So in the time I have been using SLR and DSLR cameras I dont think I ever came across a more useful picture than this one.
It's something that I recommend all serious photographers copy and print off and keep with you.
I dont know the original creator so I cant credit them.
As you can see it clearly shows the 3 things any photographer needs to know.
The f-stop numbers, shutter speed and ISO values. Quite self explanatory and a great picture.