Today’s Question: When using extension tubes does the effective lens focal length change? If the effective focal length changes does that mean that the f-stop also changes?
Tim’s Quick Answer: Yes, technically an extension tube increases focal length and therefore changes the effective f-stop. However, the metadata for your photos will not reflect this change.
More Detail: An extension tube provides a simple and inexpensive way of enabling a lens to focus at a closer distance. That, in turn, increases the potential magnification of the lens. Therefore, an extension tube provides a very affordable way to do closeup or macro photography.
An extension tube simply adds space between your camera and lens, with no additional glass to potentially degrade image quality. Because the definition of focal length for a lens is the distance from the lens’ nodal point to the image sensor, using an extension tube increases the effective focal length. And since f-stops are calculated based on the lens focal length, the effective f-stop also changes.
However, the metadata won’t reflect the changes to the effective focal length and lens aperture, because the lens is effectively reporting its focal length without taking the extension tube into account. In fact, some extension tubes don’t include electronic connections between the camera and lens, preventing the camera from fully controlling the lens and eliminating metadata about the lens from being recorded at all.
