Aspect ratio
The shape of the frame, width to height: 16:9 wide, 4:5 tall, 1:1 square. It decides what survives the crop.
See it
What it is
Aspect ratio is the shape of the frame written as width to height. The ones you meet: 16:9 (video and wide heroes), 3:2 (what most cameras shoot), 4:3 (phone cameras, compacts, Micro Four Thirds, iPads), 4:5 (the tall crop social feeds reward), 9:16 (full-screen phone), 1:1 (square, avatars), 2.39:1 (anamorphic cinema letterbox). It is shape, not size: 1600x900 and 800x450 are both 16:9.
Decide your ratios before the shoot, because you can crop in but you can never crop out. If one photo has to serve a 16:9 hero, a 4:5 ad, and a 1:1 avatar, shoot loose and compose for the tightest crop, then mark the safe area that survives all three. In CSS, the aspect-ratio property plus object-fit reserves the box so the page does not jump while the image loads.
The trap is believing one master frame covers everything. A hero composed perfectly at 16:9 becomes a chin and a shoulder at 9:16, and automatic center-cropping will happily behead your subject. Either shoot a version per ratio, or compose with the subject and the copy space parked inside the intersection of every crop you owe.
Ask AI for it
Produce one version of this shot at [target aspect ratio, for example 4:5]. Recompose specifically for that frame rather than center-cropping a wider master: keep the subject on the left third, and reserve at least 30 percent of the frame as clean, low-detail negative space for a headline. Never cut the subject at a joint or across the top of the head. Run this prompt once per ratio you owe, holding lens character, framing distance, and grade constant so the set reads as one shoot.