Today’s Question: Maybe this should be more obvious, but I don’t understand why it is so common to refer to an “equivalent focal length”. Why do we need to know what lens would be the same as ours if we were using 35mm, when we’re only using our lenses on a smaller sensor?
Tim’s Quick Answer: The equivalent focal length is a useful way to translate between different sensor (or film) sizes. While the current “standard” for equivalent focal length is based on a full-frame sensor (or 35mm film), this isn’t necessarily helpful for many photographers today, and may become anachronistic before too long.
More Detail: When digital cameras first became available, most photographers were using 35mm film. As a result, when image sensors that were smaller than 35mm became available, such as the APS-C sensor, it made sense to provide a translation of what to expect in terms of the behavior of the lens.
For example, a 50mm lens was considered the “standard” lens in photography. For photographers who mostly employed a 50mm prime lens, it was pretty common to be able to anticipate how far you would want to position the camera from a scene to get the framing you were after. Put another way, many photographers got to the point that they could anticipate what lens focal length they wanted just based on a visual evaluation of the scene.
If you had been using the 35mm format for long enough that you could anticipate what lens focal length was going to be right for a given scene, the notion of equivalent focal length would be particularly helpful if you migrated to a different sensor size.
For example, if you switched to a camera with an APS-C sensor it would be helpful to know that on your new camera a 50mm lens would behave like an 80mm lens on a 35mm camera system. Conversely, in this scenario it would be helpful to know that to achieve the same field of view as a 50mm lens on a 35mm camera but using an APS-C camera you would need to use about a 30mm lens (31.25 to be precise, assuming a 1.6X cropping factor for the sensor). So, knowing the equivalent focal length can indeed be helpful for photographers who got familiar with the framing of a particular lens focal length.
In theory, if lenses were describe based on field of view rather than focal length, we could have avoided this situation. However, many lenses can be used on cameras with different sensor sizes, and for each sensor there would be a different field of view with a given lens. So the equivalent focal length provides a reasonable proxy for field of view in the context of making calculations for different camera systems.
If any lens could only be used with a single sensor size, then field of view would be the perfect way to describe the behavior of a lens. Since that isn’t the case, the current approach of referencing equivalent focal length provides a bit of a workaround.
