The pictures here offer a comparison between each sight along with data such as ADS (Aiming Down Sights) speed and real amount of zoom given.
The ADS speed data was gathered by recording in-game animations and then count how many frames the animation needed. There might be minor errors in time due to the inaccuracy of simply counting frames versus hard data. Although it is still far more accurate than the human eye.
Speed is written as Up/Down speed. Sorted in fastest to slowest up or "ADS" speed.
0.264/0.264 - Iron sights
0.264/0.333 - Reflex/Kobra (RDS), Holographic/PKA-S (HOLO)
0.333/0.333 - M145/PK-A (3.4x), ACOG/PSO-1 (4x), Rifle Scope (6x), PKS-07 (7x), IRNV (IR 1x)
0.363/0.297 - Ballistic 8x, Ballistic 12x
0.396/0.333 - 44 Scoped