In the film days most of us chose our favourite film to use (i'm speeking mostly for landscape shooting), packed a bunch of them into the camera bag along with our second choice of film (just in case). With the film already in the camera, there where only two adjustments to be made f-stop and shutter speed. With digital we also have ISO sensitivity.
All 3 work together to expose your image correctly. However, your choice in either one may effect the final look especially when there is movement in the subject.
A 'stop' denotes a doubling (or halving) of light. So f1.8 to f2.8 is known as one fstop which means that you have 1/2 the light going through and hitting your sensor. from f2.8 to f4.0 is another stop, so another 1/2 less light. From f4.0 to f5.6 is 1/2 less again. We have gone down 3 stops which means 1/8 of the light from f1.8 to f5.6.
Shutter speed works the same way. Each doubling (or halving) of the speed is known as a stop: 100 to 200 is one stop 200 to 400; 400 to 800 etc.
ISO, ditto shutter speed.
From the above you can see that f-stop is harder for most people to get down. You basically just have to memorise them (or look at your older lens collection). f-stop increments are: 1.8; 2.8; 4.0; 5.6; 8; 11; 16; 22
Let say you put your camera in Aperature mode and point it to your fast moving subject and get a reading of f4.0 at 1/200s and you have your ISO setting at 400. If there is motion blur, you will need to increase the shutter speed. Say that you need 1/800s to achieve a good sharp image. Now, the problem is that from 1/200 to 1/800 is 2 stops less light (1/200 to 1/400 to 1/800). You will need to compensate the fstop by allowing the aperature to open 2 stops (4.0 to f 2.8 to f1.8). What happens if there is no f1.8 but only an f2.8 on your lens? You will need to set the f-stop to f2.8 (which buys us 1 stop) but also have to change the ISO to 800 to buy us the second stop (with a very slight degration of image quality).
In the days of film we might have needed to change the film (there were other work-arounds), but with digital cameras today we can save ourselves with the ISO quite easily to ISO 800, and on some cameras all the way up to 12800 and still get amazing results (and if really needed another 2 stops more!). Please understand that ISO settings do affect image quality, if all possible, it should be kept to the base (default) ISO which is ususally 100 or 200. However, in this example there was no other choice.
As a tiny aside, the lower the f-stop selected the less depth of field you will have. That is why in landscape photography, we have to readily use tripod. Landscapers want to maintain maximum depth of field. So what does that mean? Essentially, high f-stop, low ISO (for better quality) and to compensate for the little light coming through, they have to use longer exposures. Which translates to tripod or else everything shakes and the image blurs.
In the next article I will discuss how you can use all of this wonderful knowledge when shooting near pitch black conditions so that you don't waste a lot of time with trial and error.