Don't you just love those portraits where your little girl is so sharp and crisp, while all the background surrounding her is blurry and magical? That's called "bokeh" and to get that soft, blurry background, you have to have a basic understanding of aperture.
Aperture is one of the 3 components in the Exposure Triangle. In order to correctly expose your portrait, you need to have the correct aperture, the correct shutter speed, and the correct ISO (or film speed from the good old days). Let's focus on Aperture today. Aperture, also referred to as f/stop, is the size of the opening of your lens. This determines how much light is allowed into your camera's sensor whenever you release the shutter. Take a minute and locate the dial on your camera that controls the aperture (ahem, that means checking the manual perhaps?) Once you locate it, play around with it. If you look into your viewfinder as you turn that dial, you will see a series of numbers that change as you scroll. That is your f/stop. The wider your aperture, the smaller that f/stop number is. The smaller your aperture, the larger that f/stop number is.
Now different lenses have different ranges of f/stop, so what might be your lowest f/stop might not be mine. Some lenses open up to f/4 or f/4.5, while others open all the way up to f/1.4! When you set your lens to it's smallest f/stop, you are shooting "wide open". When you begin to close the lens and increase your f/stop, you are "stopping down the lens".
OK so what does all of this mean? Basically, the larger that f/stop number, (f/16 or f/32), the smaller that hole of light is. And what that means is that everything will be in focus, from your daughter to the tree 30 yards behind her. Bring that f/stop number way down (f/2.8 or f/4) and your "depth of field" will be shallow, meaning what you focus on will be in focus. Everything in front of, or behind her will be blurry.
Let's see a visual. A few years ago, I shared a lesson on aperture and took this series of photos. My 6 year old was a baby then, so these were the toys in our home. I suppose I could do this today with a football and some Lego sets. But let me have my memories. You'll see that the image on the left has a low f/stop number, or a wider open hole of light. This creates that shallow depth of field, making everything but my focal point blurry. On the far right, I've begun to "stop down on the lens" and my f/stop is higher, creating a smaller aperture. As a result more of the toys are in focus.
So how can this help you with your portraits? Well, if you have a full family, then you want to work with as high an f/stop number as possible, in order to keep everyone in focus. As well, if your kid is moving, especially from back to front (or towards you or away), then you want to keep that f/stop number high enough that he doesn't move out of focus.
But if you've got one solo child, or two kids along the same plane (side by side, so one doesn't appear in the "background blur"), then you can experiment a bit more. Open up that aperture, bring that f/stop number down lower and try to create some "bokeh". I absolutely love those shots, even if it's just my kid in his play gear. Nothing special, but open up that lens and everyday images become something special.
Take a chance and experiment with aperture. Play with the f/stop numbers on your camera and see if you notice a difference. Don't get frustrated, but keep practicing! Enjoy the learning process. Soon you'll be creating gorgeous bokeh with your camera too!