If you’re a photographer and you’d like to know more about lenses, then you’ll love this lesson taken from our free course, What Every Photographer Should Know About Lenses.
Focal Length and Field-of-View
The basic description of a photographic lens is focal length, because this determines how much of the world in front of your camera you will be able to fit on your image sensor. In other words, how much will you be able to see through your camera?
In this lesson, you’ll learn what focal length is and how focal length and field of view are related.
What Is Focal Length?
Despite what you may have thought, focal length isn’t a description of how long or short a lens is physically. Focal length is the distance in millimetres from the optical centre of the lens to your camera’s image sensor when the lens is focused at infinity.
The optical centre is the point where all light rays intersect inside the lens. Unless you’re going to pursue a career in optics, you really don’t have to go too much further than this because it’s more important to understand how focal length relates to what you see.
The focal length of a lens determines its angle of view or field of view, and thus how much the subject will be magnified or reduced for a given photographic position.
Short Focal Lengths
A shorter focal length results in a wider field of view.
Long Focal Lengths
A longer focal length (below) results in a narrower field of view.
This is one of the most important things to understand about lenses because the focal length of a lens is telling you how much of the world you’ll be able to see from your current position.
Another Example
For example, let’s say you’re trying to photograph the building below:
If you had a 50mm lens and were standing to the right, near the power pole, you would get this much of it in your frame:
In order to get more of the building in your shot, you basically have three choices: you can stand in the same spot and make multiple images and then stitch them together, stand in the same spot and use a shorter focal length, like 18mm, or use the same 50mm lens and move way back to get the building in your shot.
What this example shows is that one focal length is not good for all situations. What if you couldn’t move back? Then you would have to rely on the stitching method or use a different lens.
How Sensor Size Relates to Lens Field of View
In the example, we were using a lens with a fixed focal length, also called a “prime” lens. Most photographers will start out with a zoom lens, which is essentially a lens with a variable focal length. With almost any interchangeable lens camera system, you can get a fantastic range of focal lengths in lenses, from the super wide to the very, very tight.
You might have heard the term “crop sensor” or digital photo lens. First, let’s look at crop sensors. Digital photo cameras have sensors that record the images projected by the lens, and sensors are not all the same size.
There are four major categories that camera sensors fall into:
- Medium format
- Full-frame
- APS-C
- Micro 4/3
The ‘standard’ is full-frame, which is the 35mm film equivalent. If your camera has a sensor that’s smaller than full-frame, which most of the cameras in the world are, then it’s a crop sensor. Medium-format cameras have sensors larger than the 35mm standard.
Camera makers have different names for these sizes. Nikon calls the full-frame format FX, while Sony and Canon call it full-frame. Sony and Canon use APS-C to describe the sensors that use the APS-C format, while Nikon calls this format DX. Finally, Micro 4/3 is pretty much called Micro 4/3 by everyone.
Full Frame, Crop, and Micro 4/3 Examples
Full Frame (50mm Lens)
The word “crop” is a reference to what happens when you put a lens from a full-frame camera on the crop-sensor cameras.
If we put a 50mm lens on a full-frame camera, we would get an image that looks something like the one above.
Crop (50mm Lens)
If we put that same 50 mm lens on a camera with a smaller sensor, like an APS-C size, we would get a different image that looks like the photo below.
Micro 4/3 (50mm Lens)
And here’s another example. If we put that lens on a Micro 4/3 camera, it’ll look like this:
The lens is the same on all cameras, but the images look different.
Equivalent Focal Length or Equivalent Field of View
A lens acts like a projector. Light from the world in front of the lens is projected and brought into focus on the image sensor.
You can see from the diagram above that a full-frame camera will show more of the projected image than crop, and crop more than 4/3. The effect is that the smaller sensors are cropping compared to the full-frame sensor.
This doesn’t mean that the photos will be smaller, because these smaller sensors may have the same or more pixels. As they’re only seeing the centre portion of the projected image, they’re seeing less of the world. This means that in order to get a shot comparable to that of a camera with a smaller sensor, you’d have to use a shorter focal-length lens or move back to get the same composition.
Field-of-View Crop Factor
At some point, someone may have told you that a 50mm lens on your APS-C Canon camera is really an 80mm lens. This is sometimes referred to as equivalent focal length. Really it should be called equivalent field of view as the 50mm lens didn’t change focal length once you mounted it on a different camera. What does change is the crop and the equivalent field of view. A 50mm lens on a Canon APS-C sized sensor is similar to an 80mm lens’s field of view on a full-frame camera. This is also referred to as the Field-of-View Crop Factor.
APS-C has a crop factor of 1.6x for Canon cameras and 1.5x for Nikon cameras. Micro 4/3 has a 2x crop. A 50mm lens on a Micro 4/3 camera would have a similar field of view to a 100mm lens on a full-frame camera.
To simplify it, a crop sensor camera will be tighter, and getting ultra-wide angles will result in more distortion. Once you’ve been using your camera for a while, you’ll start to understand how to interpret this equivalent field of view much better.
For now, just remember that a lower focal length means wider, and a higher focal length means tighter or narrower.
Digital Lenses (EFS, DX, and More)
When you’re looking at lenses, you might see a term like ‘digital lens’ or ‘designed for digital cameras’. Canon calls this EFS, Nikon calls this DX, and other lens makers have different names for it as well.
These lenses are designed for smaller APS-C cameras. This enables them to be made smaller and lighter, with less glass, so they’re usually less expensive.
Some of these lenses can’t be mounted on a full-frame camera because of a rear lens element that protrudes out of the back of the lens.
Because these ‘digital lenses’ are designed to cover a smaller image sensor, sometimes they won’t fill a full-frame sensor, meaning that even though you might be physically able to fit it on a full-frame camera, the projected image from this lens wouldn’t cover the sensor: you’d get black along the edges, as in the diagram above.
More Great Resources for Photographers
Keep learning about lenses with more photography tutorials and free lessons from this course.
