F/11 and Be There

DO stands for diffractive optics, and is used by Canon. PF stands for phase fresnel, and is used by Nikon. Both are a lens element design that tends to reduce overall weight and length of telephoto lenses. 

Canon’s introduction of two long telephoto DO lenses with a maximum aperture of f/11 is sure to raise eyebrows (600mm f/11 DO and 800mm f/11 DO). The question is who really wants such a lens?

I suspect that Canon has gotten envious of Nikon’s perpetual “out of stock” demand on the 300mm and 500mm PF lenses, which are small, lightweight, and very handholdable, and which every Nikon wildlife or sports shooter has ending up buying (one or both). The Canon 600mm f/11 is 930g and 199.5mm long compared to the Nikon 500mm f/5.6E PF at 1460g and 237mm long. So the smaller aperture opening nets (1) a focal length increase; (2) a weight reduction; (3) a size reduction (at least retracted; the Nikon optic doesn't increase in length during use, the Canon becomes 269.5mm when used); and (4) a price decrease.

Canon has made DO lenses in the past, most notably the 70-300mm 4.5-5.6 and the 400mm f/4. The former is compact, a little on the heavy side for its focal lengths, and expensive. I’m not sure it really differentiates enough from all the other 70/80mm to 300/400mm lenses. The latter is a gem of a lens, but is four-and-half pounds or so in a stout, large package. The 400mm f/4 is not really a walking-around lens, though on the sidelines of sporting events, you can do far worse in terms of size/weight for the image quality. Still, I’d rather have the Nikon 500mm f/5.6, I think (I really want a 400mm f/4 PF, but Nikon doesn’t make one yet). 

But f/11? That’s a bit of a problem. The Sunny 16 rule says that on an ISO 100 camera we’ll be at 1/200 second and f/11 in broad daylight. That’s not enough shutter speed to stop action, so we quickly start using ISO to compensate. I tend to want to be at a minimum of 1/1000, so there’s 2.5 stops of ISO boost right there, again, in full sun. In deep shade, add another 3.5 stops, so we’re at ISO 6400 already. Also, note that these lenses are fixed aperture, meaning that they're always f/11 (though if you add a TC they're a different effective aperture).

As I’ve explained before, photons are random, and exposure is really the amount of light (photons) filtered by aperture filtered by shutter speed. (As always, I’m sure I’ll get pushback on that, because most of you have been trained wrong by the camera companies, marketing, press, and even photography teachers that should know better. All the aforementioned think exposure is balancing ISO, aperture, and shutter speed to a match needle in the viewfinder, the so-called “exposure triangle” [sic]. It’s not. Exposure is basically the number of photons you capture, and which will get converted to a DN (digital number). ISO is essentially a multiplication of that number, and can only be worse than the “base” exposure, as multiplication introduces math errors.). In wildlife and sports photography we have to filter out a lot of light (photons) before it gets to the sensor, because we’re using very short shutter speeds to stop motion and action. We rely upon aperture to make up for the filtering of light that the shutter speed does. And f/11 basically doesn’t give us any way to get that back.

Moreover, on today’s higher resolution sensors—e.g. the Canon R5—f/11 is diffraction impacted. I wouldn’t want to stop down more, as I’d get even more visual aliasing of data from the diffraction, which is one of the problems that occurs with long-lens-camera-with-small-sensor products, such as the Nikon Coolpix P1000. As it is, I don't tend to use a TC-14E on my Nikon 500mm f/5.6 and Z7, even though the lens does remarkably well with a TC, as I'd really tend to end up at 700mm f/11 (due to stopping down to the best aperture with the TC mounted). Note that that's pretty much dead square between the two Canon optics without TC, and I'm avoiding it.

No doubt the new DO lenses travel well. Both retract for travel into very compact sizes for their focal lengths. They're definitely bag, and thus airline, friendly. But I'm not sure that they're the right design choices. Personally, a 400mm f/5.6 DO and a 600mm f/8 DO would have made more sense to me. Of course, then there wouldn't be as clear size/weight and focal length marketing claims to be made.

What the DO lenses do have going for them (he wrote do-do, snicker snicker ;~) is price. At these low prices (US$699 and 899 respectively), I can't imagine that an RF owner won't buy at least one. In that respect, Canon has a clear winner.

The two new DO lenses are going to make a mark, if nothing else for the marketing splash they make at their low price points. A Canon RP plus one of these lenses just became a birder's delight, at least birders who wander around in sunlight.

How practical the lenses will be in use remains to be seen. 

I've previously written about the trend towards smaller apertures in the variable aperture zooms.

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