Proper form for chest press

Chest press is one of my favourite upper body exercises, and recently I started wondering whether I do them correctly. What I’m not sure about is arm position because we always have a sideways view of the trainers:

We start with our arms up and weights above our shoulders, and when we lower them, do we stop our elbows at around 90 degrees, or let them bend further? In other words, in the lowest position, are the weights supposed to be above our elbows, or above our shoulders?