Exposure calculator
Exposure is a product of three things. Change one by a stop and you must change another by a stop the other way.
1/x seconds
New shutter speed
500.00
- New shutter in seconds
- 0.002
- Exposure value at this ISO
- 11.9366
- Exposure value at ISO 100
- 9.9366
- Stops gained from the aperture
- -2.000
- Stops gained from the ISO
- 0.000
- Stops the shutter must give back
- -2.000
Show the working
- Aperture = 5.6
- Shutter speed = 125
- ISO = 400
- New aperture = 2.8
- New ISO = 400
- shutter_seconds = 1 / 125 = 0.008
- Exposure value at this ISO = log2(5.6^2 / 0.008) = 11.9366
- Exposure value at ISO 100 = 11.9366 - log2(400/100) = 9.936 64
- Stops gained from the aperture = 2 * log2(2.8 / 5.6) = -2
- Stops gained from the ISO = log2(400 / 400) = 0
- shutter_change = -2 - 0 = -2
- New shutter in seconds = 0.008 * 2^-2 = 0.002
- New shutter speed = 0.002 > 0 ? 1 / 0.002 : 0 = 500
- Stops the shutter must give back = -2 = -2
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- EV = log2(N^2 / t)
- one stop is a factor of two in light, which is a factor of sqrt(2) in f-number
The f-number sequence 1.4, 2, 2.8, 4, 5.6, 8 is powers of the square root of two, because aperture area goes as the square of the diameter.