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Canon RF 24-240mm f/4-6.3 USM IS - Review / Test Report - Analysis |
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Lens Reviews -
Canon EOS (Full Format)
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Page 2 of 3
Distortion
As you might expect with such a lens, the RAW distortions aren't something Canon can brag about. The "worst" setting is, of course, at the wide end with an excessive barrel distortion of more than 8(!)%. At 50mm, we detected slight pincushion distortion, and this is amplified to strong pincushion distortions from the middle range all the way up to 240mm. So to sum this up - don't even think of using this lens without image auto-correction.
With activated auto-correction, these worries are mostly gone. At 24mm, you can still spot a mild barrel distortion just short of 1%. At medium to longer focal lengths, the distortions are basically eliminated, though.
Vignetting
The native vignetting characteristic is ... "interesting". At 24mm, the lens produces black corners regardless of the aperture setting. Or in other words - images are basically useless without auto-correction. The vignetting is "normal" at longer focal lengths.
For the fun of it - the "vignetting" looks like this at 24mm:
Once again, image auto-correction comes to the rescue. It reduces the fall-off with decent results at max aperture already and almost negligible vignetting when stopping down a bit.
MTF (resolution at 45 megapixels)
An almost surprising aspect of the Canon RF 24-240mm f/4-6.3 USM IS is its resolution characteristic. Despite the 10x zoom ratio, it's actually pretty sharp. Its sweet spot (or better range) is the wide- to medium tele settings. Even the 24mm setting is quite decent despite the massive auto-correction at the setting. The center quality is excellent in the f/4 to f/8 range, and the border quality is still very good. The corners are somewhat softer but still very usable, even at f/4. Diffraction effects reduce the quality slightly at f/11. The quality is more even at 50mm and 100mm. The center quality is marginally reduced, whereas the corner performance improves quite a bit. Unsurprisingly, image quality takes a hit at 240mm. The broader center quality is still very good, whereas the outer image field gets softer without being terrible though. Stopping down to f/8 doesn't improve the image quality.
The field curvature is noticeable at 24mm but mild at the other tested focal lengths. The centering quality of the tested sample varied quite a bit - a typical fate in this class.
Please note that the MTF results are not directly comparable across the different systems!
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 of sharpness.
If you want to know more about the MTF50 figures, you may check out the corresponding Imatest Explanations
MTF (resolution at 30 megapixels)
If you are using a lower-megapixel body, the subjective quality perception will be "better" due to the lower pixel density. At 30 megapixels, the results are actually pretty impressive across the focal length range. This also applies to 240mm. Although it's not the last word in terms of sharpness, there's little to be desired here, even in the outer image field.
Chromatic Aberrations (CAs)
Lateral CAs (color shadows in the outer image field) are another weak spot in RAW images (only). At 24mm, the average CA pixel width is more than 3px on average at the borders and even more so in the image corner. The issue is a somewhat lesser concern other focal lengths, but, once again, Canon does solely rely on image auto-correction to handle this.
Bokeh
The RF 24-240mm f/4-6.3 USM IS can do many things for you, but it is hardly a lens that can be called a bokeh monster - it's just too slow for this. However, given its long tele range, you can still achieve some decent object isolation at closer focus distances.
Out-of-focus highlights are surprisingly clean, although there is a bit of an outlining effect that gets emphasized the more you stop down.
The shape of the highlights tends to deteriorate towards the image corners - this is due to mechanical vignetting. However, the shape deteriorates quite "late" towards the corners, as you can see below. Stopping down to f/8 restores most of the circular disc shape already.
The general image rendering in the focus transition zones isn't absolutely perfect, but it's still much better than expected. The blur is quite symmetrical and smooth both in the background (shown to the left below) and the foreground (to the right).
Sun Stars (Experimental)
Below is s sequence of 100% cropped images from 24mm f/4 all the way up to f/16 - illustrating the sun star behavior. Sun stars are an aperture effect that shows up if a bright light source is part of the scene (usually in night shots) - here, illustrated using an LED. While the lens is capable of producing star rays even at f/4, the shape isn't overly pleasant with "fan-like" rays plus sub-rays within.
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