Choosing Lenses — Focal Length and Angle of View, Perspective, Primes vs. Zooms, Cine Lenses, Depth of Field, Anamorphic

The shorter the focal length, the wider the area in frame (angle of view); the longer, the narrower. Angle of view also depends on sensor size: putting the same lens on a smaller sensor is like cropping the middle, so you see less.

To compare angles of view across sensor sizes, convert to full-frame equivalents. Super 35 varies slightly by model but is roughly 1.4–1.5×, and Micro Four Thirds is 2×; multiply by that factor to get a similar full-frame angle of view. For example, 25mm on Micro Four Thirds gives about the same angle of view as 50mm on full frame. The lens’s actual focal length doesn’t change.

Perspective comes from the camera’s position, not the lens. To frame a person at the same size on a telephoto, you have to back away, which makes the distance to the person and the distance to the background relatively closer, so the background looks bigger and nearer. This is commonly called compression. Shoot with a wide lens from the same spot, crop the middle and enlarge it, and you get the same perspective as the telephoto. A nose that looks big when a wide lens moves in close to a face is also perspective created by short distance.

• Prime — One focal length. In the same class, primes are usually faster (open wider) and smaller and lighter. To change the angle of view you swap lenses.
• Zoom — Covers several angles of view with one lens. Great for shoots where things change fast, such as documentary, unscripted TV and events. Usually bigger and heavier, with a slower maximum aperture than primes.
• Constant vs. variable aperture — On some zooms the maximum aperture changes as you zoom in. If you zoom during a take, a constant-aperture lens is easier, and also check that focus holds through the zoom.

• Gearing — The focus, iris and zoom rings have teeth for a follow focus or motors. Lenses in a matched set have similar gear positions and front diameters, so you rarely need to readjust when you swap.
• Long focus throw — The ring turns a long way so you can pull small distance changes precisely, and the distance marks are large.
• T-stops and declicked iris — Marked by actual light transmission, and the iris ring has no click stops, so you can change it smoothly during a take.
• Focus breathing — The angle of view grows and shrinks slightly as you pull focus. The frame seems to breathe, so cine lenses are often designed to minimize it.

• Aperture — The wider you open (the smaller the f-number), the shallower it gets.
• Focal length — From the same distance, the longer the focal length, the shallower it gets.
• Subject distance — The closer you focus, the shallower it gets.
• Sensor size — At the same angle of view, position and f-number, the bigger the sensor, the shallower it gets.

The focus zone usually extends farther behind the focus point than in front, and at very close distances the two become about equal. The shallower it is, the more a small move by the actor throws them out of focus, so rehearse the blocking with the 1st AC (focus puller) in advance.

An anamorphic lens squeezes the image horizontally onto the sensor, and in post it’s stretched back out (desqueezed) into a wide frame. A 2× squeeze is the classic ratio, and some lenses squeeze less. For example, shooting a 2× squeeze on a sensor area close to 4:3 desqueezes to about 2.66:1, which is then trimmed slightly at the sides to 2.39:1. Shooting about a 1.33× squeeze on a 16:9 sensor desqueezes to about 2.37:1.

• The signature look — Flares that stretch horizontally and out-of-focus highlights shaped like tall ovals.
• Monitor settings — The image looks squeezed while you shoot, so turn on the desqueeze display on the monitor.