I am a little embarrassed to admit that when I first started out I did not completely understand aperature.  I was basically teaching myself with things I could read online and everything I read said the same thing…”I shoot wide open.”  What the heck did that mean?   Through some research and reading and spending a great deal of time on the computer and google, I figured out that shooting “wide open” meant shooting at a certain aperture, a certain number that would create a blurred out background.  I liked the look of that so I started shooting at the lowest number my lens would go, thinking that was there was to it.

After shooting for a while, learning from other photographers and shooting with some, I realized that while, yes, the aperture number would make things a little blurry in the background, but the distance of the subject to the background and the distance of myself to the subject also had something to do with this look I was trying to go for.  It wasn’t always just the number.

Depth of field determines the area in focus, both in front and behind the subject.  A shallow depth of field produces a small amount in focus such as the foreground and background blurry and the subject in focus.  A large depth of field allows the majority of the photo to be in focus.  If you are trying to produce the effect of bokeh or to make your subject stand out from the background, then a shallow depth of field will help you achieve this.  A shallow depth of field works with a large aperture (or small number) such as 1.8, 2.0, 2.8.

Check out this diagram.

Diagram photo courtesy of google images.

Camera to subject and subject to background distance affects the depth of field.  The closer the subjects are to the background the more in focus the background will be and the further away the subjects are from the background the more blurry the background will be.  Where you are as the photographer to your subjects always affects the depth of field and the blurriness you are trying to create.

In this first photo, you can see the subjects are really close the the background and I am not that close to the subjects.  Now the aperture on this was 1.4 and the quilt is blurry but it does not have the same effect as the next photo.

In this next photo, the subjects were about the same spot as they were in the first photo.  However, I got closer to the subjects and produced a photo with a much blurrier background.  This is due to the aperture of 2.0 and also the shallow depth of field.

Another way to effect the amount of blur is to have the subjects further away from the background.  In the photo below, I moved the girls a little further out from the quilt and I was able to create blur on the quilt.  The aperture was still at 2.0 and I was still rather close to the girls.

So while the aperture definitely helps produce blur in photos there are a few other factors that help with this too.  Remember, the distance between you and your subject and the distance between your subject and the background will also effect the final photo and its blurriness.