When discussing lenses and focal lengths the question of sensor size usually comes up. If you’ve never considered sensor size and how it affects the way your lens behaves then keep reading, you probably need to understand this. The fact is that all cameras are not created equal and how much of the scene your camera is capable of recording through a specific lens can vary quite a bit.
Back before the days of digital SLRs, lenses were pretty straightforward. A 100mm lens acted like a 100mm lens. These days, the same lens will behave very differently based on the camera on which the lens is mounted.
This phenomenon is called Crop Factor or Field of View (FOV) crop. To understand what’s going on we first need to review a few basics:
Field of View describes the amount of your scene a given lens takes in.
A wide angle lens takes in large amount of the scene. Imagine being able to get the whole orchestra in your photograph while you’re sitting in the front row.
A telephoto lens takes in a very narrow part of the scene. Now you’re taking a close up for the first chair violinist while seated in the seventh row.
(No, I’m not a huge classical music fan, nor have I ever seen an orchestra but I thought the analogy would work so don’t start emailing me about classical music.)
How your lens works…well kind of.
Your lens is a cylinder that focuses light inside your camera, which is probably not a surprise to you. Being a cylinder, your lens projects a circle of light onto the digital sensor in the back of your camera. This is called the image circle. (fig. 1)
Film and Digital Sensors
Your imaging sensor sits inside the image circle. The lens and your DSLR camera body were designed based on the need to cover a piece of 35 mm film with the image circle. A full frame digital sensor is the same size as a piece of film (approx. 1”x1.5”) and, as a result, takes in the same amount of the image circle. (fig. 2)
Digital Sensors and their sizes
A Full frame digital sensor doesn’t alter the Field of View of the lens because it fills the image circle in the same way that a piece of film did. Not all digital sensors are the same size, however, and this affects the Field of View that your lens takes in.
The two most common sensor sizes are Full Frame and APS-C. An APS-C sensor measures roughly .8”x.5 inches, or half the size of a full frame sensor. When placed inside the same image circle, the APS-C sensor will take in significantly less of the image circle than the Full Frame sensor…specifically it’s recording the center 50% (Fig. 3)
Using the same lens on a camera with an APS-C sensor results in an image with a Field of View that is narrower than with a Full Frame Sensor. (Fig. 4) This is due to the fact that the smaller sensor records less of the image circle. The resulting image looks like you just put on a telephoto lens when, in fact, you’ve just cut off the surrounding area or cropped in on the full frame photo.
Sensors of different sizes will affect the Field of View that is recorded with a given lens. This change in the Field of View of your lens can also be called your effective focal length. To determine your effective focal length you must first know the crop factor of your sensor.
Knowing the crop factor of your camera is pretty straightforward; specific cameras have specific crop factors. Below are some common cameras and their associated crop factors.
Camera Crop Factor
Nikon D5000 series, D7000 series (APS-C) 1.5
Nikon D4, D810, D750, D600 (Full Frame) 1
Canon 70D, 60D, 7D series, Rebel (APS-C) 1.6
Canon 1D series, 5D series, 6D (Full Frame) 1
Fuji X-T1, X-Pro 1, X-E2 1.5
Determining your effective focal length is a matter of following the formula below.
Focal Length X Crop Factor = Effective Focal Length
So if you put the same 100mm lens on a Nikon D750 and a D7200 you will get the following effective focal lengths
Camera Lens Crop Factor Effective Focal Length
Nikon D750 100mm x 1 100mm
Nikon D7200 100mm x 1.5 150mm
When purchasing a lens for your crop sensor camera keep in mind that the effective focal length will be narrower than the actual focal length of the lens; a big concern when using wide-angle lenses. A 24 mm lens will end up with an effective focal length of 36mm on most crop sensor cameras.
There are plenty of arguments you can find on the Internet fueled by those who are adamant that you must own a full frame camera. I certainly won’t add fuel to that fire but in my mind there are advantages to both systems. These days, the performance differences between full frame sensors and APS-C are small if they exist at all… and after all it’s the archer not the arrow isn’t it?