Aperture to Exposure Converter
Convert aperture (f-stop) into required shutter speed at a given ISO and EV — or calculate the needed aperture for a selected shutter speed.
What the aperture to exposure converter does
The aperture to exposure converter solves the exposure equation for the setting you are missing. Enter the exposure value (EV) of the scene and the ISO, then either the aperture to find the shutter speed you need, or the shutter speed to find the aperture. Leave the field you want calculated empty and press Calculate. The tool works from the standard photographic exposure relationship used by light meters.
The formula
EV = log2(N² / t) − log2(ISO / 100)
where N is the f-number and t is the shutter time in seconds. Rearranged:
- Shutter time: t = N² / 2(EV + log₂(ISO/100))
- Aperture: N = √(t × 2(EV + log₂(ISO/100)))
The ISO term shifts the scale: doubling ISO adds one stop of sensitivity, so the camera needs one stop less light from aperture and shutter combined. At ISO 100 the term is zero and EV describes the aperture-shutter pair directly.
Worked examples
| Scene | EV | ISO | Aperture | Shutter |
|---|---|---|---|---|
| Bright sun | 15 | 100 | f/16 | 1/128 s (use 1/125) |
| Bright sun | 15 | 100 | f/2.8 | 1/4180 s (use 1/4000) |
| Overcast | 12 | 400 | f/5.6 | 1/522 s (use 1/500) |
| Indoor, well lit | 7 | 800 | f/2.8 | 1/131 s (use 1/125) |
| Night street | 3 | 3200 | f/1.8 | 1/79 s (use 1/80) |
The first row is the “sunny 16” rule: at f/16 the shutter speed is close to 1/ISO. The calculator returns an exact time; round to the nearest marked speed on your camera, since the marked speeds form a ⅓ or ½ stop series and the difference is invisible in the result.
Typical exposure values
| EV (ISO 100) | Lighting |
|---|---|
| 16 | Snow or sand in full sun |
| 15 | Clear sunny day |
| 13 | Hazy sun, soft shadows |
| 12 | Overcast, no shadows |
| 10 | Heavy overcast, sunset |
| 7 to 8 | Bright interior, shop window |
| 5 to 6 | Home interior, evening |
| 2 to 3 | Lit street at night |
| −2 to 0 | Moonlit landscape |
Your camera’s meter reads EV directly; the exposure value converter calculates it from any aperture, shutter and ISO you already have.
Trading aperture for shutter
Every full stop of aperture (f/1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22) halves the light, and every doubling of shutter time doubles it. Opening from f/8 to f/4 is two stops, so the shutter must be four times faster to keep the same exposure. Which combination to choose depends on the picture: a wide aperture blurs the background and lets you use a fast shutter in low light; a small aperture keeps a landscape sharp front to back but forces a slow shutter and often a tripod.
Limits of the formula
- Reciprocity failure. Film loses sensitivity during exposures longer than about a second; digital sensors do not.
- Lens transmission. The f-number is geometric. Real lenses lose 5 to 30% to glass and coatings, which cinema T-stops account for.
- Flash. With flash, shutter speed only controls ambient light; the flash exposure depends on aperture, ISO and distance.
- Diffraction. Beyond about f/11 on small sensors and f/16 on full frame, sharpness falls even though exposure is correct.
Common mistakes
- Entering shutter speed as 250 instead of 1/250. The field wants seconds: 0.004.
- Filling all four fields. Leave the one you want calculated empty.
- Using EV from a meter set to a different ISO. Enter the ISO the meter was set to, or convert the reading to ISO 100.
- Forgetting filters. A 3-stop ND filter lowers the effective EV by 3.
Related tools
The shutter speed converter turns fractions into decimal seconds and back. The screen PPD calculator and pixel to inches converter deal with the output side of photography, and the angle converter helps with field-of-view calculations.
External references
Frequently asked questions
What is EV?
A single number for a combination of aperture and shutter speed that gives the same exposure. Each step of EV is one stop, a doubling or halving of light.
How do I find the EV of a scene without a meter?
Use the table above, or the sunny 16 rule: in full sun, f/16 at 1/ISO seconds, which is EV 15 at ISO 100.
Why does the calculated shutter speed not match a setting on my camera?
Cameras offer a fixed series of speeds. Choose the nearest; a third of a stop is within the tolerance of any meter.
Does ISO change the EV of the scene?
Scene EV is normally quoted at ISO 100. Raising ISO lets you use a smaller aperture or faster shutter for the same scene, which the calculator handles through the ISO term.
Can I use this for video?
Yes, but video usually fixes the shutter at about twice the frame rate (1/50 s for 25 fps), so aperture, ISO and ND filters do the adjusting.