Focal length changes with focusing distance

Discussion in 'Canon EOS' started by lalon_karim, May 16, 2009.

  1. Help! I own a 450D/ Rebel XSi. Today (May 16, 2009) I bought a EF-S 55-250mm f 4-5.6 lens. This is not an internal
    focusing lens (front of the lens rotates while focusing). My interest is portrait photography and this lens covers two
    traditional portrait focal lengths 65mm (105 mm equivalent) and 83mm (135mm equivalent). But I found out that if a
    leave the lens at any focal length (55, 70, 100, 135, 200, 250 etc) and than focus (manually or auto), the perspective/
    angle of coverage changes. That is, for a same subject at a same distance using a same focal length, the closer the
    distance I focus, the viewfinder (and final image) shows a tighter angle which means the focal length increases. The
    focal length ring does not rotate while it happens.
    Why is it happening? Is it normal? Does anyone have any experience regarding this? As far as I know, it may
    happen to internal focusing lens. But why is it happening with this lens?
     
  2. For reference, I believe the phenomenon is called "focus breathing". As to which lenses do it and which do not, I'm afraid I don't know. It's mainly a problem in video.
     
  3. The other side of the coin is a change of focus when you zoom a lens. If the focus stays fixed as the zoom moves, a lens is denoted as "parfocal". All Canon L series zoom lenses are parfocal, and as a rule of thumb all the non L series lenses are not parfocal - they are varifocal.
    If a lens changes its focus as it zooms (i.e. is varifocal) I would also expect it to change its zoom as it focuses.
    I also recall that if a Canon lens is listed has having a maximum aperture that varies according to focal length (like the EF-S 55-250mm f/4.0-5.6 IS) then it is definitely varifocal. If it has a fixed maximum aperture (like the EF 24-70mm f/2.8) then it is parfocal.
     
  4. this happens on prime lenses as well. i think it is intrinsic to moving lenses to focus the image, the composition will change. But i do not know from any eral knowledge, just my own speculation.
     
  5. With modern zooms, the focal length shortens as you focus closer. Some primes also such as 100 2.8 APO Elmar for Leica. It is a trick to get shorter focus distance.
    Prime lenses EFFECTIVE focal length gets longer as you focus closer because the lens is moved further from the film. Internal elements usually do not move, so it is really the original focal, just the angle of view narrows making it effectively longer on the same size film.
     
  6. And it really is a big effect: I calculated for example that the 28-135 with its zoomring set at 135mm and the lens at its minimum focus distance, the lens has an effective focal length of just 70mm...
     
  7. For critical focus with a zoom lens designed for still camera use, ALWAYS focus at the focal length at which you will shoot. L-series lenses may be more nearly parfocal than consumer-grade lenses, but highly accurate parfocal behaviour is simply not a design criterion for still camera lenses, with cautionary implications if they are also to be used for video.
    A lens that focuses by rigid movement of the entire lens away from the film/sensor plane (sometimes called a linear focusing lens) has a narrower angle of view the closer it is focused, but that does not imply that there has been any change in focal length. Such a change can happen only if components of the lens move relative to one another. Many modern lenses, both primes and zooms, have components that move relative to one another during focusing. On a prime lens this may be to control aberration, but often it is, or is also, to effect focusing by reducing the focal length of the lens rather than by moving it rigidly. The change can be quite large: the EF 100/2.8USM has a focal length of about 70mm when focused at x1. If its focal length did not change, it would need 100mm of fousing travel to reach x1. On a zoom lens, the interaction between focus and zoom can be very complex indeed, and there is no reason to be surprised by the behaviour you are observing with your lens.
     
  8. Thank you all for your responses. What surprises me is that the change in effective focal length is very noticable with this lens and I didnt read about it in any review. If I am not wrong, at 55 mm, when focused to a distance of 4ft, it works like an 68mm lens (gives the coverage of 110mm lens instead of 88mm in 35 mm equivalent). That's a huge difference!
     

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