Trying to photograph girls' asses at the office

So working at an office building, I see a lot of pretty girls dressed up all sexy. The other day this hot blond was wearing a really nice dress. The thing I notice about her is the fact that she has one of those large, bubble asses. Normally she wears dress slacks or jeans and it sticks out. But on this day she wore the dress.
I think she was wearing it for me and knows that I have the hots for her. Because she would look in our office every time she'd walk down the hall past me.
2 times I tried to sneak a nice photo of her ass with my phone. But on both tries, the photo didn't turn out well. I want to start up this hobby of taking pics of women's asses at work. I want a collection of them so I can look at them at home. I want to feel like I have their asses in my collection and can look at them when I jerk off and think about them.
Anyway, this is the best shot I could get of her. I'll have to try again.