Understanding Camera Lens Focal Lengths & Sensor Sizes: A Comprehensive Guide
People keep asking me variations of the same photography question: “Why does my lens behave differently on different cameras?” This seemingly simple question underscores a fundamental aspect of photography that often eludes even seasoned photographers. It also involves math (which I hate).
The crux of the matter lies in the relationship between the focal length of a lens and the size of the camera’s sensor. Many photographers mistakenly believe that the focal length changes when a lens is used with different sensor sizes. The truth, however, is a tad more nuanced.
Understanding Focal Length
Focal length, measured in millimeters (mm), describes the distance between the lens and the image sensor when the subject is in focus. It is one of the primary characteristics of a lens and significantly influences the field of view — how much of the scene the lens captures.
For example, a lens with a focal length of 24mm would provide a wide field of view, capturing more of the scene, while a lens with a focal length of 200mm would offer a narrower field of view (FOV), bringing distant subjects closer.
Cropping a photo taken with a wide-angle lens can simulate the effect of a longer focal length by narrowing the field of view.
Understanding the Camera’s Sensor
A sensor in a digital camera is the electronic device that captures light entering through the lens and converts it into the digital data that is your image.
It’s basically a grid composed of millions of light-sensitive points or pixels. Each pixel records the light that hits it, and all these individual points collectively create the final image when processed.
One common misconception is that the focal length of a lens changes when used with different sensor sizes. In reality, the physical focal length of a lens remains constant. However, the field of view does change when you use the same lens with cameras having different sensor sizes. This is often expressed as “equivalent” or “effective” focal length.
This is simply because different cameras have different sensor sizes and thus capture a different amount of the image projected by the lens; a smaller sensor records a smaller portion of that image, effectively ‘cropping’ it and providing a narrower FOV—similar to what you’d expect from a longer focal length lens.
Full-frame vs APS-C Cameras and more
The most common types of sensors you will encounter are Full-frame and APS-C.
A full-frame sensor, akin to the size of a 35mm film (36mm x 24mm), offers superior image quality, better low-light performance, and a wider field of view due to its larger size.
On the other hand, APS-C sensors are smaller, approximately 22mm x 15mm, and are commonly found in more compact and cost-effective cameras. The trade-off for the smaller size is a narrower field of view for a given focal length, a phenomenon often referred to as the ‘crop factor’.
Micro Four Thirds sensors, smaller than APS-C, are another popular option, especially in mirrorless cameras. They offer a compromise between portability and image quality, and have been particularly popular for video work.
There are also medium format sensors, which are larger than full-frame, though cameras with these sensors represent a small, high-end segment of the market.
What is “Equivalent Focal Length”?
“Equivalent focal length” is a term photographers use to describe how the field of view of a lens changes when used on a camera with a different sensor size from what the lens was designed for. This change is commonly due to the aforementioned “crop factor” associated with smaller sensors, like APS-C sensors, which have a smaller field of view than full-frame sensors.
For instance, if you mount a 50mm full-frame lens onto an APS-C camera (with an average crop factor of 1.5x), the field of view will be equivalent to that of a 75mm lens on a full-frame camera. Conversely, if you mount an APS-C lens onto a full-frame camera in crop mode, a similar change in field of view occurs.
(If all of this is making your head hurt, just skip to the end where I list a table showing the conversions.)
Full-Frame Lenses vs. APS-C Lenses
Full-frame lenses are designed to cover the larger sensor size of full-frame cameras without any issues, such as vignetting (dark corners). APS-C lenses, on the other hand, are designed for the smaller APS-C sensors. When used on a full-frame camera, these may cause vignetting unless the camera is set to APS-C or crop mode.
In the end, understanding these concepts will allow you to make informed decisions about which lenses to use for different shooting scenarios and desired effects.
Below is a table that lays out the common conversions you might find. (Full frame lens on an APS-C camera)
aaaand, the inverse (APS-C lens on a full-frame camera)
In summary: the interplay between lens focal length and camera sensor size is a pivotal aspect of understanding photographic gear. The focal length of a lens doesn’t change per se, but the field of view can vary significantly depending on whether you’re using a full-frame or APS-C sensor. This results in an “equivalent focal length” that can alter your lens’s perceived zoom and perspective.
Remember, an APS-C sensor will have a crop factor, giving the effect of a longer focal length. Conversely, a full-frame sensor provides a wider field of view. By grasping these concepts, you’ll be better equipped to select the right gear for your photographic needs and creative vision.
Hope that helps :)
