Now that we know all about aperture, shutter speed, and ISO, lets talk about how to get a good exposure using these three things.
To get the perfect exposure, the perfect amount of light has to hit your sensor. If you let in too little light, your photographs will be dark and generally “gray.” If you let in too much light, your photos will be very bright and generally washed-out in color. We control how much light hits our sensor using the “Big Three” that we have learned about the last few weeks.
Think of aperture and shutter speed on either side of a balance scale. When you add to or change one side–it affects the other side of the scale and in order to keep them in balance you have to make adjustments to the other side. Inside your camera is a light meter. You can see this light meter when you turn on your camera and look through the viewfinder. It will look something like this: +…..O…..- . The light meter is the scale. It is what tells you if your aperture and shutter speed are out of balance.
Let’s say that I am outside taking photos of my kids. I want the background blurry (shallow depth of field) so I open my aperture to f/3.5. The last time I was taking pictures I had my shutter speed set to 1/60. As I look at the light meter in the viewfinder it looks like this: +lllll0…..- . Those lines to toward the plus sign tell me that I have too much light coming into the camera. In order to keep this in balance, I need to decrease the amount of time my shutter stays open. I turn the dial until the lines toward the plus sign disappear. Now my shutter speed is at 1/500 and my light meter is in balance.
f/5.6 ISO 200 1/80
On the same day, I want some photos of the gorgeous mountains near my house. For this, I need a much greater depth of field (I need everything between me and the mountain in focus) so I increase my aperture to f/16 but now my light meter looks like this: +…..0lllll-. In order to keep the light meter in balance, I will have to slow down my shutter speed. Once I get to 1/60, the light meter is in balance and I snap the picture.
f/13 1/400 ISO 200
Now you may be wondering, “So, how does ISO come into play with the balance between aperture and shutter speed?” Well, lets say now that I move inside and want to take some pictures of my kids jumping on the bed. There is a lot less light in my house than outside my house, so I open the window blinds to let in as much light as possible. I also open up my aperture, to let in as much light as possible.
I know that in order to freeze the action, I am going to need a fast shutter speed. However, even with the open blinds and the open aperture, I still can’t get get my shutter speed above 1/80 and my pictures are still a little blurry. The only way to get my shutter speed faster and still keep a good exposure is to increase how sensitive the sensor is to the light by increasing my ISO. Once I increase my ISO to 800, I am able to increase my shutter to 1/400 and thus freeze the kids “mid-jump.”
Some caveats: Your light meter isn’t perfect. The great thing about digital is that you can see what your photo looks like immediately. As you are learning, use that feedback to make adjustments. Also, you may notice that your DSLR consistently underexposes or overexposes (underexposing is much more common.) You can make adjustments by just always knowing that your camera is in balance when the light meter in your camera shows 1 or 2 bars toward the + (or whatever works for your camera.) Get to know your camera and how it exposes! Please feel free to ask questions in the comments.