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Micro Four Thirds vs APS-C: Lens Size

The common refrain is that “lenses for Micro Four Thirds are smaller and lighter”. See the prior Micro Four Thirds vs APS-C: Size and Weight Realities.

This assertion is correct, but it also ignores a key fact of reality: lens speed (f-stop) equivalence for the format in depth of field terms*.

Equivalent Focal Length and f-stop
Full-frame DSLR APS-C Compact or DSLR Micro Four Thirds**
28mm f/2.8 18.3mm f/2 14mm f/1.4
35mm f/2 24mm f/1.4 18mm f/1.0
50mm f/1.4 35mm f/1.0 25mm f/0.7
85mm f/1.2 55mm f/0.86 43mm f/0.6
150mm f/3.5 115mm f/2.5 75mm f/1.8

* Exact focal length match is debatable due to differing aspect ratios; can be computed on diagonal or horizontal.

** One should not discount the fact that real depth of field depends greatly on the correction of various optical aberrations and at best only approaches the theoretical in off-center areas.

The smaller lenses with Micro Four Thirds are smaller mainly because they cheat by being one or two or even three f-stops slower than the format equivalence. That’s fine if all you want is more depth of field, but what about less depth of field (subject separation)? And it’s fine if the lenses sacrifice speed for nearly perfect image quality wide open, but they don’t—not even close.

  • Fast (for the M4/3 format) autofocus lenses simply do not exist.
  • The best M4/3 lenses are not superior to APS-C or full frame equivalents in spite of being 1-2 stops slower (in format terms). Given the speed limitations, these lenses should be all but perfect wide open. They most definitely are not.

The fastest (brightest) M/43 lens from Olympus is f/1.4, equivalent to f/2.8 in full-frame terms and f/2 in APS-C terms. And when one looks at lenses like the 60mm f/2.8 macro it becomes absurd: this is equivalent to a 120mm f/5.6 in full-frame terms! This a depth of field viewpoint of course, since f/x is f/x no matter the format, already incorporating focal length.

Smaller and lighter yes. But not the same thing in format terms.

It’s not like the better M4/3 lenses are inexpensive: the Olympus M.Zuiko 12mm f/2 is about $875 with its optional lens shade, which is more than the Ricoh GR camera with its equally fast (for the format) and optically superior lens. and superior sensor. The value proposition does not make any sense to me: double the price (E-M5 + 12mm f/2) and triple the weight.

But of course, the fundamental difference here is interchangeable vs fixed lens. If one presupposes the need for interchangeable lenses, then it drives a lot of choices. My belief is that a bi-focal fixed lens camera at f/2 or f/2.8 would be an 80% or 90% solution for most shooters. There are always other cases one can envision: how many people own focal lengths from 14/15mm through 800mm? Very few, for good reasons.

The answer

I’d like to see a move to fixed-lens APS-C or full frame compacts with highly optimized lenses for the sensor, the bulk of which resides inside the camera body, e.g., the Sigma DP Merrill approach taken up with the Ricoh GR and Sony RX1.

With two or three such focal lengths, one can cover most shooting situations with exceptional quality on a large sensor. Separate cameras are always ready to shoot, one does not introduce dust by changing lenses, there is redundancy and accessory sharing, and the total kit can be much smaller and lighter. With a Ricoh GR size camera, one can easily keep two cameras at the ready at all times—handier than the unmount-cap-uncap-mount cycle with separate lenses.

Sigma already takes this approach with the Sigma DP Merrill series (28mm, 45mm, 75mm equivalents). Next up (I hope) is Sony, with the trial balloon Sony RX1 leading to other models: I’d like to see a 19mm and 28mm version. I’d also love to see Ricoh take the lead here and produce other GR models besides the 28mm (equiv) Ricoh GR.

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