So we have discussed, in nauseating detail, each of the three main light controls that exist on most digital SLR cameras. How does all this come together to help you control image exposure? I will relate my usual techniques for photography to help explain!
When you shoot an image, the first thing that you should be thinking about is what you "expect" the final image to look like. In most cases, you will just be looking for a normal, nicely exposed image of a subject and/or scene. I typically shoot in Program Priority or "P" mode on my camera. The camera can handle a good normal exposure in most "normal" light situations. I also typically shoot at somewhere around a 200 ISO to get the best image quality.
Let's take what might be an average daylight scene outdoors. If we're shooting at 200 ISO, with a 24-105mm lens I might get a shutter speed of about 1/500th of a second at f/11. (I'm making these numbers up as I go so don't hold me to them).
What if I want to shoot a portrait and I want the background to be completely out of focus? I'd be looking for a small f-stop number, maybe f/4. Hmmm... well, that means that I'll have a lot more light entering the camera because f/4 would represent a bigger aperture opening than f/11. So I'll need a faster shutter speed. I might have to go up to 1/4000th of a second.
What if I want to shoot a landscape and I want everything in focus? Well , then I would take my aperture to f/32. That's a much smaller opening so I'd have to slow down my shutter speed to let in more light. I might and up at 1/30th of a second for my shutter speed.
So we did a couple of portraits and a couple of landscape shots. Now, lets do a portrait of a couple walking at sunset on the beach!? Well, this offers a bit of a challenge because now you might want the entire scene in focus (a small aperture) but still want the faster shutter speed because your subject will be in motion. But the high f-stop number would render a very slow shutter speed because it's such a small opening. It would be hard to get the subject to stay still enough to avoid blurring so I'd need a faster shutter speed. A shot taken at sunset ar ISO 200 and f/32 would require a shutter speed of about 1/8th of a second. That's too slow to hand hold and too slow to even do a moving subject. If I took my ISO up to 1600, I could get about 1/60th of a second which is much better and perhaps very manageable but I'd like a faster shutter speed for a moving subject in low light. So I'd probably close my aperture (or stop down as the real pros say.. I guess) one or two f-stops to f/22, which would give me 1/125th of a second shutter speed, or f/16 which would give me about 1/250th of a second. That would be much easier to hand hold and get very sharp images.
Did you notice the relationship in the numbers for the scenarios above? Each time I opened the aperture up 1 stop (smaller f-stop number), the shutter speed increased to the next faster speed. Each time I closed the aperture down one f-stop (bigger f-stop number), the shutter speed went to the next slower speed, in order to keep the same exposure at the same ISO. The same is true of the relationship with the ISO. If you go to higher ISO number, you need less light. You can either close the aperture down (bigger f-stop number) or speed up the shutter (sensor is exposed for less time) to get the same exposure. If you go to a lower ISO number, you need more light. You can open up your aperture (smaller f-stop number), or slow down your shutter speed to let more light in and keep the same exposure.
That relationship, in summary, looks like this:
To let more light in: smaller f-stop number, slower shutter, or high ISO.
To let less light in: bigger f-stop number, faster shutter, or lower ISO.
I hope this all makes at least some sense. There are many different situations that require you to think about how the camera will react to the light it sees and how it will affect your image. Then you have to know how to override the camera to get the image you want.
So that wraps it up for the discussion about what you can do with the camera to control the brightness of your image. Next we'll start talking about the mode dial and other camera settings you can use to make it easier to get the images you want!