Before you read on I suggest that you check out my previous post: About Exposure - Part 1 (Forget about the damn "Exposure Triangle").
Let's start this section by discussing a somewhat outdated principle known as Exposure Value (EV). Since the introduction of inbuilt light meters in most cameras, calculating the EV of a scene is no longer necessary. Nevertheless, I think that a general understanding of exposure values is very useful information for all photographers.
The EV is simply a way to combine shutter speed and aperture to a single value at a given ISO. (For the math nerds out there there is an equation to calculate EV: 2^EV = N^2/t where N = f-stop and t = time)
Every scene has a corresponding number on the EV scale. The darker the scene the lower the EV number, the brighter the scene the higher the number. Theoretically the EV scale is unlimited but from a practical viewpoint you will most likely never photograph a subject darker than around -5EV or brighter than +18.
-3EV - night time landscape with close to a full moon -
0EV - late evening/blue hour
6EV - bright light indoors
11EV - foggy day
15EV - full sun with no cloud
Each number on the scale = one stop of light, i.e. there is one stop difference between 11EV and 12EV.
The important point out of all of that is that there are many possible combinations of shutter speed and f-stop that will result in the same EV.
- 1/1000" at f/2.8
- 1/500" at f/4
- 1/250" at f/5.6
- 1/125" at f/8
- 1/60" at f/11
- 1/30" at f/16
Assuming an ISO of 100, all of the above combinations produce an Exposure Value of 13 - meaning that they are all equivalent exposures. 13 on the EV scale would be typically a bright but overcast day.
Of course your light meter will do all of those calculations for you so there is no need to memorise any of it. Having a rough understanding of how it all works will help you make better exposure choices though.
While all of the above shutter speed/aperture combinations will produce equivalent exposure values, the resulting photographs will each look very different. The combination that is right for you will depend on the kind of photo that you want to make.
The first question to ask yourself is whether to prioritise the shutter speed or the aperture. Which is the most important aspect of the photo: movement, depth of field, or sharpness?
The above image was shot at roughly* 90 seconds at f/11 and 100 ISO. As you can see, the movement of the water has been made to appear smooth by the long shutter speed.
The same scene taken at six seconds and f/2.8 or, three minutes and f/16, would have had the same exposure value but would have produced an entirely different looking photograph.
* I say "roughly" as I probably timed the exposure using the clock on my phone