When is 70mm/f5 not 70mm/f5?

A photographer friend, Dave LaHaye recently shared these two pics with me and I’m hoping to find a reason for them.  When shooting portraits one day, using his Nikon D300, Dave took these two test shots–same exposures (1/250, f5), same focal length (70mm),  but different lenses (18-70mm and 70-300mm). How can two different lenses, set to the same focal length, deliver such different results? What the heck? Any revelations would be appreciated!

Best,
Troy

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3 thoughts on “When is 70mm/f5 not 70mm/f5?

  1. Focal lengths are measured at infinity. Once you focus at less than infinity the focal the rear nodal point moves away from the film/sensor plane and the magnification increases or decreases.

    http://en.wikipedia.org/wiki/Focal_length

    Also the focal lengths are approximate.

    For full disclosure these are my own opinions and do not reflect the opinions of my current employer, Casio America, Inc. where I am Director of Product Marketing or my former employer, Nikon Inc and do not constitute any official statement or fact presented by either company.

  2. What m1ckster is talking about is lens breathing.

    http://en.wikipedia.org/wiki/Breathing_%28lens%29

    I would suppose then these two lenses are breathing very, very differently.

    The D300’s 1.5x crop factor results in an effective focal length of 107mm and an angle of view of 19deg horizontal 13deg vertical and 23deg diagonal at infinity.

    It would be an interesting exercise to measure how many degrees of “breath” these lenses have at their extremes. I’ll do this on my Canon gear and send you the results.

    Saying that I wondered if someone had tested this before.. Check out bullet item #11: http://mansurovs.com/nikon-28-300mm-vr-review

    “Similar to the Nikon 18-200mm f/3.5-5.6G and Nikon 70-200mm f/2.8G VR II, the lens does suffer from a “focus breathing” problem. Basically, in order to keep the minimum focus distance shorter, Nikon made a few adjustments to the lens design, which resulted in shorter effective focal lengths when shooting close objects. If your subject is very close at minimum distance, the 300mm on the Nikon 28-300mm will be equivalent to around 135mm, which is more than twice less. ”

    Here’s another test of the 18-200 specifically:

    http://www.cameralabs.com/forum/viewtopic.php?p=185752&sid=2675ce41c8c69356efaa9dd558d8ba45

    “From some rough calculations that could probably be wrong, I made the focal length of this closely focussed image to be just over 105mm, assuming that the lens is a true 200mm when focussed at infinity.

    I think that this is actually a significant reduction! People buy an 18-200mm when actually it can behave like an 18-105mm if your style of photography means using close focussing distances.”

  3. Here’s my comparison between Canon 70-200mm 2.8L ii and 24-70mm f2.8L at both a close focusing distance and infinity.

    There’s some noticeable breathing at the near distance but it doesn’t seem as extreme as the nikon lenses above.

    I forgot to note on the images they were shot on a 5Dii (Full Frame).

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