Filled in for you if you use 📏 Measure Fish in Photo.

Choose a new photo to replace the current one.

This needs 4 taps total: first the 2 ends of the reference card's long edge (the 8.9 × 5.1 cm FishBeacon card, laid flat on the fish, same distance from the camera), then the fish's nose and tail tip. We'll try to auto-detect the card so you may only need the last 2 taps. . Nothing shows until all 4 points are placed.
🔍 Touch and hold to magnify, slide to the exact spot, then lift. Drag any point to adjust it.