Focal Length and Socrates

Image Impact to Changes of Focal Length

Several months ago the forest preserve trails were in full bloom. The green leafed out trees provided cover for any bird that did not want to be seen. As I walked through the dense foliage, I noticed two eastern bluebirds that wanted to be seen. These two birds darted from tree to tree, displaying their striking orange breasts and sky blue feathers. Calmly I slowed my pace, eventually freezing in my steps. Bringing my camera into position I took several shots. Deciding to move closer for a better perspective, I quietly advanced my position moving ever closer to my target. One of the bluebirds fluttered away into the brush but another flew onto a branch just above me.  He inquisitively peered down at me, almost daring me to get closer. Not wanting to press my luck, I cautiously raised my camera. While perched arrogantly on the branch, he allowed me to snap several pictures.  Then suddenly enough was enough and he skirted away into the brush.

It was now getting late so I decided to end my day on that positive note. Once home, I downloaded my day’s work for review, excited to see if any of the bluebird’s pictures were noteworthy. Now begins the second part of my story, the “what if” part.

There were several images that I was happy with, focus was sharp, enough of the background was present to detail the bird’s habitat. Though I was happy with my bluebirds, I begin to wonder what if I had a bigger focal length lens? What would it mean for the picture, how much of the frame would the bird fill? With some help from Socrates and a general knowledge of geometry (the halving the square proof) I discovered my answer. I realized if the lens focal length doubled, the image quadrupled in magnification.  Basically, if ones changed from a 200mm length to a 400mm lens, the image would not double but instead it would quadruple. With this knowledge, I cropped the original bluebird image into different apparent focal lengths to understand the impacts of focal length changes. The original bluebird was taken at 360mm with a 100mm-400mm f/5.6 Canon lens.  I hope this article helps in your understanding of the impacts of different focal lengths.