Lenses are an integral part of photography or filmmaking. Well, unless you’re using a pinhole camera. But field of view, focal length, and crop factors can be confusing for newer photographers. This video from The Basic Filmmaker goes over the basics of what they all mean and how to convert “focal length equivalency” for non-full frame sensors.
Kevin goes through a lot of information in the video. He uses several lenses to help illustrate his points. The Canon 24mm f/1.8, 50mm f/1.4, 85mm f/1.8 and 24-105mm f/4L. If you’re very new to photography, you might have to watch some parts several times to understand it all. But I want to explore one aspect of it a little here, and that’s crop factors.
Kevin mentions that a lens of a certain focal length acts like a lens of a different focal length when you place it on a camera with a sensor other than full frame. It doesn’t actually change the focal length of the lens. That stays the same. It doesn’t magically make the lens get longer. A smaller sensor simply captures a smaller area of what the lens sees and alters the field of view to match that of another lens when used on a full frame body.
That sounds a lot more complicated than it really is. So, let’s look at a standard 50mm lens. On a full frame camera, that lens has the field of view that one would expect from a 50mm lens.
But if you put that 50mm lens on a Nikon crop body, you effectively get a 1.5x multiplier. This means that 50mm lens now offers the same field of view as a 75mm lens on a full frame body. If you put it on a Canon body, it’s 1.6x multiplier, so you get the same field of view as 80mm on full frame. If you’re shooting Micro Four Thirds with a 2x crop factor, then that 50mm lens has a similar field of view to a 100mm lens on a full frame camera.
Again, it doesn’t actually change the focal length of the lens, just the field of view that the sensor sees.
This change in the field of view will also affect the relative depth of field – how much of your scene is in focus. You get more depth of field as you go smaller in sensor size for a given field of view and aperture combination. This is why a Nikon 105mm f/2.8 lens on a full frame body offers a shallower depth of field than a lens at 70mm f/2.8 on a Nikon crop body, even though their field of view would be identical.
Look at the photos you take on your own DSLR or mirrorless camera compared to those you take with your phone. Even though you may match the field of view, the entire world will be in focus on your phone whereas you might only get a few inches of depth of field on your “real camera” at the same aperture value. This is why a lot of people prefer larger sensors for things like portraits, so they can more easily throw the background out of focus.
There’s a whole lot more to lenses than can be put across in a 5-minute video. So, be sure to do your own research for further reading. But this is a great introduction to the terms, what they all mean, and how it can affect your photography.