Work out aspect ratio crop instantly with clear inputs, formula shown and shareable results.
Cropping to a ratio means fitting the largest rectangle of that shape inside the original. If the source is wider than the target ratio the sides go; if taller, the top and bottom go. Cropping 4:3 to 16:9 loses 25% of the pixels.
Aspect crop
if source ratio > target: width = height x target ratio; else height = width / target ratio
The 16:9 frame is much wider relative to its height, so a quarter of the image height must be discarded to reach it.
Crop when composition allows, since letterboxing wastes display area. For archival images, keep the original and crop a copy.