I am unsure how to reproduce this effect on a photographic image. Any ideas?
I believe Scott is right when he says the image was created by hand. This technique is more commonly called "Stippling".
Most software tutorials found online look unconvincing at best, but the Artistic Halftone by Pixeology looks more promising than other automated options.
For Illustrator, you may want to look into Stipplism by Astute Graphics.
I don't have a source image with the same or similar levels of contrast as the owl, but I loaded up a cat photo and increased the contrast and brightness to provide some areas that might be close.
I'm not a photoshop user, but I suspect that PS has the same features as Paint Shop Pro in many cases. This image below is using round dot halftone effects, 3 pixel dot size:
You can see the white areas have very few dots while the darker ones are heavily populated.
One could also generate masks or selection fields to limit the halftone action or increase it in desired locations.
With the search terms "photoshop using halftone," a number of YouTube tutorials appear as well as text and graphics tutorials for those who care not to listen or view interminable directions and jump to the meat of the matter.
One argument against this being a correct answer is that halftone effects are uniform and the image of the owl appears to be random, matching Scott's suggestion that it was done by the hand of an artist.
Using the search terms "random halftone effect," I found a few possible locations, one of them providing not quite the same effect as the owl. It also references using masks and gradients to provide some form of non-uniformity in the results.
Following promising leads, I found also a photoshop guru video that is about the closest to the owl yet.
As with all things photoshop, the limitations are more on the operator/artist than anything else. I suspect the patience of the operator/artist in searching and researching the infinite variations on this particular aspect also comes into play.
This effect can be done with a halftone screen. Any shape can be used as the element from dots, splotches, lines, stars, etc. Note that this is a stochastic (FM screen) as the dot sizes don't vary; but, their proximity (frequency) does.
This could be done with a gross enlargement of a (stochastic) screened area or with an effect simulating such a view.
Hi, Stochaster offers a rather convincing stippling effect, but the dot size varies according to the brightness of the image.