Canon RF 50mm f/1.8 STM specifications


May 8, 2015
I don't normally visit this site these days, but a mate clued me in that something had come up on here that I knew a thing or two about and that I might want to set y'all straight.

This is not —categorically not—the optical formula of the old Canonet 45mm f/1.7. Firstly, anybody should be able to stop just by looking at the diagrams that the spacing of the front two groups is different, the relative scales and sizes of most of the elements are different (the second element in particular), and the rear three elements are also positoned further back and spaced slightly differently. Also, the Canonet 45mm f/1.7 is too big for a 43mm filter thread to be possible; the front element alone is approximately 40mm, which at the angle of view of anything under about 100mm means a 43mm surround would encroach on the image, even without an actual filter being there. From the photos of the new lens we can see that its front element is far smaller than this. Of course the fact one is a 50mm f/1.8 (likely around 51mm f/1.9 actual, same as the other Canon cheap 50s) while the other is a 45mm f/1.7 (actually 44.2mm f/1.72) should also clue you in that this is not the same formula.
Yes, it's six elements in five groups and yes they are in[ vaguely the same shapes and order, but that's different from being the same formula, which if you're being technically pedantic about it would actually mean they were literally 100% identical.

Second, as someone who owns several Canonets including three 45mm f/1.7 variations and as someone who has repaired said lenses several times, I can tell you not only are the "coatings, tolerances, etc" going to be different, but the very material of the elements will also be different. The 45mm Canonets, of all apertures, use thorium in two of the inner elements. (Second and fourth, to be specific.) That isn't used any more (though there are rumours that Zhongyi have been sneaking thoriated glass into a couple of their f/0.95 designs, I've not seen any actual evidence of that yet) and there is quite a difference in the results you get from thoriated glass vs modern materials. (Not necessarily better or worse results, of course, with such things being so subjective.) This is the main reason why the later models moved to a 40mm lens, as well as why so many FL lenses were quickly replaced with similarly slightly different focal lengths by the late 60s (the 58mm f/1.2 being replaced by a 55mm being the most famous example). Without thoriated glass on the table, some designs had to be entirely scrapped and getting satisfactory results at the same focal length and apertures without our radioactive friends was too hard, so different focal lengths were chosen which could more easily (and safely) be made to a high standard.

So while, again, it is the same number of elements in the same grouping and in a rough sense the same kind of shapes, the actual nuances of the design show that it's not the same formula (again, saying something is the same formula means something very specific, which this is not; don't throw the term around lightly if you're trying to show off) and beyond the most obvious advancements in build, the fundamental meterials will have the biggest affect on the resulting image quality.

FWIW, of course it's impossible to tell just how sharp, contrasty, flare-resistant or whatever else a lens may be just from looking at the optical formula, but I would anticipate the same kind of improvement from the STM to this RF as there was from the mkII to the STM. In other words, perfectly acceptable for the price point and for the purpose of a cheap(ish; this looks like quite a price hike for here in the UK) 50mm lens, but nothing special in any way. Slightly below-average wide open, dead average at f/2.8 and shockingly sharp at f/4, with totally average colour and contrast throughout and quite strong vignetting, is the trend that I would expect this lens to continue given the design is not trying anything radically new. (If I were a betting person I'd put a fiver on it being slightly sharper at f/2.8 but with at least a half stop more vignetting across the whole aperture range, but I tend to assume that about any lens which shrinks the front element and filter size...)

Anyway. The Canonet 45mm f/1.7, it ain't, and I highly doubt Canon spent a single second thinking about the Canonet when designing this. Stop trying to dig up some kind of 'gotcha' 'cause there's just nothing there.


