Focal Length and Field of View

One day I was at a park taking some pictures of the Austin skyline and somebody said something that got me thinking.  There were people taking a picture of themselves in front of the skyline with a phone camera.  They then looked at the picture and remarked on how small the skyline looked behind them.  My immediate thought was that they were using a phone camera with a really short focal length and needed someone with a long lens from much farther away taking their picture.  I thought more and more about this, did diagrams and calculations with focal lengths and angles of view and finally decided to set up a little experiment.

My first experiment is with a small object in my house in an area in which I have room to move my camera around.  I chose a vase on my kitchen counter with the windows in the background.  My goal was to see how small or large the windows in the background look while keeping the subject (the vase) the same size in the picture.  I would do this by changing the focal length and the distance from the subject to the camera.

First, I diagramed the experiment and did my calculations for focal length and angle of view based on the horizontal dimension of the sensor in my camera.  The Nikon D5300 has sensor dimensions of 23.5  mm  x  15.6  mm.  With a spreadsheet I was able to find some focal lengths that would allow me to place the object and my camera on a tripod within my kitchen.  The three focal lengths I settled on were 30mm, 50mm, and 80mm, which are within the range of my 16-80mm lens.  I then diagramed the subject, a vase, and the different distances to the camera (I picked a point on the camera body that I thought approximated the location of the sensor).  See the diagram below:


In the diagram, you can see the camera placement with the three different focal lengths and their calculated angles of view.  The vase is represented by the blue circle (A).  The black line represents the wall and windows that are 144 inches behind the vase.  As the focal length increases and the camera is moved further away from the subject, the subject should remain the same size in the photograph, but the background should appear closer and larger as there is less of it in the field of view and it is ‘zoomed in’.

Below are the three photos that I took.  All were shot in Aperture priority mode at f8 and ISO100.


30mm Focal Length


50mm Focal Length
80mm Focal Length


In the photos above notice that the vase stays approximately the same size in the picture, but the windows and art in the background appear larger and closer as the focal length increases.  Demonstrating that if I want the background to seem bigger in the photo, I would choose a long focal length and back up from the subject quite a bit.

For my next experiment, I would like to try this technique downtown with buildings and with the skyline in the background.  This will be a logistical challenge and will probably require a Sunday morning when few people are about.


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