Today’s Question: When using a camera with an APS-C sensor (that we will assume for ease of calculation has a crop of 1.5X). If there is a 50mm lens on that camera, the field of view is actually equivalent to 75mm lens because of the crop factor. So when I look through the viewfinder of the camera, am I seeing a 75mm view? When I take the photo, does the photo itself take on the 50mm or the 75mm field of view?
Tim’s Quick Answer: The “crop factor” determines the field of view actually seen by the image sensor for the current lens, and also impacts what you see through the viewfinder. So with your example of a 50mm lens on a camera with a 1.5X crop factor, the viewfinder would show an angle of view equivalent to a 75mm lens, and the captured photo would reflect that angle of view as well.
More Detail: I really wish we could start using “angle of view” rather than “focal length” to describe the behavior of a given lens. In the days of film photography the general use of focal length made some sense, since 35mm film was arguably the film format used most often. But even with film photography there were different film frame sizes, resulting in different behaviors for a given lens focal length depending on what film size was being used.
Today the situation is perhaps even more confusing, with digital cameras offering “full frame” 35mm sensor sizes, “cropped” sensors of various sizes, medium format imaging sensors, and more. Thus, there is either confusion about what the focal length of a lens really is, or about what the behavior of that lens will be.
For example, the primary lens on the iPhone has an actual focal length of around 4mm (which varies depending on the specific model of iPhone), but with the small sensor size of the iPhone that lens behaves like a lens with a focal length of around 30mm (again, depending on model). Talking about “effective” focal lengths is cumbersome and potentially confusing, and talking about actual focal lengths doesn’t clearly describe the behavior of the lens.
When you are using a sensor with a size that differs from a single frame of 35mm film, the behavior of the lens will be different than a lens with the same focal length would be on a 35mm camera. That is because the image circle projected by the lens is being “cropped” due to the smaller sensor. That change in the field of view is reflected by the image captured by the sensor, as well as by the viewfinder of the camera.
Note, however, that in many cases the viewfinder of the camera is not showing you a 100% view of the actual final image. Some viewfinders, for example, show around 95% of the total field of view being captured by the sensor. But in general you can expect the final image to be cropped based on the cropping factor of that sensor, and for the viewfinder to reflect that cropped view of the scene.