I recently received the following question from a friend and thought I'd share it and my answer!
One of the things that always seems hard for me is to get the right kind of focus. I love how your pictures focus on one part of the person and the rest is beautifully blurred or bokeh'd (haha if that's even a word). How do you do it? (:
Well bokeh is technically the name of the shape that the light blurs beyond the field of focus take on due to the shape of the lens. You can actually affect the shape of the bokeh by even cutting paper into say, a star shape and shooting through it.
Bokeh Shaping Tutorial
But what you want to know about is the field of focus. :D On any SLR, you will have a lens that allows you to adjust two, maybe three, things. Shutter Speed, Aperture or F-Stop, and on zoom lenses you can adjust focal length. These all have implications for how your picture will end up looking. Aperture is the most relevant to your question, so that’s what I’ll focus on. Aperture basically refers to how big the opening of your lens will be when you click the button.
Most lenses that come with your camera have aperture settings between 3.5 and 22. Basically, the smaller the number is, the bigger the opening and vice versa. This affects a few things. When the opening is as we say, wide, at 3.5, more light will be able to get in to your camera to record the image and so you’d be able to use a faster shutter speed so that your camera will take less time to get a good exposure and the picture is less likely to be blurry. The smaller the opening (bigger number) the slower the shutter speed will need to be.
But also, the more wide open the aperture is, the shallower the depth of field, or plane of focus. In other words, if the number of the aperture is smaller, less in front and in back of what you’re shooting will be in focus. This makes the focus of the composition more obvious and eliminates background elements that could be distracting. The difference between F 3.5 and F 22 is the difference between just the coffee cup in my window being in focus and everything outside being blurry (F 3.5) and almost everything from the cup to the distant mountains (F 22).
You can start experimenting with this by putting your camera on Aperture priority (A - Nikon, Av – Canon) where you will use the wheel to choose your aperture and your camera will set the rest of the settings for the light.
As far as what I do with my photography, I use a 50 mm f1.4 lens where the widest opening is very wide which allows me to get an even shallower depth of field. Canon has a 50 mm f1.8 which is pretty cheap and I’d recommend to anyone with an SLR.