Controlling Depth of Field

The photo left was taken with a 70-200mm f/2.8 lens. The lens was zoomed to 105mm and the aperture was set at f/22.  Notice that everything is in focus; the branch which was close to me, the bridge in the middle ground and even the brick school building across the road. All are reasonably sharp and recognizable. I was actually focused on the tree branch. Frequently landscape photos benefit from small apertures which will do just what you see in this photo, keep everything in focus.

The photo above was taken with the lens stopped down. If your not familiar with the term, it refers to a lens set at a small aperture. Lenses in common use have a range of apertures from f/1.4 to f/22. Not to confuse the issue, but the smaller openings have the larger numbers. All else being equal the larger the aperture available on a lens the more expensive it is. The photo on the right was taken with the aperture wide open at f/2.8. I was still focused on the branch and now the background has become much softer which causes the branch to become the obvious center of attention in the photo.




A more common use of the ability to throw the background out of focus is shown in the portrait at left. The photo of the boy was taken with a the same lens as the two in the park. It is a fast lens, again the term fast refers to the largest aperture available on the lens. If you look closely you can see that there is a chain link fence behind him but it is so far out of focus that you only notice the subject not the background.

Below another chain link fence virtually becomes the subject of this photo. This was taken with my cell phone. No cell phone that I'm aware of has anything close to a fast lens. As we saw above small apertures keep most of the scene in focus as in this case. Is it OK? That's for you to decide when you shoot the photo. It conveys a ball game behind a fence. It's not necessarily bad but the fence has a great effect on the image. Moving close and shooting through an opening would remove the fence if desired. Shooting with a fast lens would blur the fence and make it less important but may create a hazy look as in the photo, below left.
















Here the fence was so far out of focus that it is essentially invisible, however, it's ghost is there. The shot appears to have been taken through a cloud.
Why is the fence so blurred and the background soft but still fairly well defined? That is caused by the relative distances involved. The fence is about 5 feet from the camera and the young lady probably over 100 feet away. A drastic difference in focus terms. The deep red building is another 50 feet behind our all-star ball player. That means our subject is 20 times as far away as the fence but the red building is only 1.5 times as far as the girl.