F-Stop
A numerical scale that indicates a camera lens's aperture setting, controlling the amount of light passing through the lens to the film or sensor.
F-Stop
noun | Camera & Optics
A standardised numerical value that describes the aperture (opening) of a camera lens, determining how much light passes through to the film or digital sensor. The f-stop scale - f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22 - is a ratio of the lens's focal length to the diameter of the aperture opening. Lower f-stop numbers indicate a wider aperture (more light; shallower depth of field); higher numbers indicate a narrower aperture (less light; greater depth of field). F-stops are one of the three elements of the exposure triangle, alongside ISO and shutter speed.
Quick Reference
| Domain | Camera & Optics |
| Also Known As | Focal stop, focal ratio, f-number |
| Abbreviated | f/N (e.g., f/2.8) |
| Also Used In | Post-Production (exposure metadata in RAW and log workflows) |
| Opposite | T-stop (measured transmission, not geometric calculation) |
| Related Terms | Aperture, Depth of Field, ISO, Shutter Speed, T-Stop |
| See Also (Tools) | Depth of Field Calculator |
| Difficulty | Intermediate |
The Explanation: How & Why
A cinematographer on a dimly lit interior set needs to control exposure. They have three tools: aperture, shutter speed, and ISO. The aperture is the physical opening inside the lens that admits light, and the f-stop is the number that describes how wide or narrow that opening is. Understanding what the number means - and what it does not mean - is the difference between controlling your image and guessing.
The f-stop is a mathematical ratio: focal length divided by the diameter of the entrance pupil. A 50mm lens with a 25mm entrance pupil is at f/2 (50 / 25 = 2). Stop down to a 12.5mm opening and the same lens is at f/4 (50 / 12.5 = 4). This ratio is why the same f-stop produces roughly the same exposure on any lens - f/4 admits the same amount of light per unit of sensor area whether the lens is a 25mm or a 200mm.
The scale runs counterintuitively: a lower number means a wider aperture and more light. The number is the denominator of a fraction. f/2 means the aperture diameter is one-half the focal length. f/8 means one-eighth. One-half is larger than one-eighth, so f/2 is a wider aperture than f/8. Each full stop on the standard scale - f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22 - either doubles or halves the aperture area, which doubles or halves the light. The scale follows the square root of 2 (approximately 1.414) because doubling the area of a circle requires multiplying its diameter by the square root of 2. Modern cameras also allow third-stop and half-stop increments for finer control.
Aperture directly controls depth of field. A wide aperture (f/1.4, f/2) produces shallow depth of field where the subject is sharp and the background blurs. A narrow aperture (f/11, f/16) produces deep depth of field where foreground, subject, and background are all sharp. On an 85mm lens at f/1.4 focused at 8 feet on a full-frame sensor, total depth of field is approximately 4.3 inches. Stop down to f/4 and it expands to roughly 14 inches. Stop down to f/8 and it reaches about 30 inches. This relationship is the most creatively significant control available to the cinematographer.
The f-stop has a critical limitation: it describes geometry, not transmission. Light passing through a lens loses intensity at every glass-to-air surface due to absorption and reflection. A lens with 15 elements transmits less light than a 6-element design at the same f-stop. This is why cinema lenses use T-stops - a measured value that accounts for actual light transmission. A lens marked f/1.4 might transmit light equivalent to T1.6 or T1.8. For stills photography, the camera's through-the-lens meter compensates automatically. For cinema, where matching exposure across a set of lenses is essential, T-stops are the standard. For precise depth of field calculations, the Depth of Field Calculator uses f-stop as a primary input.
Historical Context & Origin
The f-stop system was formalised in the late 19th century as photography developed into a standardised technical practice. The standard scale of full-stop values follows the square root of 2 progression because doubling the light requires doubling the aperture area, which requires multiplying the diameter by the square root of 2. The Royal Photographic Society adopted standardised f-number markings in the early 1900s, and the system spread rapidly across camera and lens manufacturers. The development of the T-stop measurement for cinema lenses is attributed to the Society of Motion Picture Engineers (now SMPTE) and became standard in professional cinematography from the mid-20th century onward. The distinction between f-stop (geometric) and T-stop (measured) remains one of the most consistently misunderstood aspects of lens specifications among filmmakers transitioning from stills to cinema work. For a detailed technical reference, see the Wikipedia article on f-number which covers the mathematical derivation in depth.
