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Long Focal Lengths and "Isolation"

jcgoodsonjcgoodson Member
edited March 2013 in general photography
In a number of Neil's posts, he says he uses a 70-200mm lens to "isolate" the subject. I'm wondering what exactly does that mean? I don't have a 70-200mm lens, but it seems to me that the same isolation/composition could be achieved at a focal length of 200mm or by a smaller focal length and a decreased distance from lens to subject. But, I suspect I'm missing something. Thanks in advance to any clarification others can provide.

Justin

Comments

  • Yes a 200mm lens will do the same thing as a 70-200 in regard to using both at 200mm.

    So the idea is to be at the longest focal length possible and still get your subject framed how you want.

    This is in regard to portraits to "isolate the subject and control the background"

    100mm works better than 70mm, 200mm works better than 100mm, 400mm works better than 200mm, etc. but then there's the limitation in where you can physically stand distance wise. Neil uses the 70-200 because of that physicall limitation of where you can stand. Backing up into the street to use 200mm might be dangerous where it's a bit safer stand in the sidewalk at maybe 100mm. But 200mm would still be preferred.
    It's also just a great lens and cheaper and lighter than just a 200mm lens.

    So the two primary things that happen.
    1. Narrower field of view
    2. More blur to the background.


    1. Field of view. The difference between lenses of wide angle and telephoto is the field of view. A 24mm lens might have a field of view of 80 degrees where a 200mm might have a field of view of 15mm. Estimate.. Zoom or fixed lens makes no difference.

    See : http://en.wikipedia.org/wiki/Angle_of_view

    So with the subject the same size in the frame "you have to change your distance from the subject" testing a 24mm lens vs say a 200mm lens the subject the 24mm lens would show more background.

    So same size subject less background.

    2. Blur of the background.
    Blurring the background causes background element to compete less with your subject for attention. Blur is not the same as bokeh.

    Blur is out of focus elements of your photo. More blur is achieved by the "relative" distance between your point of focus and whatever object is in the background.
    Without going into detail, just the further your subject is away from the background the more blur or,out of focus it will be, which is what you want to isolate your subject from a competing background.

    So between the two steps here we now have less background in the frame and have also made it blurry making our subject the main focus of our photo.

    Additionally along with field of view rides the topic of perspective distortion.
    Est described by Neil in this blog post.

    http://neilvn.com/tangents/composition-for-full-length-portraits/?utm_source=rss&utm_medium=rss&utm_campaign=composition-for-full-length-portraits

    Helpful?
  • Thank you very much for the detailed explanation -- it was very helpful. I also benefited from the four pictures of the abstract yellow structure in this Wikipedia entry:

    http://en.wikipedia.org/wiki/Perspective_distortion_(photography)

    As you pointed out, even though the compositions are very similar, the angle of view achieved with different focal lengths can very much influence the photo.

    Justin
  • Neil vNNeil vN Administrator
    As Jeff implied, it isn't so much that it is specifically the 70-200 ..
    the compression and isolation is achieved with the longer focal lengths, as compared to a 24-70

    It's more about the range of (longer) focal lengths, than it being about a specific lens.
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