Kirk Marshall Photography

We’ve been talking about composition and, in the last few posts, about lenses as they relate to composition. Now we need to expand our discussion of lenses and talk about lenses as they relate to depth of field. 

In previous posts I’ve mentioned focal length, which is the distance over which a lens converges light rays. In these previous discussions, though I didn’t explicitly state it, we assumed that the light was being focused to a single point. But that’s not realistic. Since the sensor in your camera is flat and occupies an area something larger than a single point, in some cases quite a bit larger, it follows that the lens needs to achieve not just a single point of focus on the sensor but an entire plane of focus that covers the entire surface area of the sensor. This is the focal plane. 

Lenses are typically constructed from multiple layers of glass (or some other translucent material) where each surface of each layer is ground to an either concave or convex shape to promote and achieve proper focusing. The objective is to ensure that light rays of varying wave lengths passing through both the center and edges of the lens all arrive and focus at the flat plane of the sensor. This is not a trivial task. 

Viewing an image shot through a cheap, poorly constructed lens might reveal a variety of defects including blurry edges and fringing (where different colors of light don’t focus on the same plane). In a quality lens these effects should be minimized or non-existent.