# Is there a formula to calculate ISO according to Shutter speed? [duplicate]

Note: This is not a duplicate of "Exposure Triangle things". This is more related to Camera's auto settings i.e. Aperture and human's manual settings i.e. Shutter and ISO and their co-relation.

I am a previous user of Point and Shoot camera (Canon Powershot series) and recently shifted to canon EOS 80D. In Shutter Speed Priority Mode, we adjust Shutter and ISO and camera adjusts the Aperture. Higher the Shutter speed would require higher the ISO, otherwise, the picture would be darker.

I tried to set it in Live View, but sometimes Live View is not accurately understandable in some environments.

Is there a formula to quickly calculate ISO according to shutter speed we set?

• The simple answer is "no" unless you have already measured the external light level (and know your aperture). Do you actually want to solve this problem, or do you just want Live View to work? May 22, 2017 at 10:18
• I want to set it up without using Live View, so any more knowledge is always welcome. May 22, 2017 at 10:29
• Why about tries and errors ? May 22, 2017 at 10:30
• No, its not of a duplicate of What is the "exposure triangle"? May 22, 2017 at 11:40
• Well, What is the relationship between ISO, aperture, and shutter speed? is very useful in this regard, but not totally duplicate. However, what I was looking was answered here by Philip Kndall. This was not found in other mentioned duplicates. Thanks May 22, 2017 at 11:49

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.

• I've checked the formulae there, but there's a fair chance I've missed out a minus sign or similar. Please edit to correct if I have! May 22, 2017 at 10:56
• Waooo... I will have to practice it first... May 22, 2017 at 11:33