When I change the focus distance on my Pentax 55-300mm lens, the focal length appears to change. More specifically, the lens appears to zoom in as I focus towards the minimum focusing distance. This is most noticeable at the 55mm end of the lens. Is this normal?
Answer
This is called focus breathing.
With some lenses, especially macro lenses such as the Canon EF 100mm f/2.8 Macro USM, this effect can be substantial—at the minimum focusing distance, the focal length becomes about 70mm. While this is not normally an issue in still photography, it can pose a problem in video shooting as it can cause the angle of view to change while racking or following focus. Cine lenses are generally corrected for focus breathing, but lenses designed for still photography are often not corrected for this behavior.
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