Exercise 1: Focal Length and Angle of View

The assignment was to set up the camera, mark the spot from which photos would be taken, and to then shoot three photos of the same scene (if using a lens with adjustable focal length, as I am):  one at either extreme (for me, 18mm and 55mm), and the other at whichever length best replicated the image seen with the naked eye.  

Then have the three photos printed, return with prints in hand to the spot marked, hold up the photos, and notice how close or how far they have to be held from the eye in order that the photo represents the size of the objects viewed with the naked eye. 

I didn’t understand this exercise until I saw this student’s blog.  

Then I got it:  how does the lens affect what you are seeing? 

I don’t have my own printer and was worried about the hassle of getting prints made until I found this great idea from another student’s blog:  use an iPad instead of a print.  Brilliant!

So I went out and made the photos, but found that the focal length that best matched what I was seeing with the naked eye was the extreme end of the lens, 55mm.   Was I doing the assignment incorrectly? I reread my notes and didn’t find anything amiss.  I googled a bit and was relieved to discover a student who made the same discovery.  So, perhaps I’ve done it right.

Here are the results.  First the two photos, at either extreme:  18mm and 55mm.

I resized the photos for the iPad screen (1024-by-768 pixels) and held them up to view the scene anew.  To see the size of the tree as best represented by my eye, I had to hold the 55mm shot at arm's length.  To match size with the 18mm shot, I had to hold the iPad right in front of my face.

So what are the implications?  Note that both photos represent different aspects of reality.  The photo at 55mm shows a more accurate version of the human focal length, while the one at 18 shows a more accurate version of the angle of view.  A little research shows the standard focal length of the human eye is 22mm.

For a useful discussion of some of these issues, follow this link and this one.  

While I was out shooting, I also did a little experiment to see how the camera represents a variety of focal lengths.

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