Photograph the positions you want to compare

Two views per position

A frontal and a side view are needed for each position. The frontal view carries how wide you present to the wind; the side view carries your back angle and how far forward you sit.

Both are taken with the complete bicycle, since the bike is part of what the air has to travel around.

Ride the positions you actually ride

A position you can only hold for a photograph will rank well and mean nothing. Sit as you would an hour into a ride — the comparison is only useful if every position in it is one you could genuinely use.

Keep the conditions identical

Everything except the position has to stay the same across every photograph: the same bicycle, clothing, helmet, shoes, camera position, camera height, distance, lighting and background.

The comparison is only as trustworthy as the things that stayed identical between the photographs.