Now, I could make this a really technical post. But I'm not going to.
Instead I am going to explain it in a way that I found made sense to me when I was first learning. It's probably not the best way to learn, but it worked for me, and my hope is that it works for you. It's a starting point, anyway.
And this is it:
The bigger the aperture = the smaller the number = the smaller the depth of field = the faster the shutter speed.
An aperture is represented by an f-stop. A typical DSLR camera's f-stop range will be between f/5.6 - f/22. My two favourite lenses are my 50mm/ 1.8 and my 90mm/2.8 Macro. On my 50mm, my largest aperture is f/1.8. I love using the lens wide open at around f1.8-f/2.2. But, because it's so wide open, what happens is the depth of field is REALLY shallow. Meaning, that the focal point is small, and everything else blows out of focus. Focal point becomes everything at a really wide open aperture, because if I'm really close to the subject, I can have a nose IN focus, and the eyes OUT of focus. Not great for portraiture unless it's intentional.
So, it stands to reason then, that your aperture is important when deciding HOW MUCH you want in or out of focus (depth of field). But, there's an important symmetry here: Because aperture is about how wide open the shutter is, it basically then allows either more or less light to enter into the camera. A big aperture (small f-stop) allows a lot of light to touch the camera's sensor. Great. But as you stop down, or in other words, as the aperture gets smaller (aka the numbers get "bigger"), there is less light touching the sensor, and therefore the shutter speed slows down, to allow enough light to touch the sensor. Not enough light = no picture (or at very best an underexposed picture).
There's a rule with shutter speed: anything under 1/60 and you'll get camera shake. Some of the newer cameras can do alright on a slower speed, but as a rule of thumb, this speed is the best one to work off for your average DSLR. So anything slower than this, and you'll either need a tripod, or you'll have to bump up your ISO (this on another lesson).
Here's an example to illustrate the point. Not the best photos, but you will at least see the relationship between aperture (f-stop), speed and depth of field. Note: I was using a tripod, hence why there was minimal camera shake. Again, not the best example (nor quality), but it illustrates the point:
See how at f/11 everything was mostly in focus? (Bit of camera shake here- I should've used the timer). And how at f/1.8 only the front dude is in focus? And how at f/11 the speed is REALLY slow? I should add here that the light today is quite dull, and it's late in the afternoon- hence the slow speeds.
For now, what you need to remember is that there is a relationship between your aperture and your speed. If you move one, you compensate with the other. And this directly effects the overall look and feel (and focus) of your picture. If it helps- just keep saying it over and over again. That really helped me.
Next week, we'll talk a bit about some of your modes on your camera, and then hopefully you can start practicing.
