taking pictures of you

the first one was taken at Buckingham palace.  I stood behind some tourists taking picures with the palace and guards in the background. I have pulled my camera and snapped a picture of the guy who was taking a picture of his wife. Firstly she posed and then, seeing me - looked straight into my lens. Now the picture is long time gone but idea has grown. I do not always catch the hostile look. I just take pictures of people taking pictures of people.