when is a similiar work a derivative work or not

[michaelnotar]

what is the line between a derivative work and © infringement and when is it not infringement? when it it close to the origional, but not infringement.

im a commercial photographer and art directors usually find a photo they like and ask me to reproduce it or something like it, style, lighting, tones etc.

i seek to follow the law and want to know where to draw the line...

from what i can find, a derivative work is like a book that has also been made into a movie, a sitcom series, a play etc. i would love to get the definition of a derivative work in laymens terms.

i also ask this question regarding music parodies like weird al, which i am pretty sure have to be ok'd by the original artist. i hear parodies that i think are not authorized or perhaps they are getting away with a loophole in the system but i doubt it.

i guess my question is to understand derivate works laws. from what i recall studying, i the © of an image have the right to control the creation of derivative works? or can some take my image and manipulate it enough to make it original and © that new image as their own with my permission or even knowing about it?