Why do cameras record f-number instead of the actual aperture diameter?
Asked 4/15/2014
4 views
2 answers
0
Photo metadata usually lists shutter speed, f-number, ISO, and focal length, but not the physical aperture diameter. Since f-number is focal length divided by aperture diameter, why is f-number considered the more useful value? Wouldn’t aperture diameter plus focal length be more precise, and which information matters more for understanding a photo?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
15
The f-stop is more directly relevant to photographers because it normalizes out the focal length. It then gives you a measure of how bright the image will be on the sensor relative to the scene brightness.
For example, if a scene is well exposed with a 50 mm lens at f/8 and 1/200 second, then it will be well exposed with any other lens at f/8 and 1/200 second. Therefore (ignoring secondary effects like light loss in the glass of the lens), f-stop, ISO, and shutter speed tell you the exposure. Focal length then tells you the image width angle, but exposure differences due to focal length have already been factored out by the f-stop value. That is why f-stop is used instead of raw aperture diameter.
Raw aperture diameter does matter for things like diffraction effects, but that's another secondary issue in most cases.
Originally by user7603. Source · Licensed CC BY-SA 4.0
user7603
12y ago
0
Generated from our catalog & community — verify before relying on it.
Photographers use f-number because it’s the value that directly predicts exposure across different lenses.
The physical aperture diameter alone doesn’t tell you how bright the image will be, because the same diameter on different focal lengths passes different amounts of light to the sensor. F-number normalizes this by relating aperture size to focal length.
That means if a scene is correctly exposed at 1/200 s, ISO 100, and f/8 with one lens, it will also be correctly exposed at 1/200 s, ISO 100, and f/8 with another focal length (ignoring small real-world transmission losses). Focal length then mainly tells you angle of view, not exposure.
F-number is also practical: it gives photographers a consistent scale that works the same on every lens and matches the familiar stop sequence used to trade aperture against shutter speed.
If you really want the physical opening, you can calculate it:
aperture diameter = focal length / f-number
Example: a 50mm lens at f/4 has an aperture diameter of 12.5mm; a 100mm lens at f/4 has 25mm. Different diameters, same exposure effect.
So for understanding exposure, f-number is usually more relevant than raw aperture diameter.
Recommended products
UniqueBot
AI12y ago
Your Answer
Related Questions
Why is aperture area expressed using focal length and f-number instead of just πr²?
Why does a smaller f-number mean more light, even if the aperture diameter stays the same?
Does close focusing or focus breathing reduce a lens’s effective aperture?
Why does f-number depend on focal length, and do longer lenses collect more light at the same f-stop?
Why do 60mm and 105mm f/2.8 lenses have the same f-stop if their aperture diameters differ?