I’ve been examining the Canon RF 24-50mm STM lens on full-frame bodies and noticed something important: this lens, often recommended by housing manufacturers, does not actually project image corners onto the sensor at 24mm and not even fully at 28mm in uncorrected RAW. In other words, the much-discussed corner sharpness that underwater photographers focus on isn’t even present at those wide ends, because the image simply doesn’t cover the full sensor corners natively.

Even a fancy underwater correction optic such as the Nauticam WWL-1 WACP FCP or Marelux Aquista 110 or 130 or cannot fix this. The corners of the frame are just not recorded and a pure digital fantasy mashup of reality. This improves slightly at 28mm and is gone at 35mm.

However, there are reasons why this lens is still a favorite for underwater setups with water-contact corrective optics. Let me summarize the key points:

• Compact and Lightweight: It’s extremely small and light, making it easy to handle in underwater housings.

• Small Front Glass Element: It’s one of only a few Canon RF lenses with a very small front glass element (just 37 mm), which makes it suitable for water-contact optics.

• Retracting Design: At 24mm, the lens is fully extended, and as you zoom to 50mm, it retracts inward. This makes it physically convenient for flat port designs and is a unique trait among Canon RF wide-angle zooms.

In essence, while the lens doesn’t project full image corners at the widest focal lengths in uncorrected RAW, the practical benefits—compactness, compatibility with corrective optics, and convenient physical behavior—explain why it’s still a favored choice in the underwater photography community. Is it to you?

I bought the lens a few month ago and also got a Marelux Zoom Gear for it, but ever since sinking it in a bathtub it is giving me a lot of headaches. This is why I called it „the shard“ in another forum thread here. I hope somebody can change my mind and give some value to this piece of glas.

If you’re looking to have a flexible wide-angle zoom that you can combine with underwater contact optics or teleconverters, here’s a quick comparison of the zoom factors for some of these solutions:

• Canon RF 24-50mm STM (without limiter): 2.08×

• Canon RF 24-50mm STM (with 28mm limiter): 1.79×

• Canon EF 8-15mm Fisheye with 2.0× Teleconverter: 1.88×

So, these are some theoretical findings and empirically validated facts. I’d be interested in reading your practical experiences and thoughts on the Canon RF 24-50mm STM lens. If you’ve found better alternatives or reasons why this lens stands out as the ultimate wide-angle solution for you, please share your images and thoughts !

I'm using the RF10-20 and i'm happy with it. ok with a 9" dome.

I don't understand why only a RF24-50... 24 is really far from WA underwater.. Sorry for this question/remark

1 hour ago, CaolIla said:

I don't understand why only a RF24-50... 24 is really far from WA underwater.. Sorry for this question/remark

Sorry for maybe not being explicit enough about this in my thread opener: the Canon 24-50 STM is intended to be used with underwater corrective optics, which will turn it into approx 130-70 degree FOV being equivalent to your 10mm with the RF10-20.

By the way, you highly expensive RF 10-20 suffers from a similar flaw at even higher level:

With disabled distortion- and vignetting compensation, the image corners are black at 10mm.

https://opticallimits.com/canon/canon-rf/canon-rf-10-20mm-f-4-l-is-stm-review/

1 hour ago, Adventurer said:

By the way, you highly expensive RF 10-20 suffers from a similar flaw at even higher level:

With disabled distortion- and vignetting compensation, the image corners are black at 10mm.

https://opticallimits.com/canon/canon-rf/canon-rf-10-20mm-f-4-l-is-stm-review/

Wouahhhh Thanks for the information.. I didn't know that... but with the correction I didn't notice any problem... --> It's ok for me ;)

First of all neither Marelux or Nauticam support the RF 24-50mm STM lens for use with WET WIDE lenses behind a flat 67mm port. This eliminates use with WWL-1/1B, Aquista 110 and 130. This lens does however work behind dome ports from 180mm or greater with the proper extension. The lens also works with wet wide lenses like WACP-C, WACP-1/1B and FCP-1 using the proper port extension and in some cases will not zoom through the entire zoom range. WACP-C and WACP-1/1B also work on Marelux housings with the NA to MX N120 and N100 port adapters.

The best Canon RF lens for zooming through the entire range using wet wide lenses behind a flat port remains the 18-45mm. for APS-C.

Edited 19 hours ago19 hr by Phil Rudin

Yes, per Nauticam port chart, with wet lenses the zoom range is limited to 28mm

7 hours ago, Phil Rudin said:

First of all neither Marelux or Nauticam support the RF 24-50mm STM lens for use with WET WIDE lenses behind a flat 67mm port.

Sorry Phil, but that is wrong.

Nauticam lists WWL-1C combination with Canon RF 24-50mm STM in their port chart: https://drive.google.com/file/d/17vlICB0Gn6bbeTGaIR9yd3HHOIfIG9kK/view

It is quite common to see vignetting like this in lenses for mirrorless cameras, the manufacturers will do this to produce a cheaper lens and correct the errors electronically. I found a review on line with an image both corrected and uncorrected and it appears what Canon has done is the lens uncorrected has a field of view a bit wider than 24mm and as the lens has a lot of barrel distortion which stretches the corners that they pull in to create a 24mm wide frame electronically. Doing this they avoid having to correct for the strong barrel distortion.

( of course I don't have the lens in hand and it's equally possible that the correction produces an image that's not as wide as a true 24mm lens - this would be a good project to compare the 24mm field (corrected) with that of an L lens like the 24-105)

That aside a lot of reviews regard this lens as not the sharpest knife in the drawer so unfortunately FF Canon users are stuck with a wet optics lens that is less than ideal. The Nikon equivalent 24-50 is reportedly much sharper. Which brings up another point, that wet optics like the WWL can't improve a soft lens. One of the claims when the WWL first appeared was that domes degraded images enough that you could match the performance of a top line 16-35 lens in a large dome with the kit lens mated with a WWL. In some cases you could, but not all kit lenses are created equal and the RF 24-50 is a lot less equal than something like the EF 28-70 lens that can be mated with the WACP or the Sony 28-60. Having said that one or two reviews said the optics were OK - perhaps there is sample variability with these lenses or those reviews were not representative??

So the sad truth is that for Canon users who want to use wet wide optics - at least from Nauticam is that the 24-50/WWL-C combination provides flexibility but at the cost of less than stellar performance and it seems the WACP-C is a better option?