T-Stop

T-Stop

noun | Camera & Optics

A transmission stop is a calibrated measurement of a lens's actual light transmission to the film or sensor, accounting for light lost to internal glass elements, coatings, and reflections within the lens barrel. While an f-stop is a geometric calculation (focal length divided by aperture diameter), a T-stop is a measured value: the actual quantity of light that passes through the lens and reaches the imaging plane. Cinema lenses are marked in T-stops rather than f-stops because T-stops allow accurate exposure matching across different lenses in a set.

Quick Reference

The Explanation: How & Why

Every glass element inside a lens absorbs a small amount of light. A lens with many elements - a complex zoom, for instance - may absorb significantly more light than a simple prime with fewer elements, even if both lenses are set to the same f-stop. Two lenses at f/2.8 may pass different amounts of light to the sensor, producing different exposures on the same shot. For photography this is usually acceptable. For cinema, where multiple lenses are used on the same production and shots must match seamlessly in the edit, it is a serious problem.

The T-stop solves this through direct measurement. A light meter is placed at the imaging plane, a known light source illuminates the lens, and the aperture is adjusted until the meter reads a specific value. The aperture position at which each specific light level is achieved is marked as the corresponding T-stop. Every lens marked T/2 delivers the same amount of light to the sensor, regardless of its internal optical complexity. The relationship is expressed as: T-stop = f-number / sqrt(transmittance). Most cinema prime lenses have a T-stop that is 1/3 to 1 full stop slower than their nominal f-number. A 50mm lens with f/1.4 optics commonly has a T1.6 or T1.8 marking, meaning it transmits 2/3 to 1 stop less light than f/1.4 implies.

On a simple, high-quality prime with few elements and excellent coatings, the T-stop value is very close to the f-stop value - perhaps T/1.4 corresponds to f/1.3. On a complex zoom with many elements, the T-stop may be significantly lower: a zoom might be marked f/2.8 but only transmit enough light to be T/3.2 or T/3.5.

A cinematographer working with a matched set of cinema primes sets each lens to T/2.8 and knows the exposure will be consistent across every focal length. When they switch from a 25mm to a 50mm to a 100mm, the exposure does not change. If they were using f-stops, they would need to adjust exposure individually for each lens. On a fast-moving shoot, that individual adjustment for each lens change would be impractical.

T-stops control exposure; f-stops control depth of field. Since T-stops and f-stops are closely related values (T-stops are slightly lower than f-stops for the same aperture position), depth of field calculations are sometimes made using f-stop values even on cinema productions. A focus puller calculating depth of field for a lens set to T/2 would typically use approximately f/2 in their calculations. For a Cooke S4/i 50mm T2.0, the geometric f-number is approximately f/1.9. The DoF difference from this 5% discrepancy is small - typically under 3% - but it compounds with lens breathing and other factors. For a detailed analysis of how T-stop discrepancies affect DoF calculations, see the Tools for Film blog post on lens breathing and T-stops.

Historical Context & Origin

The T-stop standard was developed by the Society of Motion Picture Engineers (SMPE, later SMPTE) in the mid-20th century. The need for a more accurate expression of photographic speed than the f-number ratio emerged around 1940, as cinema production became more technically demanding. The proposed American Standard PH22.90, published in the Journal of the SMPTE in 1953, established the photometric method for aperture calibration that became the T-stop standard. E. W. Silvertooth's 1942 paper "Stop Calibration of Photographic Objectives" and Emmanuel Berlant's 1946 "A System of Lens Stop Calibration by Transmission" laid the groundwork.

The standard is now universal across professional cinema lens manufacturers. Cooke, Zeiss, Leitz, Panavision, and ARRI all calibrate their cinema lenses in T-stops. Still photography lenses from Canon, Nikon, and Sony continue to use f-stops, which is one of the practical distinctions between still and cinema optics. The ANSI PH22.90:1964 standard formalized the method for determining aperture calibration of motion-picture lenses.

