Next we apply the same Lens Blur effect to the background. Since depth-of-field blurring is more noticeable in the distance, we'll use an even higher setting. Heck, let's go nuts and turn it up to 100!
Because the blur effect will only be applied to the areas outside out mask, the towers we painted over will be unaffected, and will stand out against the blurred background. The finished effect is quite striking.
If you want to try this for yourself, bear in mind it only works on certain types of picture. Ideally you want a shot that is taken from an angle that looks appropriate if it was really a shot of a tiny model, and which has a clear foreground and background, which for buildings and other large objects usually means an aerial photograph, but the effect can be applied to more than just buildings. You could, for example, make your friends look tiny by photographing them from an upstairs window and using this effect.