The Relationship Between F-Stop and Shutter Speed

 

Now that we have gone over the three crucial camera settings used to achieve the proper exposure, f-stop, shutter speed, and ISO, we need to talk about how they work together. To start I’m only going to address f-stops and shutter speeds first. I will address the third component ISO in another post

The f-stop, shutter speed and ISO are completely tied together for exposure. When I’m starting a shot the first thing I decide is how much depth of field I want in the shot. This decision will dictate my f-stop. When you are setting your f-stop there is a huge range to choose from. The smaller the f-stop number the smaller the depth of field also called shallow depth of field or selected focus. The larger the f-stop number the larger the depth of field. I tend to shoot about 95% of all my images with very shallow depth of field so that the foreground and background are out of focus and only the area I want the viewers to look at first is in focus. I use depth of field to control where I want the viewer to look.

Here is an image showing the same shot with F-5.6 for shallow depth of field on the left and then F-16 on the right for more depth of field. Its a subtle example but you can see the difference with the front strawberry and the background strawberry in the back of the shot.

The image on the left was exposed with the following settings:  F-5.6 with a shutter speed of 1/2 second AND an ISO setting of 100. I am not changing my ISO here, just the f-stop and shutter speeds for simplicity.

The image on the right was exposed with: F-16 with a shutter speed of 4 seconds and the same ISO (100) in order to achieve the same exact exposure but just with different f-stops.

In order to change the f-stop and have the same exposure I had to change the shutter speed to accommodate the fact that f-16 is exactly THREE stops darker than f-5.6. So I had to slow the shutter speed down by three stops. What do I mean by that? Well, there is a standard scale for f-stops and shutter speeds that I talk about in those posts and depending on your lens your range for f-stops might be:

f-1.4, f-2.8, f-5.6, f-8, f-11, f-16, f-22 – (some lenses only open up to f-5.6 and won’t go to 2.8 or 1.4)

Each of these numbers represent a Full Stop of light traveling down the lens. F-1.4 is one full stop brighter than f-2.8, f-2.8 is one full stop brighter than f-5.6 and so on. Now, if you have your camera in front of you this will help make more sense. When adjusting the f-stop setting on your camera when you turn the dial (or button) one click  you are only going 1/3 of a stop which is what most cameras are set to (or with some cameras 1/4 of a stop) so if you were set to f-5.6 and change the setting by one click you are only changing the setting by 1/3 of a stop and not an entire stop. So if you were “closing down” your f-stop then you would be going to f-5.6 and 1/3 OR with some of our digital cameras this number might say f-7. Then another click would say f-7.5 and then the third click, which is one stop down from f-5.6 would now say f-8. So to go from one full stop of light to the next stop of light you have to adjust your f-stop dial by three clicks (or four clicks). One full stop of light is three (or four) clicks on your settings dial.

In the image above on the right I had to turn my f-stop dial 9 clicks to go from f-5.6 to f-16. Then to have the correct exposure accordingly I also had to turn the shutter speed dial by nine clicks in order to slow the shutter speed down by three stops. This means I went from a half second exposure, 1/2 sec all the way down to 4 seconds long (obviously I’m on a tripod).

Please keep in mind I did not change my ISO setting in this example. I only changed the f-stop and shutter speed to keep the same visual exposure. When you also change your ISO the same rules apply – one click is one third of a stop so three clicks would be one full stop of light either darker or lighter.

In my next post I will talk about the need to change the ISO to achieve a proper exposure and how to compensate that with f-stops and or shutters speeds.