Lens f-stop (f/1.4, f/5.6, etc.) — controls exposure, depth of field, and light gathering. Lower number = wider opening.
The aperture value determines how much light falls through the lens onto the sensor—and thus three of the most important decisions on set: exposure, depth of field, and light efficiency. It is expressed as a fraction (f/1.4, f/2.8, f/5.6), where the number in the denominator gets smaller the wider the aperture opens. This seems counterintuitive, but it's mathematics: f/1.4 collects four times as much light as f/2.8, and ten times as much as f/4. You notice this immediately on set—in low light, you can only achieve clean images without excessively high ISO values with fast lenses (f/1.4, f/2.0). Each stop of aperture (from f/2.8 to f/4, or f/5.6 to f/8) halves the amount of light that enters.
Depth of field is the second playing field. A wide aperture (f/1.4) creates a razor-thin focus range—perfect for close-ups, portraits, or for blurring backgrounds. But it's also tricky: even a 10 cm deviation and your main subject is already out of focus. Closed apertures (f/8, f/11) give you deep focus—important for landscapes, group scenes, or when actors are moving. In documentaries or handheld scenes, a smaller aperture value (f/4, f/5.6) saves you from constant refocusing problems.
Practically: Cinema lenses often have aperture ranges from t/2.8 to t/22. The 't' stands for transmittance—it accounts for how much light the glass elements actually let through (a t/2.8 is less bright than an f/2.8, but more honest). With digital cameras, you talk about f-numbers; these are theoretical. The connection to exposure runs through the exposure triangle: aperture, shutter speed, and ISO. If you double the aperture value (f/2.8 → f/4), you have to halve the exposure time or lower the ISO value—or both.
On set: Fast means flexible. With f/1.4 lenses, you can also work in available light (daylight, neon lights, candlelight) without setting up artificial light. This saves setup time and gives you freedom of movement. But consider: the faster the aperture, the more expensive and often larger/heavier the lens. A zoom f/2.8 is in a different weight class than a prime f/1.4. For shoots with fixed lighting conditions (studio, controlled set), f/5.6 is perfectly adequate. For street scenes, night shots, or when improvisation is required, you pay for speed.