F-Stop Scale vs. T-Stop
Comparing 2 standards across 11 specifications. 11 key differences identified.
The F-Stop Scale is a standardized geometric progression of lens focal ratios (N = f / D, where f is focal length and D is entrance pupil diameter) governed by powers of √2 ≈ 1.4142. Advancing one full stop (e.g. from f/2.8 to f/4.0) halves the cross-sectional area of the optical entrance pupil, cutting light transmission precisely in half (-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
11 Parameters| Parameter / Spec | ||
|---|---|---|
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Mathematical Definition
|
F-Number N = Focal Length (f) / Entrance Pupil Diameter (D) | — |
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Geometric Progression Factor
|
Powers of √2 (~1.41421356...): √2⁰=1.0, √2¹=1.4, √2²=2.0, √2³=2.8, √2⁴=4.0, √2⁵=5.6, √2⁶=8.0, √2⁷=11.3, √2⁸=16.0, √2⁹=22.6 | — |
|
Standard Full-Stop Scale
|
f/1.0 -> f/1.4 -> f/2.0 -> f/2.8 -> f/4.0 -> f/5.6 -> f/8.0 -> f/11 -> f/16 -> f/22 -> f/32 | — |
|
Standard 1/3-Stop Increments
|
f/1.4, f/1.6, f/1.8, f/2.0, f/2.2, f/2.5, f/2.8, f/3.2, f/3.5, f/4.0, f/4.5, f/5.0, f/5.6, f/6.3, f/7.1, f/8.0, f/9.0, f/10, f/11, f/13, f/14, f/16 | — |
|
Light Transmission Rule
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Each full f-stop increase cuts light passing through the lens by exactly 50% (1 stop / 1 EV) | — |
|
Diffraction Limit Physics
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Airy disk diameter = 2.44 × λ × N. At f/16 and f/22 on high-res digital sensors, diffraction visibly softens fine detail | — |
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Mathematical Formula
|
— | T-Number = F-Number / √(Transmittance Percentage as a decimal) |
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Physical Meaning
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— | 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 |
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Complex Zoom Glass Loss
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— | 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) |
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Cinema Iris Ring Markings
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— | Marked strictly in T-stops with linear mechanical spacing and clickless 0.8 MOD gears for follow focus rigs |
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Shot-to-Shot Matching
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— | 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 |