Just tried and it doesn't seem like you can half press.Can you half press on a volume rocker like an point and shoot camera?
edit: I remember when I played around with a Lumia 520 that users can half press and full press the dedicated camera key despite having a terrible camera.
If you hold down one of the volume buttons it does let you take burst shots.
Would've liked a separate button for photo functionality (even considered a z5 or lumia back when I was looking) but it seems manufacturers in general are moving away from that.