How It's Used in Practice
Scenario 1 - Exposure Setting (DP / Camera Operator): A DP shoots an interior scene on an ARRI ALEXA 35 at ISO 800 with a 1/48 shutter (180 degrees at 24fps). After metering the scene with a Sekonic L-858D, they set the Cooke S7/i 50mm to T2.8 for correct exposure. The T2.8 marking corresponds to approximately f/2.6 on this lens. The choice of T2.8 produces a moderate shallow depth of field that separates the actor from the background. If they needed deeper focus at T5.6, they would gain roughly 4x the depth of field but lose 2 stops of light, requiring either ISO 3200 or additional lighting.
Scenario 2 - Creative Depth of Field Choice (DP / Director): A director wants a dreamy, isolated close-up. The DP selects a Zeiss Supreme Prime 85mm at T1.5 (approximately f/1.4) on a Sony VENICE 2 in full-frame mode. At 5 feet, total depth of field is approximately 2.1 inches - one eye sharp, the other soft. The DP adds a 0.6 ND filter to maintain T1.5 under the existing lighting without overexposing. The shallow focus isolates the actor and creates a cinematic quality that separates the production from video-standard deep-focus imaging.
Scenario 3 - Consistency Across Lenses (DP / 1st AC): A production uses a set of ARRI Master Primes with T-stop markings. The DP sets each lens to T2.8 for consistent exposure across all focal lengths - 25mm, 35mm, 50mm, 75mm, and 100mm. Without T-stop calibration, each lens would require individual exposure compensation because their transmission characteristics differ. The 1st AC pulls focus using the marked T-stop and the depth of field tables calculated for each focal length at T2.8.
Usage Examples in Sentences
"Set the lens to f/2.8 and you will get the depth of field I want. Stop down to f/8 if we need the background sharp."
"The f-stop scale runs backwards from what you expect. A smaller number is a bigger opening. Remember: f/2 is twice as much light as f/2.8."
"Cinema lenses use T-stops, not f-stops. T-stops are calibrated measurements, not geometric calculations. They are more accurate for matching exposure across a set."
"Every stop you close down halves your light. Every stop you open up doubles it. The relationship is exact."
Common Confusions & Misuse
F-Stop vs. T-Stop: An f-stop is a calculated ratio of focal length to entrance pupil diameter. A T-stop is a measured value of actual light transmission through the lens, accounting for glass absorption and coating losses. For photography, f-stops are standard because the camera's TTL meter compensates for transmission differences. For cinema, T-stops are standard because they allow accurate matching of exposure across different lenses in a set. Two lenses marked f/2.8 may transmit different amounts of light - their T-stops might be T3.0 and T2.9. When matching exposure across camera setups, always use T-stops. For depth of field calculations, use the geometric f-number, not the T-stop, because DoF formulas use the physical aperture size.
F-Stop vs. Aperture: Aperture is the physical opening in the lens. F-stop is the numerical value describing the size of that opening relative to focal length. Colloquially, "aperture" and "f-stop" are used interchangeably - "change the aperture" and "adjust the f-stop" mean the same thing in practice. Strictly, the aperture is the thing; the f-stop is the measurement of the thing.
Variations by Context
| F-Stop | Aperture Size | Light Level | Depth of Field | Typical Use |
|---|---|---|---|---|
| f/1.4 | Very wide | Maximum | Very shallow | Low light; subject isolation |
| f/2.8 | Wide | High | Shallow | Portraits; controlled separation |
| f/5.6 | Medium | Moderate | Moderate | General shooting |
| f/11 | Narrow | Low | Deep | Landscapes; deep focus |
| f/22 | Very narrow | Very low | Maximum | Bright exteriors; maximum depth (diffraction risk on high-MP sensors) |
Key People & Films
The f-stop system traces to 19th-century photographic standardisation efforts, but its cinema application was defined by lens manufacturers including Cooke Optics, Carl Zeiss, and ARRI, all of which mark their cinema primes in T-stops while publishing the corresponding f-numbers in technical documentation. Cinematographer Roger Deakins regularly shoots at wide apertures (T1.4-T2.8) on ARRI Master Primes for films including Blade Runner 2049 (2017) and 1917 (2019), exploiting the shallow depth of field for visual isolation. Gordon Willis earned the nickname "The Prince of Darkness" for his preference for stopped-down exposures on The Godfather (1972), deliberately underexposing and printing down for rich blacks. Rachel Morrison used T2.0-T2.8 on Cooke S4/i lenses for Mudbound (2017) to maintain usable depth of field in naturalistic low-light conditions.
