With the age of different sized digital sensors that has flooded the market since the 2000’s, a lot of questions about what is the actual focal length of a lens when it is compared to a full frame 35mm camera, which has been the standard measurement for consumer cameras for decades has been asked. Some want to know what is the “field of view” compared to a full frame 35mm lens, and what full frame lens would you use on a full frame 35mm to get the same “field of view.” This is understandable because we were all trained on full frame 35mm cameras for the most part, so we already know the different focal lengths cover. That is: 24mm = 84 degrees, 35mm = 64 degrees, 50mm = 46 degrees. But when the size of the sensor is smaller than 24×36 (A crop camera as they are referred to), then the lens will cover a smaller area for the picture, and a shorter focal length can be used to achieve the same “field of view” as a camera with a larger sensor or full frame 35mm with a lens with longer focal length. This is referred to as the “effective focal length” as is just for reference only.
Example: Full frame 35mm negative size (24mm x 36mm), a normal lens is a 50mm lens (46 degrees). On a medium format camera like a Hasselblad where the negative size is 60mm x 60mm the normal lens is a 80mm lens (46 degrees). BOTH are normal lenses. If I was using a 35mm with a 50mm lens, and I had a Hasselblad with a larger negative size — or a larger sensor) what would be the equivalent focal length to get the same field of view?” It would be 80mm, so the full frame 35mm (24mm x 36mm) is a 1.6 crop camera compared to the 60mm x 60mm image area. That is, you multiply the crop factor by the lens focal length on the smaller sensor camera to get the effective focal length on the larger sensor camera. (50mm x 1.6 = 80mm). The 50mm lens is still a 50mm lens, that is the manufactures specs, and that does not ever change. What changes though is the lens for the smaller sensor has a smaller image circle so it covers the sensor w/o much overflow. The Image Circle is in direct relationship to the diagonal of the sensor, just as the lenses focal length is in direct relationship to the diagonal of the sensor, all the lenses for a certain size sensor have the same size image circle. Lens with larger sensors will have larger image circles, therefore they will have longer focal lengths to cover the same field of view as camera with a smaller sensor.
The marked focal length on the lens is the actual focal length of that lens, that will never change. But, to help explain or understand what lens this would be on a FF 35mm, the crop factor was marketed since most of us understand FF 35mm lens coverage. What you do is multiply the focal length of the lens on the smaller sensor camera to see what lens on a FF 35mm would be used to get the same field of view. It is just a conversion operation that’s all, it does NOT affect the actual focal length of the lens. The reason it is called a Crop Factor is so you can understand in FF 35mm terms what the lens covers in terms you familiar with already. You multiply the lens’s marked focal length by the crop factor (2x, 1.5, 1.6) to see what FF lens you would need place your 35mm SLR or RF to get the same field of view, this also has been called effective focal length though this term has caused all kinds of misunderstanding, leading to many that think the original focal length changes to the effective focal length, IT DOES NOT! FIELD OF VIEW is a much better term way explaining it.
I have a camera with a 1.5 crop factor, the lens that is on it is a 23mm f/2, to understand what lens with the same field of view would I have to have on a FF 35mm, I multiply the 23mm x 1.5 and I get 34.5mm (35mm). So, I have a field of view of a 35mm on a FF 35mm with a 23mm on a 1.5 crop camera. Or, the 23mm has an effective focal length COMPARED TO A FF 35mm lens of a 35mm lens.