Aperture capacity of a lens — expressed as f-number or T-stop. f/1.4 gathers more light than f/2.8. Critical for low-light work and depth-of-field control.
The lens speed of a lens determines how much light passes through the aperture onto the sensor. An f/1.4 lens gathers four times more light than an f/2.8 lens — this is the fundamental difference between dimly lit locations and the need to haul spotlights. On set, you'll notice this immediately: with a fast lens, you can still shoot by candlelight at ISO 800 with reasonably clean results. With a slow zoom, you suddenly need ISO 6400 for the same scene, and the sensor will be full of noise.
Important: f-numbers are counterintuitive. The smaller the number, the larger the aperture opening. An f/1.4 is fast, f/16 is practically stopped down — this leads to endless confusion for assistants. The so-called T-stop is more precise because it accounts for transmission losses. A high-quality f/2.0 lens can practically be a T/2.2 — the coatings and lens elements simply absorb light. With zooms, the problem becomes acute. A 24–70mm f/2.8 is truly T/2.8 only at the 24mm end, but at 70mm it's more like T/4.0 or worse. Inexpensive zooms are often f/4.0 throughout, which in practice means: acceptable in daylight, worthless at night without additional lighting.
Your lens speed also directly influences depth of field. With f/1.4 at 50mm and a distance of 2 meters, practically only the eye level will be in focus — a dream for portraits, a nightmare for group scenes. With f/5.6, it becomes more relaxed. This isn't an aesthetic gimmick, but a workflow reality: your focus puller has to track what's in focus on the monitor with slow lenses. With the aperture almost wide open, this becomes a pain. Conversely, premium primes with f/1.3 or f/1.1 for digital cameras allow you minimal depth of field and high frame rates without heavy ND filters. This is expensive, but ultimately saves production budget.
In practice: Always take fast prime lenses with you on location. A 35mm f/1.4 and a 50mm f/1.4 are your insurance against bad weather and dark rooms. Expensive zooms with a constant f/2.8 are great, but not necessary everywhere. And remember: a slow lens with good optics is better than a fast, cheap zoom that ruins the entire image.