How close can you crop before losing image quality?

When I took this photo, I planned to crop in tight like this and make a black and white.

I had my 50mm prime lens on the camera, and you can only focus so close with it... I knew I had to shoot wide and crop later. If I had planned to print this I don't think I could go very large, but for use online it might not matter so much. The image as you see it here is 2000 x 1125 pixels.

When I saw this spray of flowers, the little cluster in the centre seemed to stand out from the background. I'll show you the full original shot at the end.

This is my entry for the daily #monomad photo contest, sponsored by @brumest & @monochromes

Nikon D750, 50mm, f/2.8, 1/1600 second, ISO 1000 (Click for full-screen)

Here is the same crop in colour

Here is the original scene that caught my eye. My lens could not focus closer than this.

Thanks a lot for taking a look. Make sure to check the latest post from @monochromes for the contest rules and description:

https://steemit.com/monomad/@monochromes/monomad-daily-picks-june-29th-winners-and-delegations-upvote-rates