Apple Photos isn't for Photos
Today I learned that Apple Photos does not display your photos. Instead, it displays a compressed version thereof. I assume this is done to save space and time, but I think that it defeats the purpose of an application meant to display your photos.
For example, the next photo is a screen shot of a photo (cropped) that I took with my Sony a5000. The first copy is from Photos, the second on Preview:
Apple Photos at 100%
Preview at 100%
Now, zooming in some:
Apple Photos zoomed in
Preview zoomed in
As you can see from this completely unscientific result, using Apple Photos results in a lot of pixelization and looks worse even at 100%. I would not reccomend using Apple Photos for viewing photos.