Kirk Marshall Photography

In my post last week I mentioned that in between wide angle lenses and telephoto lenses are normal lenses. What is a normal lens? 

Mathematically, a normal lens is one where the focal length is approximately equal to the diagonal of the sensor. 

Traditionally, 35mm cameras are the common, base standard against which many things are referenced, so let’s start there. 35mm film was 35mm wide and 24mm tall; doing the math we find that it has a diagonal of 42.4mm. So a normal lens on a 35mm camera would be somewhere around 42mm. In practice a normal lens can range from 40 to 50mm (and sometimes a bit beyond that). 

Practically, a normal lenses is one that renders the scene in a perspective that mimics our normal perception of the world. To better understand a normal perspective let’s consider an abnormal one; like the cover from Don McLean’s album American Pie.

How many people do you know who’s thumb is as big as their head?  

In this image the artist used an abnormal perspective to emphasize the American Flag painted on Mr. McLean’s thumb. The distortion of the relative sizes of the thumb, fist and head was accomplished using a wide angle lens that renders things close to the lens as large and things farther away as small.

Again, a normal lens is one that renders the relative size of objects in a manner that mimics our natural vision.

As you might expect from the previous discussion, a telephoto lens does the opposite of a wide angle lens and renders objects in an image visually compressed, making the resultant scene appear somewhat flat. This image of the Utah State Capitol building and State Street was shot with a 200mm telephoto lens. 

(Sony a7RII, Canon EF70-300 @ 200mm (w/ Metabones converter), 3.2s, f/14, ISO 400)

Since the definition of a normal lens is dependent on the size of the sensor it follows that what is a normal lens for one sensor wouldn’t be normal for a different sized sensor. For example, the sensor in my cell phone is approximately 8.5mm and therefore a normal lens as discussed above wouldn’t be normal on my cell phone. 

The choice of lens can have a tremendous impact on the final image, rendering the scene in a normal perspective or distorting it in one way or another. Choosing the appropriate lens to help you convey your vision is a choice that should not be ignored and must be made early in the photographic process.