There are lots of things people say about camera equipment that just aren't true. The first example that comes to mind is the tired, old assertion that the only reason the four thirds sensor cameras were invented and introduced was to make cameras smaller and lighter. No. Not at all.
Olympus had real engineering reasons for selecting the smaller sensors at a time when other camera makers were chasing APS-C and full frame sensors. Earliest digital cameras were mostly being used with lenses designed for film. Film didn't care about what angles the light came from, it just recorded the light. Early Canon and Nikon users (up to about 2004) were mostly getting lens that were either coming directly from film days or were re-packaged, but largely optically unchanged, and users quickly found that they would get images that were less sharp on the edges and corners. And not just a little unsharp but a lot unsharp. With gobs of vignetting tossed in too.
Olympus wanted to reinvent digital photography by designing lenses that were specifically made to deliver collimated light that hit the sensors (and their deep little wells) head-on. They were called Tele-centric designs. The idea being that light hitting at a straight 90° angle would better fill all the pixel wells evenly and would not cause radical vignetting in the system. Also, since ample anti-aliasing filters were required to prevent moiré on low resolution sensors the collimated "pipe" of light didn't have differential travel through the filter packs. This cut down radically on edge distortion. But to make all this work and keep lenses compact enough and inexpensive enough the engineers had to keep the image circles small enough. So the small sensors were part of the design parameters. The idea being that the improved performance of the whole system would offset the difference in sensor sizes and would deliver a bonus in that smaller sensors were available at that time for far less money per item than larger sensors.
The first generations of four thirds sensor cameras were designed for professional use and had rugged bodies which were early exemplars of weather proofing and shock resistance. All features that required a fairly large outer shell.
At a certain point the overall growth of the camera market meant that larger sensors were falling in price as quantities ramped up. Consumers made the choice of purchasing cameras with larger sensors in spite of theoretical and actual engineering advantages of the smaller sensor cameras. By the end of the 2010 camera makers were bringing to market lots of newer lenses that were designed to work around the earlier limitations of digital and this basically castrated the market for smaller sensor cameras among advanced amateurs and professionals. But, the smaller sensors were never primarily about reducing the size and weight of systems.
The micro four thirds cameras, which did away with mirrors altogether, ushered in the whole mirrorless camera revolution but again, I would argue that for early adapters, at least, the size and weight was less important than the new freedom and control of using EVFs and being able to easily adapt a wide range of lenses onto the cameras. The smaller size resulted from eliminating moving mirrors and glass pentaprisms but size reduction was an offshoot of the engineering and not a primary goal of the makers.
Olympus embraced the marketing differentiation of smaller sized gear as a way of staying relevant in the market at time when the relevance of tele-centric lens design and smaller chip geometry faded away. Interestingly, the pro level cameras that Olympus offered (OMD EM-1 etc.) and which were adopted by many were very often coupled with battery grips and various body extensions when used by professionals and advanced amateurs which clearly showed that many felt cameras had become too small and required more grip space in order to be used comfortably.
Another myth that people throw out all the time is that it's somehow wrong to buy a Leica body and then use an inexpensive lens from a different brand on that body. The idea being that once you've splashed out big money on an M or SL series camera you have some sort of obligation to use it only with a lens of the same brand. Having a bit of historic perspective I find this either amusing or witless. In the days when Leica and many other brands shared screw mounts for lenses photographers mixed and matched lenses and cameras with impunity. At one point in the 1950s Nikon launched a line of lenses that cemented their reputation for optical quality for decades to come. The screw mount Nikkor 50mm f1.4 was a legendary performer and many Leica camera users rushed to use that lens on their cameras.
It was the introduction of the R series Leicas, and indeed dedicated lens mounts from all major camera makers, that shifted the market away from people eagerly adapting the best options to being locked into one lens system. Each camera company designed their reflex cameras with different lens mount flange to film plane distances and each mount also had different mechanical linkages to effect exposure metering and auto stop down. Those differences made it much more difficult; impossible in some cases, to mix and match lenses and cameras from different sources.
The massive move since 2012 to mirrorless cameras, as well as the introduction of Leica's SL lens mount system for Leica users meant that adapting any of the lenses from the film days was now possible and allows users to choose the best from among a wide range of suppliers. And by "best" I don't mean just the most expensive but also the best cost-to-performance ratios for various imaging needs.
Leica M and SL camera users are taking advantage of different use profiles to choose the lenses that best fit their budgets and their projects instead of buying only the (very pricy) Leica branded products. And that's core to the promise of mirror-free?EVF enhanced camera systems. That's one of the many reasons why makers are inexorably moving to kill mirrored cameras and provide deeper systems of the newer tech. That, and the fact that mirrorless cameras can be so much less expensive to make....
