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. The images below illustrate the deterioration of the out-of-focus highlights towards the image corners. Stopping down to f/2.2 to f/2.8 maintains the circular disc shape - at least in the image center. Out-of-focus highlights are nicely rendered with no outlining and a very smooth inner zone. And the Canon RF 35mm f/1.8 STM IS macro doesn't disappoint us here. If you decide to buy a fast prime lens, you also do so for superior bokeh compared to the zoom lenses. This is usually neglible and the remaining traces can be easily auto-corrected without any harmful side effects. Lateral CA (color shadows at the image borders) stay just below an average pixel width of 1px at the image borders. If you want to know more about the MTF50 figures you may check out the corresponding Imatest Explanations The chart shows line widths per picture height (LW/PH) which can be taken as a measure for sharpness. Please note that the MTF results are not directly comparable across the different systems!īelow is a simplified summary of the formal findings. The centering quality of the tested sample was good. f/16 sees a more pronounced drop in resolution and f/22 should be avoided (as usual). 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. The peak performance is reached between f/4 and f/5.6 with an excellent center and very good borders/corners. The broader center zone has excellent quality at f/2.8 and there's a slight boost in the outer image field as well. At maximum aperture, most of the image field is very sharp and the corners are at least on a good level. 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. The issue is pretty much resolved from f/2.2 onward already albeit a strong amplification is, of course, still required till f/4. 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. With auto-correction, you can still spot some light falloff at f/1.8 (1.3EV). You have to stop down to f/4 in order to tame the issue down to acceptable levels here. Given the very small front element, it seems likely that the lens is a bit underdesigned in this respect. 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). Activated image auto-correction is advisable when it comes to vignetting.
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