About the F-Stop Converter
The Camera F-Stop Converter shows how much light a given aperture setting lets in compared to a wide-open f/1.0 lens, and how many stops down that represents. It's useful for photographers comparing lenses with different maximum apertures or trying to understand exposure math beyond memorizing the standard stop sequence.
How It Works
Enter an f-stop value, such as 2.8. The calculator computes the light-gathering percentage relative to f/1.0 using the inverse square of the f-number, and separately calculates how many stops down from f/1.0 that aperture represents using a base-2 logarithm.
Formula & Methodology
Light-gathering area is proportional to the square of the aperture diameter, and the f-number is defined as focal length divided by aperture diameter, so light intake scales with the inverse square of the f-number. That's why relative light is calculated by squaring the f-stop and inverting it. The stops calculation uses log base 2 because each full stop represents a halving of light, and multiplying by 2 accounts for the f-number itself scaling by the square root of 2 per stop rather than by a factor of 2.
Examples
f/2.8
At f/2.8, relative light works out to about 12.76% of what f/1.0 admits, and the aperture is about 2.97 stops down from f/1.0.
f/4
At f/4, relative light drops to 6.25% of f/1.0, and the stop count comes out to exactly 4 stops down from f/1.0, since f/4 is two full stops darker than f/2.
Advantages
- Converts an f-stop number directly into a concrete light percentage, which is easier to compare across lenses than the f-number alone.
- Quantifies the stop difference between any two apertures precisely, instead of relying on memorizing the standard 1.4, 2, 2.8, 4 sequence.
- Helps explain why the f-stop scale looks uneven, since the underlying relationship is a square root progression, not a linear one.
Common Mistakes
- Assuming f-stop numbers scale linearly with light, when going from f/2.8 to f/4 cuts the light in half rather than by a proportional amount close to the number difference.
- Comparing two lenses' maximum apertures by the raw numbers alone without recognizing that a full stop of extra light-gathering makes a meaningful difference in low light.
- Confusing relative light-gathering with total exposure, which also depends on shutter speed and ISO, not aperture alone.
Edge Cases to Watch For
- An entered value of 0 is treated as 1 to avoid an undefined division, since an f-stop of 0 has no physical meaning.
- The percentage is always relative to a theoretical f/1.0, a very fast aperture that few real lenses reach, so most everyday lenses will show single-digit or low double-digit percentages.
- This measures light-gathering ability only, not overall exposure, which also depends on shutter speed and ISO.
Common Use Cases
- Photographers comparing how much faster one lens is than another in absolute light-gathering terms.
- Videographers matching exposure when switching between lenses with different maximum apertures.
- Photography students learning why the f-stop sequence follows a square root of 2 progression instead of even increments.