F-Stop Scale vs. ISO Speed Standards
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).
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).
Specification Comparison Table
11 Parameters| Parameter / Spec | ||
|---|---|---|
|
Mathematical Definition
|
F-Number N = Focal Length (f) / Entrance Pupil Diameter (D) | — |
|
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
|
Each full f-stop increase cuts light passing through the lens by exactly 50% (1 stop / 1 EV) | — |
|
Diffraction Limit Physics
|
Airy disk diameter = 2.44 × λ × N. At f/16 and f/22 on high-res digital sensors, diffraction visibly softens fine detail | — |
|
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) |