How It's Used in Practice

Scenario 1 - Lens Set Consistency (DP / Camera Department): A production uses a set of five Cooke S4/i prime lenses: 25mm, 35mm, 50mm, 75mm, 100mm. The DP meters the scene at T/2.8 using a Sekonic L-858D light meter. Every time the lens is changed, the focus puller adjusts to T/2.8 on the new lens. The exposure is identical on every focal length without any individual adjustment. This is what T-stop calibration delivers.

Scenario 2 - Zoom Lens Compensation (DP): A production uses an ARRI Alura LWZ 15.5-45mm T2.8 zoom for a specific sequence. The DP notices that at the telephoto end, the exposure is slightly darker than at the wide end. This is a known characteristic of some zoom lenses - the T-stop value changes slightly across the zoom range. The DP notes the compensation needed at each end of the zoom range or works with the 1st AC to confirm the lens's specific T-stop behavior using a test chart and light meter.

Scenario 3 - Mixed Lens Sets (1st AC / DIT): A production combines ARRI Master Primes (T1.3) with Cooke S7/i lenses (T2.0) for different sequences. The 1st AC tests both sets with a light meter at the imaging plane to confirm that T/2.8 on the Master Primes and T/2.8 on the Cookes produce identical exposure. If there is any discrepancy, the AC notes the offset and communicates it to the DIT for on-set look management. The DIT applies matching exposure in Pomfort Silverstack to ensure dailies are consistent across lens sets.

Usage Examples in Sentences

"Set it to T/2. Not f/2. The T-stop is what actually controls exposure on this camera system."

"Every lens in this set is calibrated. T/2.8 on the 25mm gives you the same exposure as T/2.8 on the 100mm. That is what T-stops are for."

"Still lenses use f-stops. Cinema lenses use T-stops. The difference matters when you are cutting between focal lengths in the edit."

"The zoom loses half a stop at the long end. Know your glass."

Common Confusions & Misuse

T-Stop vs. F-Stop: An f-stop is a geometric ratio - focal length divided by aperture diameter. A T-stop is a measured light transmission value. F-stops are used on still photography lenses. T-stops are used on cinema lenses. The values are similar but not identical - a T/2 typically corresponds to an f-stop between f/1.8 and f/2 on the same lens, depending on the lens's optical efficiency. Using f-stop and T-stop interchangeably introduces exposure errors of 1/3 to 2/3 stop, which is visible on professional cameras with 14+ stops of dynamic range.

T-Stop vs. ND Filter: A neutral density (ND) filter reduces the amount of light reaching the sensor in full stops without changing the aperture or depth of field. Changing the T-stop changes both exposure and depth of field. They both control light, but through entirely different mechanisms with different visual consequences. An ND filter is external to the lens. A T-stop is a property of the lens itself.

T-Stop as DoF input: Entering the T-stop value into a depth of field calculator that expects an f-stop introduces a small error. The geometric f-number is slightly wider than the T-stop for the same aperture position. For a Cooke S4/i 50mm T2.0, the geometric f-number is approximately f/1.9. The DoF difference is typically under 3%, but it compounds with lens breathing at close focus distances. For critical focus work, use the manufacturer's published f-number rather than the T-stop marking.

Variations by Context

Key People & Films

The T-stop standard emerged from the work of E. W. Silvertooth, whose 1942 paper in the Journal of the SMPE established the theoretical basis for transmission-based aperture calibration, and Emmanuel Berlant, whose 1946 paper proposed the practical system. The Cooke Optics company, based in Leicester, England, has been calibrating cinema lenses in T-stops since the 1950s and remains one of the most respected manufacturers of T-stop-calibrated cinema optics. Roger Deakins, ASC, BSC, working with ARRI Master Primes on 1917 (2019) and Blade Runner 2049 (2017), relied on T-stop calibration to maintain consistent exposure across frequent lens changes in long, complex sequences. Hoyte van Hoytema, ASC, NSC, used Zeiss Master Primes and ARRI Signature Primes on Tenet (2020), where T-stop precision was essential for matching exposure across multiple cameras shooting simultaneously.

