"At its most basic" means at its essential fundamentals. The most simple way to describe what it is.

'Most basic' means the element of photography that is the very first part, the integral part of what makes photography happen. For example "the most basic element of being alive is breathing". You need to use its most basic, as the its is a determiner which means the most basic element belongs to photography. You are really saying: "At photography's most important stage photography is a chemical..."