How do shutter speed and ISO relate in Shutter Priority mode?
Asked 5/22/2017
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On a Canon EOS 80D in Shutter Priority (Tv) mode, I choose the shutter speed and often the ISO, and the camera adjusts the aperture. I understand that using a faster shutter speed may require a higher ISO to keep the exposure from getting darker if the aperture can’t compensate enough.
Is there a practical formula or rule for calculating the ISO needed for a chosen shutter speed? I’m looking for a quick way to understand the relationship between shutter speed, ISO, aperture, and scene brightness.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
2 Answers
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Yes if you know the aperture and light level. The "reference point" is that at an exposure value of 0, you will get a correct exposure with a shutter speed of 1s, an aperture of f/1.0 and ISO 100. From there, it's just a matter of counting stops to work out where you need to be: for example, if your light level is -2 EV (2 stops darker), your aperture is f/2.8 (3 stops darker) and you're at ISO 6400 (6 stops brighter), you need a shutter speed 6-2-3 = 1 stop faster or 0.5s.
Mathematically, you can calculate it all via:
- sISO = ln(ISO / 100) / ln(2)
- sAperture = -ln(Aperture) / ln(√2)
- sShutter = EV + sISO + sAperture
- Shutter speed = 2-sShutter
But by the time you've actually gone as far as measuring your light level, there are probably better ways to have solved all this.
Originally by user11371. Source · Licensed CC BY-SA 4.0
user11371
9y ago
0
Generated from our catalog & community — verify before relying on it.
There isn’t a formula for ISO from shutter speed alone. You also need the scene brightness and the aperture being used (or available).
Exposure is based on stops: if you make the shutter 1 stop faster, you must compensate with either 1 stop more ISO or 1 stop wider aperture to keep the same exposure. So the practical rule is:
- shutter 2× faster = ISO 2× higher, if aperture stays the same
- shutter 2× slower = ISO 1/2 as high, if aperture stays the same
A more formal reference is EV 0 = 1 second, f/1.0, ISO 100. From there, exposure is just counting stops between shutter, aperture, ISO, and scene light level.
In real use, though, once you know the light level, the camera meter usually solves this faster and more reliably than manual calculation. In Tv mode, the camera will open the aperture as needed until it hits the lens limit; if that still isn’t enough light, you must raise ISO or use a slower shutter speed.
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