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I think you’re about 2 or 3 years too late with this comment. The switch to mirrorless is already here, Nikon and Canon have the majority of their core lens lineups out already for these platforms so it’s unlikely they’d replace them again for the next decade or longer. And yes, they are mostly better than previous generations across the board. Which is pretty cool, that something based on such a simple idea that’s b…
What is the deal with mirrorless? (no mirror, but why does this improve optics?)
Mirrorless in practice really just means the camera works more like your phone camera. You view the image on a screen instead of through an optical viewfinder. That brings some nice benefits, none of them are necessarily a game changer on their own but together it makes for a major step forward in the experience. Now the viewfinder shows you the picture more like it will actually look, the processor can see the image as you’re composing it before you take the shot and provide hints and show you that information overlayed on the image, the processor is involved in auto focusing and can do things like use ai models to focus on eyes, plus the physical benefits like there’s a big complex moving piece the camera no longer needs.
Camera makers then used this change to make a bigger platform shift at the same time. Just like a major version bump in software, if you have breaking changes you’ve been waiting to make you might want to make them all at once. They widened the mount diameter of lenses, which combined with the shorter distance to sensor because of the lack of a mirror gives more flexibility in letting in more light to the sensor. And they built a new set of lenses around it, whereas a lot of the lenses in their old lineups were sometimes decades old and not designed for the crazy high megapixel sensors we have now and in the near future. Most of these new lenses released in the past few years have been very high quality, so to answer your question that’s really where the improved image quality comes from, this new generation of lenses.