real focal length of EF 70-200/4L

Discussion in 'Canon EOS' started by idoy, Jun 19, 2005.

  1. I'm considering buying the EF 85/1.8 or EF 100/2 for portrait
    photography indoors. Since I'm not sure whether 100mm may be too long
    for indoors, I decided to try these 2 focal lengths using my EF 70-
    200/4L. My body is Elan 7, by the way.

    The problem is that I'm not sure whether I can trust the focal length
    scale on the lens, because I found something strange: it seems like
    the short focal length of the 70-200 (supposed to be 70) is similar
    to the long focal length of my EF 24-85/3.5-4.5 (supposed to be 85).
    They give the same picture when the camera is mounted on a tripod.

    I compared the picture I get with the 24-85 at 50mm to my EF 50/1.8
    and they're the same, so it looks like the 24-85 is "ok".

    Did anybody notice this issue? Can it be true? How can I really know
    what the 85 and 100 mm focal lengths will look like?
     
  2. One thing I found out when I did something similar to what you are trying to do is that the focal length varies quite a bit depending on how far the lense focus is set. I tested the field of view of my 24-135mm tameron at 135mm to my 70-200mm at 135mm at around 6 ft and the difference in FOV was huge! When others on this forum told me to try it from farther away (around 20 ft) the FOV was much closer between the two lenses.

    So I guess, to answer your question, it varies for each lense depending on subject distance.
     
  3. Kenneth, this is strange. I don't remember the FOV changing when changing the focus, but I'll check it. Anyway, I tested it around 6 ft too, which is about the maximum distance I'm going to use indoors.
     
  4. IF/RF lenses work by reducing the focal length (by moving components relative to one another) in order to focus, rather than by moving the lens away from the film plane. Marked focal lengths are often not precise, and if you want to compare them for different lenses, do it with the lens focused at infinity. On the whole, I suspect that prime lenses are closer to their advertised focal length than zoom lenses. If you have a particular application such as portrait photography in mind, obviously you will also want to make comparisons at the likely working distance. But provided you have a reasonable amount of flexibility over camera-sitter distance, the difference between 85mm and 100mm is unlikely to be critical, and either will be excellent for head-and-shoulders to upper-body coverage on full-frame (as would be the 100/2.8USM macro), so you can't really go wrong. For tightly framed shots of faces, 135mm may be a better focal length, and if you have that in mind as well, the 50-85-135 pattern is probably slightly preferable to 50-100-135 for general use.
     
  5. I don't know if this helps but photodo seems to suggest that the actual focal lengths of the 24-85 and 70-200 are 71-190 and 25-82 respectively. You lose a bit at each end but I doubt that would be the sole cause of your problem.
     
  6. The focal length indicated on a lens is its focal length when focused at infinity. As you focus a lens on closer objects, the lens moves further away from the film plane. As the lens moves further away, its focal length increases, in accordance with the distance. So, if you have a 100mm lens focused at infinity, and move it 5mm further from the film plane, you now have a 105mm lens, and the increased magnification of a 105mm lens.
     
  7. Nearly right, Peter. A conventional linear-focusing (that is, not IF/RF) lens does NOT change its focal length as you focus it out. What does change is the angle of view.
     
  8. I think we've got an issue here of semantics. The actual focal length of any lens is of course always fixed. And it's always related to infinity focus. The point I was trying to make is that if you take a 100mm lens and move it 5mm forward of its position when focused at infinity, the magnification of objects at infinity will be the same as that of a 105mm focal length lens. The difference will be that the 105mm lens will be focused, and the 100mm lens will not.
     
  9. Er.. Rookie question here: Are the terms "infinity focus" and "focused at hyperfocal distance" the same? If yes, would a lens calculator be the only sure shot way of determining whether or not inifinity focus is achieved on the 70-200F4L specifically ?

    TIA

    - Harman
     
  10. If you want to measure focal length you should read this article
    Measuring focal length isn't trivial but it doesn't need an optics lab.
    Many internal focus zoom lenses show huge focal length changes when close focused. Some 28-00 type zooms can turn into 150mm lenses when close focused at 300mm.
    The marked focal lengths should be good to +/- 5% with the lens focused at infinity.
     
  11. Well, I tested the lenses at home and here are the results: indeed, the FOV changes as the focus changes. The change isn't consistent between lenses, though.

    On the 24-85, when the focus changes from infinity to closer distance, the FOV increases (the "effective" focal range decreases), while on the 70-200, when the focus changes from infinity to closer distance, the FOV decreases (the "effective" focal range increases).

    This can explain why when focused at around 6 ft, the 85 decreases and the 70 increases, so they seem the same.

    Thanks everyone for your answers!
     
  12. Hi Ido, are you from Israel? If so, you are welcome to try my 85/1.8.


    Happy shooting,
    Yakim.
     

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