Adjusting your camera’s f/stop (aka aperture, f-stop) not only controls the amount of light entering the camera by opening and closing the hole through which the light. The choice of aperture also has another side effect: control of the depth of field, or the depth of the photograph that will be in focus.
I was taking a few photos while friends of mine worked on a project and realized later that I had accidentally taken excellent comparison examples of two different depths of field. So, I will put them up as a way to see more clearly how altering the f/stop changes photographs.
This first photo was taken at f/4. Now, f/4 is by no means a wide open aperture, but is a world of difference compared to the f/1.4 photo below. The f/1.4 shot below has, at most, two letters in focus, whereas the f/4 shot above has up to six letters in focus.
And that is not all. Even though the f/4 photo still has a large portion of the photo out of focus, compare the out of focus regions in the two photographs. The out of focus areas of the f/1.4 shot are wildly out of focus, especially on the top and bottom of the photo, which is further from and nearer to the camera from the sweet spot of the focus. The f/4 photo still gives the feeling of being in focus, but more subtly draws the eyes to the in-focus area.
Keep this in mind when taking photographs. The f/stop is more than just letting in more or less light. Portraits often use these out of focus areas to their advantage: eyes in sharp focus, but other areas of the face which are not in that focus area are lightly softened by the lack of sharp focus, thus making a (often) more pleasing photo.
The further away from the camera a subject is, the less selective that focus area will be. My example shots above are dramatically different (even though they are only a couple aperture stops apart) because the subject is so close to the camera. For example, if I shot the same photo with a longer lens (a telephoto) and backed up to frame the same photo in my viewfinder, the depth of field would be much less selective, making more of both photos in focus.
This may not be a comprehensive look at aperture, but it helps begin to understand how we can use these settings to our advantage, or possibly look out for potential problems with too selective of a focus.
