"Dead on" means accurate or exact.
I don't find that the meaning fits its usage here well:
Avoid frontal shots with straight linesMany camera phone lenses suffer from barrel distortion (straight lines appear to bow), especially when you get close to your subject. Avoid taking dead‑on photographs of building façades. Try to keep straight lines away from the edges of the frame, where the distortion will be greatest and most obvious. If you shoot from the ground up, buildings will probably appear to narrow near the top in your photograph. Unless you want this effect, photographing building corners and avoiding dead‑on shots of façades make perspective distortions less noticeable.
Source: The Camera Phone Book, by Aimee Baldridge
What does it mean in the text then? Thanks.