π΅ Aperture f-stop Calculator
Calculate f-numbers, entrance pupils, T-stops, and aperture light ratios.
Enter two values to find the third.
Results
How to Use This Calculator
The f-number tab calculates any unknown when you know two of the three values: focal length, entrance pupil diameter, and f-number. The T-stop tab converts an f-stop to a T-stop using the lens's light transmittance. The chart tab lets you compare aperture areas side by side across the full f-stop range.
On the f-number tab, enter any two values and leave the third blank. The calculator solves for the missing one. For example, enter focal length and f-number to find the entrance pupil diameter.
On the T-stop tab, enter your f-stop and the lens transmittance percentage. Most modern lenses transmit 88 to 95% of light. Cinema lenses often list T-stop ratings directly on the barrel.
Check the aperture area in the f-number results. This shows exactly how much larger the aperture opening is at f/1.4 versus f/2.8, which directly explains the exposure difference.
Use the f-stop chart tab to see the relative light-gathering difference across the whole range at once. The bar widths represent aperture area, not diameter.
f-stop and Aperture Physics
N is the f-number. f is the focal length and D is the entrance pupil diameter, which is the physical opening of the aperture as seen from the front of the lens. The area ratio formula tells you exactly how many times more light a wider aperture admits. A T-stop is more accurate than an f-stop for video and cinema work because it accounts for light lost to glass absorption and reflection inside the lens elements.
Real-World Examples
When You Need This
If you're shooting video and matching multiple cameras or lenses on the same production, T-stops matter. Two different 50mm lenses rated at f/2.8 may have different T-stops because they use different numbers of glass elements. One lens at T/3.0 and another at T/2.8 will produce slightly different exposures even at the same f-stop setting, which creates continuity problems in an edit. Cinema lenses are rated in T-stops specifically to avoid this issue.
Understanding the entrance pupil size is also useful when working with flash. At f/8, a 100mm lens has a 12.5mm pupil. At f/2.8, it's 35.7mm. This affects how much of a softbox or reflector the lens "sees" from the subject's position, which subtly changes the quality of the light in the final image. Photographers who build custom lens rigs sometimes calculate pupil diameter to decide whether a specific filter holder or matte box will vignette at wide apertures.
Frequently Asked Questions
What is an f-stop (f-number)?
The f-number is the ratio of the lens focal length to the diameter of the entrance pupil: f/N = focal_length / pupil_diameter. f/2.8 means the pupil is 1/2.8 of the focal length in diameter.
What is the difference between f-stop and T-stop?
F-stop is a geometric ratio based on pupil size. T-stop (transmission stop) accounts for light lost to absorption and reflection in the glass elements. T-stop = f-stop / β(transmittance). Cinema lenses are rated in T-stops.
How much more light does f/1.4 let in vs f/2.8?
Each full stop doubles/halves light. f/1.4 to f/2.8 = 2 stops = 4Γ the light difference. f/1.4 admits 4Γ more light than f/2.8.
What are the standard f-stop values?
The standard full-stop sequence is f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22. Each step doubles or halves the aperture area (and thus the light). Third-stop increments fill in between.
What is the "sweet spot" aperture?
Most lenses are sharpest 2β3 stops below maximum aperture. A f/1.8 lens is typically sharpest around f/4βf/5.6. At very small apertures (f/16+), diffraction softens the image.