Why is my photo taken with a SLR? It's quite clear on my mobile phone, but it's like a circle of friends.

If you have played Baidu Post Bar, you will know. The reason is the same-compression.

When you transfer the photo from the SLR to the mobile phone, the size of the photo has not changed, so the effect of enlarging the photo at will at this time is not much different from that on the computer. But when you send it to a circle of friends again, the system will automatically compress your photos. The size of the photo has changed from a few MB to dozens of KB, and then the photo will be blurred if you enlarge it again.

I hope it works for you.