The Sony a9 is a masterpiece of technology. Even if you have no intention of ever dropping $4500 to buy one, you have to admit that its key specifications are impressive. Aimed squarely at action photographers, it’s much faster than the a7R II, with a more sophisticated AF system, but it can’t match the older camera for sheer resolution.
In this article, we’ll be comparing the a9 and a7R II directly, looking in detail at exactly where their differences lie. For some photographers, the a9 might meet their needs admirably, whereas for others, the older a7R II might be just as good – or better. Read on to decide for yourself.
Resolution
Perhaps the most fundamental difference between these two cameras is their sensors. The a9 offers a resolution of 24MP, putting it in the middle of the pack in terms of full-frame cameras’ pixel-count.
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https://www.dpreview.com/articles/2661726126/alpha-better-sony-a9-versus-a7r-ii