From the course: Lighting for Photographers: Flash Exposure

Unlock the full course today

Join today to access over 22,500 courses taught by industry experts or purchase this course individually.

Understanding why guide number math works

Understanding why guide number math works

From the course: Lighting for Photographers: Flash Exposure

Start my 1-month free trial

Understanding why guide number math works

In this movie, I'm going to take a moment to review how the flash to subject distance and the aperture scales relate to one another. And then we're going to look at how we can use this information to maintain equivalent exposures when we're working with small strobes. Remember, the square root of 2 is an integral part of the Inverse Square Law, which we use to calculate the change in illumination when the flash to subject distance changes. It's also an integral part of calculating the size of an aperture. And an aperture is nothing more than a circle, and you may recall that the formula for the area of a circle is Pi R squared. Because Pi is a constant that doesn't change no matter how big or how small the circle, we can ignore it while determining the relative change in the size of the aperture. You probably already know that if we double the area of the circle, we'll create a one stop increase in the size of the aperture. And life would really be simple if the formula read area…

Contents