Our next assignment in my photography class was Depth of Field. This is what will be in focus in your picture and you use the aperture setting (or F-stop) to set your depth of field.
F-stop has always been a very confusing issue for me. I still have to think hard about it but here is what you need to remember.
Bigger Aperture (or small F-stop) = shallow depth of field (less in focus)
Small Aperture (or large F-stop) = deep depth of field (more in focus)
Three things effect depth of field
1. camera to subject distance. The farther away you are the more depth of field
2. focal length of the lens. The shorter the focal length the more depth of field.
3. Aperture setting. Bigger aperture more depth of field.
For the assignment. Take two photos of the same scene. Keep the focal length of the lens and distance from subject the same. Use extreme aperture settings to really see the difference.
f/5.0 1/500 sec. ISO 400 35 mm lens
f/25.0 1/60 sec ISO 800 35 mm lens
I wanted the tree in the forefront to be in focus both times. When you change your F-stop you must also change your shutter speed (if you are in manual mode). The shutter speed and F-stop have a reciprocal relationship. That means if you move your aperture down one F-stop, you need to increase you shutter speed by one stop too. The reason the ISO changed in the photos was because I couldn't get the photo I wanted at ISO 400 with that large of an F-stop. I just didn't have enough light, so I increased the ISO to help.
Play around with your aperture and see what results you get. Remember if you use Aperture Priory setting on your camera, you only need to set the aperture and the camera will set everything else for you. This is a good way to start to get out of the Automatic Mode on your camera.