[nayuki]

Amazing website. If I didn't know better, I thought this content came from Canon itself. A couple of comments from me:

> The following comparison chart shows all of the autofocus drive types that have ever been used in Canon's EF, EF-S, RF, and RF-S lenses.

They missed mentioning EF-M, Canon's foray into mirrorless interchangeable-lens cameras.

> Although the ultrasonic noise cannot be perceived by the human ear, it can be picked up by sensitive microphones which can be problematic during video shootings.

I used several Ring USM lenses and can hear the noise when focusing. It's a soft swishy kind of white noise, not harmonic. The Ring USM definitely generates noise in the audible range, not only ultrasonic.

> The latest types of autofocus drives, including the Stepper Motor, Nano USM, and Voice Coil Motor, offer focus-by-wire.

Though I have nothing against focus-by-wire on principle, the implementations have not been good. Having used several EF-M (all STM) and RF (some STM and some Nano USM) lenses that have focus-by-wire, I find that the motor actuation significantly lags behind the user turning the focus ring (which is a digital encoder), and the granularity of the focus steps are visible.

Meanwhile, the old EF Ring USM's full-time manual (FTM) was really good by comparison; it didn't take much force, was fully responsive with no lag, and was fully analog when turned by hand and had no discrete steps. I miss that, as new lenses don't use this design.

> Canon EF 15mm F2.8 Fisheye (et cetera)

Please change the syntax to "f/2.8"; this is even in Canon's official pages. https://global.canon/en/c-museum/product/ef263.html

Why? Because f is actually a lowercase italic variable that denotes the focal length (15 mm in this case), and "/" (slash) really means division. The size of the aperture is (15 mm / 2.8) = 5.4 mm; that is the real diameter of the hole that light passes through. The absolute aperture is 5.4 mm and the relative aperture is f/2.8. https://en.wikipedia.org/wiki/F-number

[ExAr]

Thanks nayuki for the detailed comment and positive feedback.

Thanks for pointing out the missing EF-M, I am going to add that during the next polishing round. With regards to USM noise, the ultrasonic whine can be picked up by very good ears. I think the Nano USM is excited in the range of 60+ kHz, so at least that type of USM motor is absolutely inaudible.

Syntax-wise with F2.8, you got me. I know it's not 100% correct, but it was a deliberate choice that I made. Historically, there is either 1:2.8 or f/2.8 which will always be correct representments of the aperture (exit pupil) diameter. However, pursuing a cleaner look, I avoided the division or slash characters because the relation between the focal length and the f-number is known by every photographer. This is why Canon has also made the transition to the syntax "F2.8" in all their RF and RF-S-lenses - at least their product names and labels printed on the lens barrels. (The same is true with the focal length where Canon doesn't print "mm" on the barrel - to achieve a cleaner less mathematical look). So I believe what I did here was just go with the trend :-)

[nayuki]

> With regards to USM noise

Here is a video showing the "whooshing" noise of Ring USM, and it is clearly in the audible range (not ultrasonic): https://imgur.com/a/canon-ring-usm-autofocus-sound-YCrV1CR

> the relation between the focal length and the f-number is known by every photographer

I'm going to disagree with this. I think very few photographers understand what an absolute aperture size is. Here's a good test: If you take a "constant (relative) aperture" zoom lens like a 24-70mm f/2.8, what happens if you keep the aperture "constant" at f/2.8 and zoom the lens in and out? I think the answer will surprise many people.

I think most people don't even know that the f (or incorrectly "F") refers to the focal length; I think the vast majority of photography education just treats "F" or "f/" as some piece of fixed notation or incantation without explaining what it means. (Kind of like if I asked you, what does the "degree" in "degree Celsius" mean?)

> Canon has also made the transition to the syntax "F2.8" in all their RF and RF-S-lenses - at least their product names and labels printed on the lens barrels

Oh wow, I didn't notice this one. Your observation is correct; Canon did change their official naming of RF lenses to use the "F2.8" syntax instead of the previous "f/2.8". Oh well, I still disagree with it because it is bad mathematical syntax and spits in the face of tradition.

> The same is true with the focal length where Canon doesn't print "mm" on the barrel - to achieve a cleaner less mathematical look

True, and I have subconsciously noticed this.

> So I believe what I did here was just go with the trend :-)

Please don't change the naming of old lenses. Also, I would prefer new lenses to be hammered back into the old naming scheme, but that's more open to debate. Curiously, the camera shop Vistek sometimes uses old naming for RF lenses: https://www.vistek.ca/store/434924/canon-rf-1535mm-f28l-usm-... "Canon RF 15-35mm f/2.8L IS USM Lens"

[ExAr]

Thanks for following this up.

I have checked the video you've linked. My suggestion would be that this whooshing noise is not generated by the piezoelectric part but rather from the connected focusing mechanism (especially the helical focusing barrel which turns, and the focusing lens cell which slides back and forth. These parts sit inside the stationary lens barrel, and so a bit of friction is unavoidable. Just my guess.)

Syntax-wise, I will have to think about it. As with all my other articles, I like being very precise with physical formulas and mathematical expressions. But here is my point: These values in photography are not pinpoint precise anyway. When optical engineers come up with lens designs, the precisely calculated values for the aperture are something like f/4.63 which is then simply rounded to the next possible f-value specified on the lens barrel, and calculated focal length is often 25-49mm and Canon simply names that lens 24-50mm on the barrel (other manufacturers do that in a similar way). So, this is just my personal opinion, but it appears to me that in the context of photography, these values are just halfway decent guide values, almost like approximate descriptions of the lens behaviour. For that reason, I do understand why lens manufacturers move away from the precise syntax, and treat these values more like an abstract product description. Of course, if used in a strictly scientific context, I would rather prefer to use f/4.5. But I might be pretty alone with this view.

PS: I have planned a new chapter explaining all this, including the interesting fact that you've mentioned, where the diameter of the exit pupil actually changes according to the focal length, even with a constant aperture lens. But it will probably be end of summer when this is going online.

[nayuki]

> this whooshing noise is not generated by the piezoelectric part but rather from the connected focusing mechanism

Indeed. I never implied that the piezoelectric part makes an audible whooshing noise. I only meant to say that the entire Ring USM system makes that noise, and so it will affect audio recorded on the camera's on-board microphone. Either way, that noise explains the motivation for the development of STM AF and I guess Nano USM.

Agreed with everything else you said. Thanks again for your fantastic articles and replies!

[m463]

> EF-M, Canon's foray into mirrorless interchangeable-lens cameras

Canon's foray into SMALL mirrorless interchangeable-lens cameras.

The R series are mirrorless, but not that small. too bad.

[rpearl]

EF-M is fairly dead at this point isn't it?

[dboreham]

I still use mine occasionally since there is no direct replacement in the RF product line (they're all much larger).