no longer circular from the mid image field which is fairly "early" and the shape is clearly distorted in the corners. As usual, the shape improves when stopping down but the corners are still not fully corrected at f/2.8.

A positive aspect is the general bokeh as you can see below. The background blur (shown to the left) is especially silky. The less critical foreground (to the right) shows a double-lined blur though.

Bokeh Fringing (LoCA)

Bokeh fringing - also referred to as LoCAs - is a colour fringing effect on the Z-axis. It shows up as purplish halos in front of the in-focus zone and greenish beyond. You should be easily able to spot the effect in the image crops below. They are most obvious at f/1.8 but traces are still visible at f/4. While this is not positive, it is rather normal. Just a handful of lenses are corrected in this respect and they tend to have a higher price tag.

However, what you can also observe when scrolling through the images below - the focus point moves towards the rear the more you stop down - without a change of the initially set focus distance. These are so-called "residual spherical aberrations" (RSAs) and they may explain why we had some issues with the accuracy of the AF as mentioned in the introduction.