Open editions refer to photographs by photographers who do not declare an edition size, however, the photographer effectively limits an open edition through time and price structure because they will only print a photograph as it is sold.
Limited editions create rarity and are, therefore, more highly valued to a collector. The smaller the edition, the more highly valued and collectible it is. Edition sizes are usually less than 100 with many less than 50.
In an edition, there always exists the very first print by the photographer, known as the Artist Proof. The market and its collectors generally favor purchasing the Artist Proof, #1 in a small edition and will even pay a premium for the last one in an edition on the basis that it represents the final available one for purchase. As with all forms of art, the artists set the pricing at the outset but the market determines demand and appreciation.