Why Thom is wrong...

Started Sep 25, 2013 | Discussions thread
Matsu Senior Member • Posts: 2,113
Mirrorlessness and legacy systems

They really are two separate, but entwined discussions.

Let's start with the mount dimensions. The flange focal distance within a digital system is in and of itself not necessarily a guarantee of smaller lenses. Sony's NEX and Alpha mounts provide an interesting example. The kit 18-55 is virtually the same size in either because of a number of reasons - they're both zooms for the same size image circle (APSC). APSC DSLRs built on legacy (full frame) mounts have an extra long flange focal distance to image diameter ratio compared to mirrorless cameras and even to full frame DSLRs.

I'll just round the numbers, but they go something like this:

Full frame SLR's tend to have registers about 45mm deep for an image circle about 43.27mm in diameter, a roughly 1:1 ratio.

APSC SLRs have the same register, but an image circle of only about 28mm, so the ratio longer, about 1.5:1

Curiously, the old 4/3rds mount had an even longer ratio, about 1.8:1.* And this is worth noting because Kodak did a lot of research, and had most of the best early knowledge of the properties of digital sensors for most of the early days of digital, but we'll come back to that.

Micro4/3 has basically reduced the ratio down to about 1:1, the same as a full frame DSLR, but with a smaller sensor and a smaller image circle, about 21mm each.

NEX and Fuji X have ratios that look more like full frame film Rangefinders did, about 0.6:1, or roughly 20mm registers, for roughly 28mm image circles. It will be interesting to see how a full frame NEX works with the same mount - which would give it about a 0.5:1 ratio.

When you compare a Leica M lens set to a film era SLR set, the Leica's are smaller, but not all of them. The difference is more dramatic for wide primes, but some of these are very symmetrical and do not deal as well with digital sensors unless other corrective measures are employed. Even Sony's NEX has some issues with wides.

Back when Kodak and Olympus were planning 4/3, telecenricity was a major concern. They overestimated the requirement, and since adjusted with Micro4/3, but it's interesting nonetheless because it gives us some clues about how big full frame mirrorless lenses would have to be. As Does Sony's NEX/Alpha comparison.

Basically, wide primes should be smaller on a system with a shorter flange distance, but difference in lens size lessens as focal ranges and zoom ratios increase.

How big or small the system is will depend on how large a sensor you want, AND what sort of lenses you want. Big fast zoom ratios will be big, on either a long register or a short one. Wide primes will be smaller on a "mirrorless" design.

So what can Nikon and Canon do to protect and leverage their legacy mount? Easy, move more people over to Full Frame, hence the cheaper models like D600 and 6D, and perhaps even more aggressive pricing to come. APSC DSLRs are a compromise, not a bad one, very convenient for users of the whole system, but the advantage of their mount is in housing the bigger sensor and lens selections tailored to it. Also why there has never really been a proliferation of DX/APSC DSLR glass, despite Hogan's and many of our own protestations

If one day they delete the mirror, and I would expect that one day they would, they can the visit the question of reducing the register. A simple adaptor and on chip PDAF would be all that's required to maintain compatibility, much like Olympus has already shown in the transition from 4/3 to m4/3.

Marketing, installed base, and lens selection and support will be far more important than the exacting dimensions of any mount, they are all compromises, just different sorts.

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