How to make an image 20% brighter?

I am doing an experiment and I am having trouble figuring out the math involved of making an image 20% brighter in PS. If I am understanding correctly one full stop increase in brightness will give an image twice as bright, but I don't want to make the image twice as bright. If I could take a pic an increase the brightness by approximately 20% just using the camera that would best, but it seems that I could get a more accurate increase by using PS.

So how would I go about increasing the brightness by 20%?

ashdavid wrote in post #8751750
I am doing an experiment and I am having trouble figuring out the math involved of making an image 20% brighter in PS. If I am understanding correctly one full stop increase in brightness will give an image twice as bright, but I don't want to make the image twice as bright. If I could take a pic an increase the brightness by approximately 20% just using the camera that would best, but it seems that I could get a more accurate increase by using PS.

So how would I go about increasing the brightness by 20%?

If you mean 1:1 brightness (0=>10, 245=>255 style), then just use the bightness image property layer...

If not, 20% depends on what you want to call 20%, either average of 20%, or all parts 20%, etc. 10=>12 is a 20% increase, but so is 100=>120, yet 100-120 is much more noticeable than than 10-12.

Thanks I will give it a try.

Anyone for a maths lesson? (If not, skip to the last line for something that may be useful).

new = old x 2^n

Where the new exposure value = old exposure value x 2 to the power n, where n is the number of stops.

So 1 stop will increase the exposure by 2, 2 stops by 4, etc.

What you want is a value of n such that 2^n = 1.2 - for that we need logs. However, we need the log to the base 2, and most calculators won't do that. But we can use a nice little trick where

log(base2) n = log (base10) n / log(base10) 2

So, log(base2) of 1.2 = log (base10) of 1.2 / log(base10) of 2 = 0.263

Lets's check with new = old x 2^0.263 = old x 1.2

QED.

So 20% extra brightness is set by using 0.263 stops overexposure.


For general use = if you want to increase exposure by a factor of A then you need log(A)/0.301 extra stops exposure.

Huh...oh yeah, I see