|Changing aperture and focus settings.|
Depth of field is controlled by a number of factors, but the three which the photographer has direct control over are focal length, aperture, and point of focus. I shot three images of Kathryn, seen above, using my 150 - 600 mm birding lens. Set at the 150 mm mark, it had a relative focal length of 225 mm on my Nikon digital crop sensor camera.
The first image would be the one I would normally go after, with foreground and background both blurred so draw your attention to the subject. The key here is a longer focal length lens with a small f/number (wide aperture). Larger sensors produce images with less depth of field given the same relative focal length, so using a full frame camera will produce narrower depth of fields than a compact camera will. Focusing on the subject renders most other objects out to focus.
The middle image used all the same settings except that the point of focus was changed to the red poppy indicated. The wide aperture and closer point of focus led to a very narrow depth of field, causing my subject to be very blurry. If you notice though, your eye goes to the poppy without much prompting - this is a terrific way to draw your eye. I will go into this in anther blog.
The last one used a small aperture (large f/number). I increased the ISO so that my shutter speed would not be too slow. The lens had vibration reduction, and I was three stops under the recommended hand held shutter speed without vibration mitigation technology. Since I had this turned on, I was within its effective range, as long as the subject wasn't moving. Point of focus placement is important, and I chose a spot about 1/3 of the way from the front poppy to the person. You can see that depth of field was increased significantly.
This is one of the reasons I suggest shooting in aperture priority instead of program or an auto mode; you have much more control over aperture. As you become more familiar with your camera its many controls you will be able to make better choices regarding the composition of your images.