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?
Answer
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.
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