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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?

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possible duplicate of Does changing the focal length change focus? –  mattdm Oct 19 '11 at 17:11
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No. This is the other way around. –  DragonLord Oct 19 '11 at 17:11
    
Oh! Well, it's still basically the same thing from the other side of the equation, right? But also see How can a prime lens focus on more than one plane?, which also asks a similar question in a slightly different way. –  mattdm Oct 19 '11 at 17:17
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2 Answers

up vote 9 down vote accepted

This is a phenomenon called focus breathing. With some lenses, especially macro lenses such as the Canon EF 100mm f/2.8 Macro USM lens, this effect can be substantial - at the minimum focusing distance, the focal length becomes about 70mm. While this is not normally a problem 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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Actually, a reduction in focal length while focusing closer, as with most internal-focus (or "rear-focus") lenses, is the correction for changing field of view. (Additional floating correction elements may interfere somewhat with a constant FoV in some designs.) In a traditional unit focus lens (where the physical length of the lens changes as you focus) the field of view narrows substantially as you focus closer. A traditional 100mm macro lens would have a 200mm operating length at 1:1. –  user2719 Oct 19 '11 at 19:25
    
@StanRogers, are you implying that (focal length != field of view) when the lens is not focused at infinity? –  DragonLord Oct 20 '11 at 2:56
    
No, I'm stating it clearly, not implying it. The lens's focal point (the infinity focus point, if you will) is moved away from the recording medium (sensor), and the resulting image is a smaller crop of the lens's image circle. On unit focus lenses, the field of view will be that of a lens whose focal length is ($infinity_focus + $lens_extension) focused at infinity; IF lenses keep ($infinity_focus + $lens_extension) constant by reducing ($infinity_focus) while increasing ($lens_extension), so FoV is maintained. –  user2719 Oct 20 '11 at 7:30
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Yes, it is normal.

Closer focusing lenses exhibit this more but it depends on the design of the particular lens. In any case, there is almost always a slight change in focal-length as focus changes. The difference will be more noticeable at the wide-end of a zoom but even prime lenses do that too.

When manufacturers specify things like focal-length and angle-of-view it is always (in my experience) with the lens focus set to infinity.

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Focal length (and thus angle of view) and aperture are by definition given at infinity focus. Which can lead to slightly silly numbers, for example in the case of the Canon MP-E 65 dedicated macro lens which cannot focus to infinity in the first place :) –  Staale S Oct 19 '11 at 22:24
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