Most photographers understand the logic of selecting the highest performing lens for the focal length range that they use most often. And most photographers do have a decided predilection for a certain focal length. Henri Cartier-Bresson and I both favored the classic 50mm while many, many street photographers embrace the 35mm. There are even long arching styles of lens choice with the 28mm (on full frame) in current ascendency. In the 1960's the focal lengths of 85, 90, 105 and 135mm were tremendously popular.
In some circles there is a fixation with ultra wide zooms that range from 12-24 to 14-24mms.
I think most users of systems like to buy lenses that are either made for their mount and actually of the same brand where their primary and most used lens is concerned. But I think it's for not for the snobbery of owning a particular brand as much as it's the reliability of the lens when it comes to autofocusing, handling and exposure accuracy. There is a presumption that the "native" lens will better match those interrelated linkages.
But I think even the most ardent, professional Leica user might buy a few Leica primes or one good, standard zoom for most of their work while readily, happily buying and using high quality lenses from third parties (at a quarter the price) for all the focal lengths that they feel need to be "covered" but which are used sparingly. For me this would include all the wide lenses. If I needed a wide angle zoom I would immediately see what Sigma has in their art series. I'd buy their 14-24 Art lens for L mount in a heartbeat and wouldn't even consider the Leica 16-35mm. Why spend the money on a lens I'd use on less than 10% of the shots I take? The same applies to longer lenses as well. Once I've crested the 90mm or 105mm focal length range I wouldn't consider the very expensive Leica offerings if I could instead make use of a different brand at a lower cost. That's why I have a Panasonic S-Pro series 70-200mm f4.0 instead of Leica's 90-280. I only use my current 70-200mm a couple times a month and rarely for anything really demanding. Spending an extra five or six thousand dollars makes no sense given my shooting habits.
Never say never again. If I got hired to shoot swimming competitions all the time; if that was my main job, I'd probably pick up the 90-280mm by the end of the week. I have no doubt it's a great lens but you have to fit the tool to the tasks you enjoy photographing and not let your enthusiasm for a lens drive your content.
Finally, there is the presumption that fast auto focusing is everything. I hear people say it all the time. Even my good friends who work professionally get all wrapped up in focusing speed as a primary feature for their cameras; and none of my friends (that I know of) shoot anything that moves quickly. Many don't shoot moving subjects at all. But they are convinced that, "Pros use Sony exclusively because the focusing outperforms everything else on the market."
If you shoot car racing or kid's soccer you'll get no arguments from me. The most recent Sony cameras do focus very well when used in C-AF. But the reality is that Canon outsells Sony almost two-to-one when it comes to professionally capable, fast focusing cameras. And I'd venture to say that Nikon will catch up to Sony's performance in the very next generation of cameras they release. At that point Sony's advantage in focusing will be blunted and they'll have to compete on more practical features instead. They lag in most of their cameras by offering 8 bit in-camera video files instead of 10 bit files. They lack menu clarity. They lack in haptics and comfort. Some would also argue that they lack in color science but I'm not in that camp.
I guess at the heart of all this is that people say stuff that's either a result of groupspeak, inexperience or ignorance of the past, present and future. It pays to try stuff for yourself. It pays to test the gear personally to see if something awkward will reduce your pleasure in using a camera. It pays to check and see if the lenses you use less frequently can be sourced from companies other than the brand of camera you use. And even more important, to understand whether or not a less expensive product can be successfully used for your purposes. I would say that in today's world image quality is no longer limited by (tiny) differences in lens performance between top brands but by the constraints of the target medium and the user methods that may themselves limit the ability to see a difference. If you are trying to squeeze the best from a lens are you using a tripod? Are you selecting optimum apertures? Are you focusing carefully? Is your shutter speed high enough to freeze motion of both your camera and your subject? Are you using a shutter speed that provides the best performance for the system (aieee! Shutter shock!).
If I see a Leica M10 being used with a $369 TTArtisans M mount 35mm f1.4 on the front my first question is not about whether the photographer could not afford the Leica version of the lens but about how the photographer shoots. If the photographer works mostly on the street and uses a hyperfocal distance strategy for achieving focus I'll assume he or she has figured out that at f5.6 or f8.0 or f11 the difference between the quality of the lenses is totally limited by so many other factors as to be immaterial. If that's the case why spend six or seven thousand more dollars on one lens over the other?
I've been buying used Leica SLs lately. I do so not because I need a platform on which to use Leica SL lenses but because I like the build quality, the feel and especially the logic of the menus and the simplicity of the controls. I buy them because, used, they are in the same price range as a camera such as the S5 from Panasonic. And I buy them because they can be used with a wide range of lenses from Sigma, Panasonic and Leica, as well as a huge range of older adapted lenses.
People say a lot of stuff for which they have no real basis in fact. It's good to do your own research before you buy into their assumptions. It's just a hobby for most but we should still try to get stuff right.