Category search was removed, but you can still do it, using this link:
https://500px.com/search?q=beach&type=photos&categories=Nude&sort=relevance
this is a search for beach. If you want to search for something else, replace the word beach with your own search term
https://500px.com/search?q=BEACH&type=photos&categories=Nude&sort=relevance
Want to search for 2 words? Put them in here, with a + sign between them.
E.g. sunshine+mountain
https://500px.com/search?q=WORD1+WORD2&type=photos&categories=Nude&sort=relevance
Want to search based on latest apperance?
Use this link:
https://500px.com/search?q=beach&type=photos&categories=Nude&sort=newest
categories types advanced search