How the exposure triangle works
Aperture, shutter speed and ISO together decide how bright a picture is. One stop doubles or halves the light: opening the aperture by one stop, using a shutter speed twice as long or doubling the ISO each make the picture twice as bright. To keep the brightness while changing one value, another value moves by the same number of stops in the opposite direction. That is how equivalent exposures arise: f/8 at 1/125 s and ISO 100 gives the same brightness as f/4 at 1/500 s and ISO 100, with a different depth of field and different motion blur.
EV and the formula
The exposure value (EV) describes a combination of aperture and shutter speed in one number. EV 0 is f/1 at 1 s, and every increase by 1 EV halves the light. To compare ISO settings, the EV is stated for ISO 100: each doubling of the ISO subtracts 1 from it.
EV = Av + Tv - Sv = log2(N² / t) - log2(ISO / 100)
Av = 2 log2(N) is the APEX aperture value (N is the f-number), Tv = -log2(t) the time value (t in seconds) and Sv = log2(ISO / 100) the ISO value relative to ISO 100. The Exif standard defines the aperture value and the shutter speed value in this form.
| Aperture | Shutter speed | ISO | Exposure value (at ISO 100) |
|---|---|---|---|
| f/1 | 1 s | 100 | 0.0 |
| f/2.8 | 1/60 s | 100 | 9.0 |
| f/8 | 1/125 s | 100 | 13.0 |
| f/16 | 1/125 s | 100 | 15.0 |
| f/5.6 | 1/125 s | 400 | 10.0 |
What each value does
- Aperture
- Controls how much light passes through the lens and how deep the zone of acceptable sharpness is: a larger aperture gives a shallower depth of field, a smaller one a greater depth of field.
- Shutter speed
- Controls how long the sensor is exposed: a longer time records movement as blur, a shorter time freezes it more. Camera shake becomes visible at longer times.
- ISO
- Controls the amplification of the sensor signal: a higher ISO allows shorter times or smaller apertures, and image noise usually becomes more visible.
Limits
Equivalent exposures give the same brightness, not the same picture: depth of field, motion blur and noise change. The calculator uses the standard series of whole, half and third stops. Camera values such as 1/125 s or f/5.6 are treated as exact steps, as cameras do (1/125 s counts as 1/128 s); calculated directly, f/8 at 1/125 s gives EV 12.97 instead of 13. The series run from f/1 to f/64, from 1/8000 s to 1 h (Bulb) and from ISO 50 to 102400. If a value would fall outside, it stops at the end and the tool shows how much brighter or darker the result is.
FAQ
What is the exposure triangle?
The exposure triangle is the relationship between aperture, shutter speed and ISO. Together they determine how bright a photo is. Changing one value requires an opposite change of another to keep the brightness.
What does EV mean in photography?
EV stands for exposure value. One number describes all combinations of aperture and shutter speed that let in the same amount of light; f/8 at 1/125 s and ISO 100 is EV 13. It is usually stated for ISO 100.
How do I keep the same exposure when I change the aperture?
Change the shutter speed by the same number of stops in the opposite direction, or change the ISO. Opening the aperture from f/8 to f/4 is two stops, so 1/125 s becomes 1/500 s.
What is a stop?
A stop is a doubling or halving of the light. One stop of aperture is a factor of about 1.4 in the f-number (f/5.6 to f/8), one stop of shutter speed halves or doubles the time, and one stop of ISO doubles or halves the ISO value.
Why does the tool show EV 13.0 for f/8, 1/125 s and ISO 100?
Cameras treat the nominal value 1/125 s as one exact step from 1/250 s, which is 1/128 s in the stop series. The tool does the same. Calculated with exactly 1/125 s the result would be 12.97.
What happens when the offsetting value reaches the end of its range?
Every series has an end, for example ISO 50 or 1/8000 s. The tool then uses the closest possible value and shows how many stops brighter or darker the result is than the starting exposure.