Kirk Marshall Photography

Let’s assume our lens provides a focal plane that is free from edge distortion and achieves even focus across the entire sensor (which is what I discussed in my last post). That’s all good and fine and we’ll get great pictures if the subject we’re photographing is a flat object. For example, if we’re taking a picture of a painting it’s a simple matter to square up the camera/lens in front of the painting, focus the lens, push the button and get an image that’s in focus from edge to edge. 

However, the focal plane applies to both sides of the lens: both the inside of the camera and outside of it, looking forward toward the subject. 

The focal plane on the inside of the camera (i.e. the sensor) is always flat. The focal plane on the outside, the subject, rarely is. When you focus the lens on a subject 20 feet away, that same point on the sensor will be in focus but… every other point in the scene, that’s not 20 feet away, will be out of focus. 

Consider this image of my nephew, Cody, performing at Kilby Court in Salt Lake City, Utah. The narrow depth of field is apparent as the microphone and his face are sharp and in focus while the head of his guitar and the backdrop are very out of focus.  

(Canon 5DMII, 50mm, 1/60s, f/4, ISO 1600)

Mathematically lenses can only focus at one focal length but we can use other characteristics of the lens to achieve acceptable focus across a depth. 

Now consider this image of salt encrusted posts in a pond adjacent to the Great Salt Lake in Utah. This image has a much more extensive focal depth. Notice the reflection of the nearest post is reasonably sharp all the way down its length and the salt layers at the base of the post are quite sharp well into the upper third of the image. 

(Canon 5DMII, EF24-105 @ 65 mm, 1/320s, f/16, ISO 400)

(Neither of these images look very sharp as rendered in low resolution through this blog, but trust me, they are as described.)

Notice in both of these images that the focus fades slowly from sharp to semi-blurry to blurry, although at different rates. The rate of fading allows us to achieve acceptable focus over the critical or important part of the image.