Automotive, Motorsports, Commerical, & Real Estate
CR Pro
Apr 2, 2015
I think you'll find that the "new optical formula" is at least 56 years old if you have a look at the first page of this thread ;)

I'm well aware the Double-Gauss design is old and the design we see here is very similar to some older formulas. But I'm saying I do not want a recycled EF 50mm lens formula again. I think the design shows a UD element and we already saw what coats can do to improve ghosting a flare between the STM and non STM versions. So yes, I'm happy to see a new formula being used for this lens and not the same exact formula we had in the last "Nifty Fifty".
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David - Sydney

CR Pro
Dec 7, 2014
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H. Jones

Aug 1, 2014
Pre-ordered the RF 50mm F/1.8 STM the second the page went live. At $200 there's absolutely no reason to not buy one. This will be such an excellent, tiny pancake of a lens for when I'm out and about and not focused on photography.


How can you stand out, if you do like evrybdy else
Mar 25, 2014
I guess it will be optional, as usual with non L lenses.
By the way, I have and use the 40mm pancake with its Canon lensshade (ES 52), no issues outdoors.
Thanks for letting me know. I didn't know about it. Good stuff.


How can you stand out, if you do like evrybdy else
Mar 25, 2014
A good 52mm screw on hood for the 40mm pancake (and the EF-S 24mm and RF 35mm as well) is Canon's LH-DC20 designed for the Powershot S* IS series. You can probably find them used and cheap. I don't know why Canon doesn't market it for these lenses.
Thanks, will try one.


CR Pro
Jan 5, 2013
This is good news from Canon.....finally, a nifty RF fifty and at a great price. This lens will undoubtedly allow a lot of people to embrace the Canon RF series of cameras, especially those with limited funds as they have been waiting in the wings to jump on the mirrorless format, if only Canon would provide a reasonable venue for them. This lens should help them accomplish that. So happy to see that Canon managed to keep the price down, as well. Good job, Canon....just hope you have an ample supply when they start shipping.


Sep 29, 2018
This will be a hit for people rocking an RF body on a gimbal or some thing like a Head One on a slider.

Michael Clark

Now we see through a glass, darkly...
Apr 5, 2016
No it's a pure simple double-gauss design. So it's pretty much identical in capabilities to the EF 50mm STM. Slightly better macro (0.3m MFD vs 0.35m = 0.25x instead of 0.21), same number of diaphragm blades, and same weight.

Both are modified double Gauss designs. (Gauss is capitalized because it is a proper noun - Johann Carl Friedrich Gauss invented the two-element Gauss telescope objective lens.) The RF version is less symmetrical due to it being much closer to the sensor than the EF counterpart. The 44mm registration distance of the EF mount is very near the 50mm focal length, while the 20mm registration distance of the RF mount is much shorter than 50mm. The 24mm difference is partially, but not fully, offset by the RF lens' elements being further forward of the flange in the RF lens than in the EF lenses. In the case of the EF lenses, the optical center of the lens is about 7mm forward of the flange (it's actually a 51mm lens in at least the EF 50mm f/1.8 II version). In the case of the RF lens, the optical center will be around 30mm forward of the flange.

A "pure" double Gauss has four independent elements. Two positive meniscus lenses on the ends and two negative meniscus lenses in the interior. Both sides are symmetrical.

In 1895 Paul Rudolph, working for Carl Zeiss Jena, replaced both single interior negative menisci with cemented doublets. This was improved by Taylor, Taylor, and Hobson into the slightly asymmetrical "classic" double Gauss design, with both interior negative menisci still two sets of cemented doublets. Thus the "classic" double Gauss design has 6 elements in 4 groups.

The EF 50mm F/1.8 (in the original, II, and STM versions) had the front interior negative menisci with an air gap between the two elements and the rear interior negative meniscus group with the two elements still cemented together.

The RF 50mm f/1.8 reverses that and the front interior negative meniscus group is a cemented doublet with the rear interior negative menisci having an air gap between the two elements.
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Landscape and Architecture, R5
CR Pro
Mar 15, 2020
Des Moines, IA we have a shipping date for this yet? I thought I had seen Dec.4, have also read middle of Dec.
Anyone have any additional insight?
Dec 6, 2020
3 we have a shipping date for this yet? I thought I had seen Dec.4, have also read middle of Dec.
Anyone have any additional insight?

I just received mine a few days ago. Looking forward to testing it out tomorrow on my R5, along with my newly purchased RF 24-105 f/4!
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