Does the Lens Mount Matter?

First, let's put blame where it belongs: the marketing departments of Canon and Nikon. When you make a claim—our new mount is better—it's up to you to prove it. Worse still, Canon and Nikon have both opted to point to lenses that don't really effectively speak to the claim (Canon 28-70mm f/2, Nikkor 58mm f/0.95). 

So let's back up a minute. 

For optical design purposes, there are two factors that are important in a lens mount for lens design: (1) how close it is to the focal plane (flange distance); and (2) how wide the opening is (throat; measured at smallest non-obstructed point). 

In the film/DSLR era, we had the following:

  • Canon EF: 44mm flange, 50.6mm throat
  • Nikon F: 46.5mm flange, 44mm throat
  • Minolta/Sony Alpha: 43.5mm flange, 42mm throat

The common myth is that the throat diameter is what determines how fast a lens you can put on the mount. For years, this was expressed by (incorrect) statements like "Canon can make f/1.2 prime lenses and Nikon can't." Yes, there is a bit of a relationship between aperture and throat, but it's not a direct "larger throat means faster lenses" relationship. 

So the marketing fumble started long, long ago. 

Today, we have the following situation in mirrorless:

  • Canon RF: 20mm flange, 50.6mm throat
  • Nikon Z: 16mm flange, 52mm throat
  • Sony FE: 18mm flange, 43.6mm throat

Nikon went from having the most restrictive mount—in terms of allowing optical design options—to having the least restrictive one. 

So what does that mean: least restrictive? Consider the following illustration of an off-axis light path (from Bill Claff's excellent site that lets you play with patented optical designs).

bythom offaxis

The lens mount is going to be generally in that gap between the last element and the focal plane (labeled FL in this Nikon 35mm f/1.8 Z lens patent). (I used the word "generally" because technically you can have a lens element that is beyond the mount in mirrorless designs, but the ray angles then would start to get extreme and the filter over the sensor starts to come into play.) A less restrictive lens mount simply gives you optical design options you wouldn't have with a more restrictive mount. 

So, the real problem is this: neither Canon nor Nikon have actually demonstrated the difference the new mount gives them in optical design. Their marketing departments point to a pair of lenses, but they actually don't show how those lenses couldn't have existed in their previous mount. And they're not very specific about what's different.

And there's the rub. If Canon and Nikon did go out of their way to show how the new mount is better, then by definition the old mount they're still selling is worse! Neither company really wants to deprecate their existing DSLR lineup (and both have DSLR mounts that are effectively more restrictive than their mirrorless mounts). Thus, the two companies marketing folk talk in generalities, and even in futures ("...will allow us to..."). 

Ironically, Sigma's CEO, Katuto Yamaki, was able to do a bit better than either Canon's or Nikon's marketing in one response to a question from dpreview: "I’ve been very impressed by Canon’s new lenses for RF. The 50mm F1.2 and 28-70mm F2. Very impressed - and a little jealous! They’re possible due to the wide diameter and short flange back. Otherwise such lenses would be very difficult or impossible. Having the larger elements at the rear of the optical system makes it easier to achieve good performance at large apertures."

Yep: "makes it easier to achieve good performance at large apertures." Not that it can't be done, but that it opens up optical decisions that are easier to create in the manufacturing plant.

So let's make the Sony fanboys both happy and unhappy ;~). I mention them in particular because they're the ones running around screaming that the mount doesn't make a difference all over the Internet. 

Okay: start with unhappy Sony fanboys. The mount does make a difference. Canon and Nikon engineers now have room to explore many new optical designs, including moving the entrance pupil far forward of its usual position and not having to use expensive and complex aspherical elements to move light radically. Optical designs at the rear of the lens can involve larger elements. Sony, not so much. 

Okay: let's create happy Sony fanboys. Nikon happily existed and made excellent lenses for over 50 years in their old most restrictive lens mount. Sony will, too. 

And that's the bottom line here: it isn't that you can't design a good lens with a restrictive mount. You can, but your design choices are more limited, you may have to resort to trickier glass choices, and there are light ray paths you can't contemplate or use. At present, we don't know just how far Canon and Nikon will go in exploring the limits of their new mounts. It very may well turn out like it did for film SLRs and DSLRs: that most of the common lens choices users want are adequately done with all the existing mounts. 

As it stands, I'm not sure I want to give up 24mm and carry a three pound weight off the front of my mirrorless camera body (Canon 28-70mm f/2). And I know I'm not much interested in a manual focus 58mm f/0.95 lens. I consider these to be more design explorations by Canon and Nikon than truly practical lenses we're all going to want in our gear kit. That's not to say that those companies might not at some point come up with something interesting that couldn't be done before and is a practical lens. Just that they haven't yet. 

Meanwhile, as my reviews indicate, Sony and Zeiss have been creating perfectly fine lenses for that "most restrictive" FE mount. 

So, if someone tells you that you have to choose a camera based upon lens mount efficiencies, I'd say balderdash. Nope. Plenty of useful lenses will be available for all the mounts. 

Meanwhile, I—and the rest of the world—are waiting for Canon and Nikon to actually prove that a less limiting mount truly makes a real difference. It's possible that they will. But they haven't so far, and I see no sign that they will any time soon.

text and images © 2018 Thom Hogan
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