Equipment / Tools Reference
Cinema prime lens sets that display T-stop markings (with f-stop data in documentation): ARRI Master Primes (T1.3-T22), Cooke S7/i (T2.0-T22), Zeiss Supreme Primes (T1.5-T22), and Sigma Cine Primes (T1.5-T16). Photography lenses that display f-stop markings: Canon RF series (f/1.2-f/22), Sony FE series (f/1.4-f/22), Nikon Z series (f/1.8-f/22). Light meters for exposure measurement: Sekonic L-858D Speedmaster (incident and reflected, with cine frame rates), Sekonic L-308X (compact flash and cine). On-set monitoring tools that display T-stop metadata: ARRI WVM (Wireless Video Monitor), SmallHD Cine 7 with camera metadata integration. For depth of field calculations at any f-stop, the Depth of Field Calculator on this site provides instant results for any lens, sensor, and distance combination.
Standards & Specifications
The f-number is defined mathematically as N = f / D, where N is the f-number, f is focal length, and D is the entrance pupil diameter. The standard full-stop scale follows N = (square root of 2)^k, where k is an integer, producing the sequence f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22. ISO 517:2023 specifies camera aperture calibration and f-number marking conventions. Modern cameras typically allow adjustment in 1/3-stop increments, matching the ISO system of film speeds. The T-stop is defined as T = N / sqrt(transmittance), where transmittance must be measured physically - it cannot be calculated from focal length and aperture alone. SMPTE standards govern cinema lens marking conventions, requiring T-stop markings on professional cinema lenses.
Common Questions / FAQ
Q: Why do cinema lenses use T-stops instead of f-stops?
A: Because f-stops describe geometric aperture, not actual light transmission. Two lenses at f/2.8 may transmit different amounts of light due to differences in element count, coating quality, and glass type. Cinema productions need matched exposure across a set of lenses, so T-stops - which measure actual transmission - are the standard. A Cooke S4/i 50mm at T2.0 transmits exactly the same light as a Cooke S4/i 25mm at T2.0, even though their f-numbers differ slightly.
Q: What f-stop should I use for cinematic shallow depth of field?
A: There is no single "cinematic" f-stop, but T1.4-T2.8 on a fast prime at 50mm or longer on a Super 35 or full-frame sensor will produce visibly shallow DoF in close-ups. The look depends on subject distance, focal length, and sensor size working together. An 85mm at T1.5 focused at 5 feet on a full-frame sensor gives roughly 2 inches of total DoF - a classic cinematic close-up look.
Q: Does diffraction make small apertures like f/16 or f/22 softer?
A: Yes. Diffraction limits resolution at small apertures. The diffraction limit in f-stops is approximately pixel pitch (micrometres) / 0.55. A 45MP full-frame sensor (pixel pitch ~4.3 micrometres) hits its diffraction limit at roughly f/7.8. Shooting at f/11 or f/16 on such a sensor will soften the image across the entire frame, partially offsetting the depth of field gain. On lower-resolution sensors (12MP, pixel pitch ~8.3 micrometres), diffraction does not become visible until roughly f/15.
Related Terms
- Aperture - The physical opening that the f-stop numerically describes
- Depth of Field - The creative outcome most directly controlled by f-stop selection
- ISO - The second element of the exposure triangle; works with f-stop to determine overall exposure
- Shutter Speed - The third element of the exposure triangle; works with f-stop and ISO
- T-Stop - The professional cinema equivalent of f-stop; measured rather than calculated
See Also / Tools
The Depth of Field Calculator uses f-stop (or T-stop) as a primary input for calculating the precise depth of field for any combination of lens, aperture, sensor size, and subject distance. For a deeper look at how the gap between f-stops and T-stops affects real-world depth of field calculations, see our blog post on lens breathing, T-stops, and DoF accuracy.
