by Klaus Schroiff, published April 2023
Reviewing a “super zoom” lens is both heaven and hell. It’s heaven because the world is full of shooting opportunities with this much zoom range. And it’s hell for the same reason when having to do the formal testing across many focal lengths in the lab – with results that are often not all that exciting. Anyway, these lenses are popular, so they are an “inevitable” part of the game. This time, it’s about the Canon RF 24-240mm f/4-6.3 USM IS. Canon has a bit of a mixed history regarding super zoom lenses. There is, of course, the mighty EF 28-300mm f/3.5-5.6 USM L IS or its predecessor, the EF 35-350mm f/3.5-5.6 USL L – both loved and well respected within the photojournalist community. On the other hand, there was the miserable EF-S 18-200mm f/3.5-5-6 IS or the terrible EF 28-200mm f/3.5-5.6 USM. So let’s see which category this new RF lens falls into. Given its rather steep price tag of a whopping $900/1000EUR, it better delivers something decent …
In terms of mechanical quality, there’s nothing wrong with the RF 24-240mm f/4-6.3 USM IS. Yes, it’s mostly made of plastics, but it feels quite sturdy. This – admittedly subjective impression – may also relate to the rather heavy weight of 750g. The lens doesn’t feature any weather sealing, which is a bit meh, given its price tag. Typical for its class, it extends quite a bit when zooming toward the long end of the range – using 2 inner lens tubes.
Even so, our sample didn’t exhibit any wobbling even when fully extended. You can feel the extreme zoom range when turning the zoom ring, but the zoom action is pretty good. The custom control ring, also used for focusing, operates smoothly as well.
The petal-shaped lens hood is optional. This is common practice with non-L Canon lenses, but for a penny article, it’s disappointing nonetheless.
The Canon lens uses a Nano-USM for autofocusing – this is both very fast and noiseless. As usual, manual focusing works “by-wire” which works very well. The built-in image stabilizer is rated at 5 f-stops and is also optimized for smooth video stabilization (“Dynamic IS”).
|21 elements in 15 groups (1 aspherical & 2x UD elements)
|Number of aperture blades
|min. focus distance
|0.5-0.78m (max magnification: 1:3.8)
|petal-shaped (bayonet mount, optional)
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.
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.
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.