Equipment / Tools Reference

ARRI Master Prime lenses (T1.3) are the industry standard for productions requiring maximum T-stop consistency across a wide focal range. Cooke S7/i lenses (T2.0) are favored for their warmer optical character and i/Technology metadata that records T-stop and focus distance for post-production. Zeiss Supreme Prime lenses (T1.5) offer full-frame coverage with precise T-stop calibration. The Sekonic L-858D light meter is the most widely used incident meter on professional sets, capable of measuring both flash and continuous light with T-stop display. Pomfort Silverstack on-set software records T-stop metadata from the lens (via LDS or Cooke/i protocol) for each clip, ensuring the DIT and colorist have accurate exposure data. The ARRI ALEXA 35 records T-stop metadata directly from LDS-equipped lenses in the clip headers.

Standards & Specifications

The ANSI PH22.90:1964 standard formalized the method for determining aperture calibration of motion-picture lenses using photometric measurement. This standard, derived from the earlier SMPTE proposed standard PH22.90 (1953), defines the T-stop as the f-number divided by the square root of the lens transmittance. The ISO 517 standard for photographic aperture measurements references the T-stop concept for cinema applications. SMPTE standards for lens metadata (LDS, Cooke/i) include T-stop as a required metadata field for cinema lenses with electronic interfaces. The ARRI Lens Data System (LDS) and Cooke/i Technology protocols both transmit T-stop values to the camera body and to on-set monitoring systems, ensuring that T-stop data is embedded in the recorded file metadata for post-production reference.

Common Questions / FAQ

Q: Why do cinema lenses use T-stops but still lenses use f-stops?

A: Cinema productions cut between multiple lenses on the same scene, so exposure must match exactly across focal lengths. T-stops guarantee that T/2.8 on a 25mm and T/2.8 on a 100mm produce identical exposure. Still photography rarely cuts between lenses on a single scene in the same way, so the small exposure differences between f-stop-matched lenses are corrected individually in post.

Q: Can I use f-stop values in a depth of field calculator when shooting on cinema lenses?

A: You can, with a small caveat. The geometric f-number is slightly wider than the T-stop for the same aperture position. For a Cooke S4/i 50mm T2.0, the geometric f-number is approximately f/1.9. The DoF difference is typically under 3%. For most practical focus pulling, this is absorbed by the depth of field tolerance. For critical close-focus work, use the manufacturer's published f-number.

Q: How much light does a typical cinema prime lose compared to its f-stop?

A: Most cinema primes lose 1/3 to 2/3 stop relative to their geometric f-number. A 50mm with f/1.4 optics commonly has a T1.6 or T1.8 marking. Complex zooms with many elements can lose a full stop or more. The loss comes from absorption and reflection at each glass-air interface in the lens design.

Related Terms

• F-Stop - The geometric equivalent; T-stops replace f-stops on cinema lenses to provide measured rather than calculated values
• Aperture - The physical opening that both f-stops and T-stops describe; the T-stop is the measured result of that aperture's actual light transmission
• Depth of Field - The creative parameter that aperture controls; T-stop and depth of field are directly related but not identical, since T-stop is a transmission value and DoF depends on geometric aperture
• ISO - The sensor sensitivity that works alongside T-stop to determine overall exposure
• Exposure - The outcome that T-stop calibration is designed to make consistent and predictable across different lenses
• ND Filter - Reduces light reaching the sensor without changing aperture or depth of field, unlike changing the T-stop which affects both

See Also / Tools

The Depth of Field Calculator uses aperture (T-stop or f-stop) as a primary input. Entering the T-stop value of the working aperture gives an accurate depth of field calculation for focus pulling and compositional planning. For a detailed analysis of how T-stop discrepancies, lens breathing, and diffraction affect real-world DoF, see the blog post on lens breathing, T-stops, and DoF accuracy.