Text and Images by Chuck Place
Recently I gave one of my photography classes a lecture on lens focal lengths—their strengths, weaknesses and their creative potential. Because I listed the focal length of the lens used to create each image on a full chip camera body in our presentation, I reminded everyone that the effective focal length of a lens can change with the size of the sensor in their camera, as we had discussed in our first class a couple weeks before.
One of the great failings of on-line classes, in my view, is that I can’t see a wave of confusion sweep over my class when I make a statement like that.
An email arrived the next day asking for more information on effective focal lengths. I realized I needed to cover the subject in greater detail and felt a new post on my photography blog was the best solution.
Most 35mm cameras contain one of two sensor sizes. Full frame, or full chip, camera bodies contain sensors the same size as 35mm film—24x36mm. Most small chip cameras from Canon, Nikon and Sony come equipped with sensors that are roughly 15x23mm with a crop factor, or magnification factor, of about 1.5X.
Let’s look at an example of how effective focal length works.
A 50mm lens mounted on a full frame camera is considered a “normal” lens because it’s angle of view is similar to that of the human eye. A normal lens is also the dividing point between wide angle and telephoto lenses. If we mount that 50mm lens on a small chip body with a crop factor of 1.5X, it is still a 50mm lens, but the angle of view is narrowed by the smaller sensor, effectively creating the equivalent angle of view of a 75mm lens, a short telephoto, on a full frame body.
50mm X 1.5 = 75mm
If we take that 50mm lens and multiply it by .66, or 2/3, we get the focal length lens that will give us a similar angle of view on a small chip body, roughly 33-35mm. That would be a normal lens and the dividing line between wide angle and telephoto lenses for a small chip body with a crop factor of 1.5X. Essentially every lens focal length gets longer when mounted on a small chip body due to the cropping effect of the smaller sensor.
Knowing how to convert lens focal lengths is useful if you shoot both a full chip body and a small chip body, although most photographers shoot one or the other. It is also useful to be able to equate a lens with a similar angle of view on a small chip body during a lecture like mine where focal lengths are given for a full chip body.
But in any case, we all get used to using certain lens focal lengths for particular effects and subjects. We don’t need to do conversions in the field as long as we stay with one sensor size in our camera bodies and start to anticipate the way our camera sees the world.
To avoid future confusion, I think I’ll change the labels on my presentation PowerPoint slides to wide angle, normal and telephoto lenses and get rid of the actual focal lengths. Simpler.