Mirror Lessons' title is a great traffic lead-in, not to mention a good read, but, with regards to the larger picture, I believe it to be a bit short-sighted. Every photographer makes compromises when purchasing new gear. Today's dSLR user is beset by massive cameras and ridiculously obtuse control schemes. Meanwhile, lenses for once-small mirrorless systems grow larger and larger. In fact, today's fast mirrorless lenses are just as large, and often much more expensive, than their equivalents for FF 35mm dSLRs.
Their popularity, which, in the main, hinges on their being smaller, is no longer assured. Price is another area where mirrorless simply doesn't compete. Not every mirrorless user agrees. Mirror Lessons's Mathieu is one.
“If you use your DSLR without a battery grip and mainly with prime lenses, you can indeed improve the portability of the system. Four years ago, I decided to limit myself to the D700 and the Nikkor 50mm f/1.8 for my summer holidays and I had no issue carrying it around with me everywhere. The bottom line is that DSLR systems can be portable but you have to accept compromises to make it so: no extra accessories that aren’t essential, and prime lenses over zoom lenses. Another option of course could be a good f/3.5-5.6 zoom if you are not obsessed with fast apertures. I’ve seen more than one professional photographer carry around two DSLR bodies with one wide angle prime and one telephoto prime lens. It can work for some but not necessarily for everyone.
With MILCs, you have more freedom to carry around different lenses and still have a reasonably light system inside your bag. Indeed some body/prime lens combos are simply unbeatable in terms of size and weight (think of an Olympus E-M10 with the 45mm f/1.8). To say that an oversized lens doesn’t suit CSCs almost seems like a natural conclusion.”
Small-sensor MILC camera systems, whose lenses can be smaller, allow you to carry around more lenses. However, they rarely, if ever, offer equivalents to fast FF glass. To wit, even today's f/1,2 small-sensor MILC prime lenses, at best equate to f/1,8 or f/2,0 budget-conscious FF prime lenses on FF systems. MILC also lenses often cost more, and are just as heavy and large.
At the lens level, a mirrorless user must ignore equivalence in order to gain size/weight benefits.
Small-sensor systems are unable to create the same amount of background blur, or shallow depth-of-focus, of fast lenses on large sensors. The same goes for FF 35mm vs. FF medium format, and larger sensor systems. For many users, the only thing that matters is light intensity. And for them, an f/2,8, or faster, lens, is all the spec that matters.
Compromise is something many are willing to accept. But many others believe they're getting all the benefits of size, and somehow, price, with no drawbacks.
Mathieu's essay is for both people.
Yes, a 40-150/2,8 m43 lens will be much, much smaller than a 80-300/2,8 lens for FF 35mm would. But, then again, the m43 mirrorless system and lens returns the same effective image as a hypothetical FF 80-300/5,6 mounted to a FF camera. Of course, on FF, such a slow lens would be a hard sell. On the plus side, it would be as cheap as chips. Small-sensor mirrorless equivalents, which must use expensive, exotic, glass in order to achieve similar results as moderate-speed glass for larger sensors, are almost always more expensive, sometimes egregiously so. The same goes for MILC cameras. And, with equivalence in mind, size favours no one.
Despite this, mirrorless lenses and systems bring several advantages when compared to FF equivalents.
1. Modern optics (to effectively slow lenses)
2. High technology (OIS, AF, fast focus motors)
3. Better lens construction*
Typically, slow effective lenses have been budget lenses with the build materials, and quality to match. With the advance of small-sensor cameras, large-aperture, but slow effective lenses are made to top spec. Their deep field allows portrait photographers to shoot wide-open head-and-shoulders portraits and still get an entire faces in focus without inducing blade-defined background blur shapes. Of course, FF users could always purchase a small-aperture lens, bump up ISO, and shoot with the same DOF. The larger sensor will return less perceived noise across the image, and the lens itself, will be less expensive.
Which is why I disagree with Mathieu on the following point:
“If you are a MFT shooter, you will chose the 40-150mm f/2.8 because it is a 2.8 lens. The same with the 70-200mm f/2.8 if you are a full frame DSLR shooter. This conclusion might be less scientific, but it reflects in my humble opinion formed from practical experience out in the field.”
The user of f/2,8 FF zooms is harder to pin down than the user of the f/2,8 aperture small-sensor camera zooms. Why? FF f/2,8 zooms top out the range. They are followed by f/4 options. Both are professional and made to top spec. Both effectively isolate subjects better than the fastest, and most expensive of m43 zooms, while the f/4 version on an FF camera matches the fastest f/2,8 zooms on an APS-C camera. Embarrassingly, Nikon's sub-700$ 70-300 G (f/4,5-5,6) is both less expensive, and even more capable of isolating subjects than the 40-150/2,8 m43 lens, not to mention Fujifilm's 55-200 variable aperture APS-C lens. FF users can choose expensive, large, professional lenses. They can choose compact professional lenses. They can choose consumer price-bombs.
M43's users are stuck with expensive, or kit. And neither comes close to being able to isolate the subject, or smooth out distant backgrounds as well as price-equivalent FF lenses on FF cameras. Small sensor users have fewer choices.
The good news is that photography is what you make of it. If weight/size savings are your thing, and all else is irrelevant, today's small-sensor mirrorless cameras are your best bet for keeping your system small. Today's dSLRs, both APS-C, and FF, are too big for their own good. But small-sensor cameras, which, when marked against the half-truth equivalence of their marketing materials, serve size/weight savings only in their bodies. If/when today's dSLR makers release size-conscious, professional FF mirrorless cameras based on existing SLR mounts, small-sensor cameras will be totally outclassed, and their expensive sometimes-equivalent, sometimes-slower systems, will be hard-pressed to argue anything in their favour.
Marketing based on spec, and price, especially when neither favour small formats, is bad marketing. Choosing a camera based on marketing is even worse.
* When compared to certain modern plastic equivalent lenses for FF cameras.