ISO Speed Standards vs. T-Stop
Comparing 2 standards across 10 specifications. 10 key differences identified.
ISO Speed (standardized under ISO 12232 for digital sensors and ISO 6 / ISO 2240 for film, combining historical American ASA arithmetic and German DIN logarithmic scales) defines the sensitivity of an imaging medium to light. Operating on an arithmetic doubling progression (ISO 100 -> 200 -> 400 -> 800 -> 1600 -> 3200 -> 6400), each step doubles sensitivity, requiring half the exposure time (+1 EV).
The T-Stop (Transmission Stop, standardized under ISO 517 for cinematography) is a photometric measurement of a lens's true light transmission (T = f / √Transmittance), correcting the theoretical geometric f-stop for physical light lost to glass reflection and optical element absorption. Standardized across all cine primes (Cooke, ARRI, Zeiss, Angénieux).
Specification Comparison Table
10 Parameters| Parameter / Spec | ||
|---|---|---|
|
Standard Arithmetic Progression
|
ISO 50 -> 100 -> 200 -> 400 -> 800 -> 1600 -> 3200 -> 6400 -> 12800 -> 25600 -> 51200 -> 102400 | — |
|
1/3-Stop Increments
|
ISO 100, 125, 160, 200, 250, 320, 400, 500, 640, 800, 1000, 1250, 1600, 2000, 2500, 3200 | — |
|
Dual Native ISO Architecture
|
Two distinct analog gain circuits, switched rather than blended (e.g. Sony A7S III/FX3 in S-Log3: Low Base ISO 640 / High Base ISO 12,800 -- exact pair varies by camera and picture profile) | — |
|
Base ISO Signal-to-Noise
|
Maximum dynamic range and 14+ stops of tonal latitude typically occurs at the camera's lowest native hardware ISO (e.g. ISO 100 or ISO 64 on Nikon D850/Z8) -- dual native ISO sensors are the exception, with a second local peak at their high base ISO | — |
|
Historical Standard Fusion
|
ISO 400/27° fused American ASA 400 (arithmetic) with German DIN 27° (logarithmic) | — |
|
Mathematical Formula
|
— | T-Number = F-Number / √(Transmittance Percentage as a decimal) |
|
Physical Meaning
|
— | A lens set to T2.8 transmits the exact same photometric illumination to the sensor as an ideal 100% transparent lens at f/2.8 |
|
Complex Zoom Glass Loss
|
— | A 18-element zoom marked f/2.8 with 80% transmission has an actual transmission rating of T3.1 (0.3 stops darker than f/2.8) |
|
Cinema Iris Ring Markings
|
— | Marked strictly in T-stops with linear mechanical spacing and clickless 0.8 MOD gears for follow focus rigs |
|
Shot-to-Shot Matching
|
— | Switching from a 24mm wide lens to a 85mm telephoto on a movie set, both set to T2.8, is designed to hold exposure and luminance effectively constant across the cut |