What can Tiger Woods do, you can’t? If you have to hit a ball and put it in a hole one foot in front of you, the difference between Tiger Woods and you might look small. But if you play a real game, and the difference is enormous.
A DSLR (or mirrorless camera) is a tool that enables you to make every picture you image, in the highest possible quality, at every possible moment. A smartphone is a tool with which you can make about 1% of these photos in a quality that varies between quite ok and very bad. (Quite ok means: looking good at the screen of your smartphone but just so-so on a 4k screen let alone if you want to do some post-processing to make contrast and colors look right.)
Yet most people make that 1 % of the pictures and don’t care to make better pictures. They don’t see that the faces of the people in the pictures are distorted, that their noses are much too long e.g.. If they would want to make really great pictures regularly (and not just now and then by accident) they would have to invest a lot of time in learning how to take pictures, even with a smartphone. And if you do that, why would you settle for gear that limits you to 1% of your capacities?
The very best smartphones are quite good – compared to smartphones from a couple of years ago. They took over large parts of the market for compact cameras. That was easy, after all, compact cameras use the same sensors and don’t have exchangeable lenses.
But physically smartphones differ a lot from DSLRs and mirrorless cameras. That has three consequences: 1. They’re extremely limited in lens size. 2. They’re extremely limited in sensor size. 3. They’re extremely limited in ergonomics.
Smartphone manufacturers have done a lot to compensate for this. The most important effort was in the field of marketing. That’s why many people ask this question. They don’t understand the differences, look at some pictures made with DSLRs but used in adds for smartphones and think they are almost equal.
The second important effort is that smartphone manufacturers spend a lot of money on smart combinations of extra hardware features and software, to mimic the effects of exchangeable lenses and larger sensors. That works – hence the 1%. But the differences are still enormous and one should never forget that those technologies triple down to mirrorless cameras (and to a lesser extend to DSLRs) so there will always be a large difference.
Maybe, one day, smartphones become as good as DSLRs and mirrorless cameras are today. Maybe, because with regards to lenses smartphones are already at the limit of theoretical capacities and the limits of the capacities of CMOS-sensors are also very near. It’s hard to say if and when smartphones become as good as DSLRs and mirrorless cameras are today, but it will, in any case, take longer than is relevant for the gear you buy and use in the next decennium.