A&P Question about the effect of focal length on depth of field.

Discussion in 'Lifestyle' started by hootpie, Jun 22, 2009.

  1. hootpie

    hootpie New Member

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    If I'm sitting at f2.8 and 17mm, will the focal plane be wider than f2.8 at 50mm or does it have more to do with the distance between you and the subject?

    I'm trying to get pictures where a depth of let's say 3' deep (2 rows of people) is in focus at f2.8 with nice background blur. I seem to be getting a very narrow focal plane and I can't figure out if it's my zoom level or the distance between me and the subject(s).
     
  2. eighteen_psi

    eighteen_psi Active Member

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    The wider you shoot, the greater your depth of field.
    The further you are from your subject, the greater your depth of field.
    The smaller the aperture, the greater your depth of field.
     
  3. NOR*CAL

    NOR*CAL OT Supporter

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  4. vwpilot

    vwpilot New Member

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  5. hootpie

    hootpie New Member

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    Ok so based on the sample numbers I put in (10' from subject, 50' from subject, comparing 17mm and 50mm focal lengths and combinations of f2.8, f5.6, and f11) I've come to this conclusion.

    If your subject is close to you, the farther you are zoomed (e.g. 50mm) in the narrower your focal plane (less of the subject will be focused) and greater your depth of field. In order to maintain a usable focal plane you must stay at a wider focal length (e.g. 17mm).

    If your subject is far from you, your focal plane will be relatively large (more of the subject will be in focus) at 17mm and 50mm with f-stops 2.8-11.

    So if I want to take a picture of some people and have a nice background bokeh, but still have all of them in focus, I should be far away and zoomed in on them, right?
     
  6. NOR*CAL

    NOR*CAL OT Supporter

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    Right. Try out an 85mm prime, and since you're on a crop camera... make sure you have a ton of room to back up :hs:.
     
  7. hootpie

    hootpie New Member

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    Yeah 85mm on a crop body is pretty far in. How many feet back would you say I'd need to be?
     
  8. vwpilot

    vwpilot New Member

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    use the calculator I posted. How much DOF do you need?

    If you know you need 3 ft, you know your focal length, plug them in the calculator and it will tell you how far you need to be to get that 3 feet.
     
  9. tetsuo

    tetsuo And shepherds we shall be...

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    i love my 85 on my crop body, much more so than i did my 50. :dunno: but i primarily use it for portraits or stage shots of my daughter's dance competitions/recitals.
     
  10. MawcDrums

    MawcDrums Active Member

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    :werd: I rented the 85 f/1.2 L and i was like HOLY CRAP at how far away I had to be..
     
  11. ssabripo

    ssabripo Banned

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    :bigthumb:

    makes it so much easier...thanks brosephs!
     
  12. hootpie

    hootpie New Member

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    Where depth of field = amount of "things" in focus, right?

    If I have this correct, being far from your subject and zoomed in on them while having them far away from what's behind them is the best way to get nice bokeh.
     
  13. eighteen_psi

    eighteen_psi Active Member

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    Well 'nice' bokeh comes from the quality of your lens, mostly shape and number of aperture blades. But yeah, DOF is the range of stuff in focus.

    You're reasonably correct in any case, if you shoot at a very long focal length, with the subject being far-separated from their background you'll throw the background way out of focus. More so if you shoot at a large aperture.

    On the flip side, shooting a very wide angle lens stopped down a little bit requires no focusing at all in many situations because your depth of field is from a few inches off the lens to somewhere near pluto.
     
  14. hootpie

    hootpie New Member

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    When you say wide angle stopped down you mean a higher f-value (e.g. f11, f22, etc.) or a lower f-value (f2.8, f3.2, etc.)?
     
  15. mojito

    mojito New Member

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    ^
    higher numbered

    also take into account the actual size of the lens. a 70-200mm at 70mm will have less DOF than a 24-70 at 70mm.
     
  16. tetsuo

    tetsuo And shepherds we shall be...

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    really? so if i took a 24-70 2.8 and a 70-200 2.8 and shot them both at 70 2.8, the dof will be less on the 70-200 picture? that doesn't seem logical. :o
     
  17. mojito

    mojito New Member

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    yup.
     

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