Q: Problem with "Photo"

If you turned iCloud Photo Library on, and set your local Photos Library yo be Optimized, then the full-resolution needs to be downloaded from iCloud before you see it at high resolution.

Are you shooting RAWS or JPEGS?

Try rebuilding the thumbnails:

https://discussions.apple.com/docs/DOC-8521

Region_154 wrote:

 

Thanks for your piece of advice. My iCloud Photo Library is always turned off. It's strange, but with photos, which were taken with an iPhone and the other cameras everything is ok. Problem is only with Canon, which takes photos with the highest resolution compared with the other cameras. Maybe Photo doesn't support my camera

Are you seeing the problem in the thumbnails or when you double click on the thumbnail to see the larger version? 

What are the pixel dimensions of these Canon photos? 

How much memory do you have and how much free space on your boot drive?

If you're only seeing the problem in the larger version of the image does it go away if you wait for a bit or resize the Photos window a bit.  It could be just a rendering issue due to the size of the image.

I have no such issues with my T6S.

If you change the image size slightly with the slider at the top does the image improve?

and

does it go away if you wait for a bit or resize the Photos window a bit

LN

That's an issue that will have to be addressed by Apple.  Tell Apple about the problem via https://www.apple.com/feedback/photos.html.