How to calculate new shutter speed base on F stop increment or decrement?

Started Oct 1, 2008 | Discussions thread
Staale S
Senior MemberPosts: 1,108
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Re: How to calculate new shutter speed base on F stop increment or decrement?
In reply to Soo Yee Seong, Oct 1, 2008

It's quite simple really. If you double the size of the aperture opening, letting twice as much light on to the sensor, you must halve the time you keep the shutter open to get the same exposure as before. And the other way around, if you halve the size of the aperture opening, you must double the time you keep the shutter open.

There is a slight complication: Aperture numbers, f-stops, specify the diameter of the aperture hole, not the area. As a result, the "whole" aperture numbers are given by multiplying the last one by 1.4, not 2.0. The f-stop sequence goes 1.0, 1.4, 2.0, 2.8, 4.0, 5.6, 8, 11, 16 and so on. If you double the f-stop number you are actually increasing the aperture size 4 x.

So, if, for example, you start out at f/5.6, 1/500 second....

f/4 1/1000 second (double the aperture size, half the speed)
f/8 1/250 second (half the aperture size, double speed)

or, for that matter
f/2.8 1/2000 second (four times aperture, one-quarter speed)
f/11 1/125 second (one-quarter aperture, four times speed)

all give the same exposure as you started out with.

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