Talking about DOF using different focal lengths.

Discussion in 'Photography Beginners' Forum' started by prodigy2k7, Apr 24, 2008.

  1. prodigy2k7

    prodigy2k7 No longer a newbie, moving up!

    Joined:
    Apr 22, 2008
    Messages:
    1,665
    Likes Received:
    22
    Location:
    California, USA
    Can others edit my Photos:
    Photos OK to edit
    Okay I understand the other stuff like a wider aperature (lets say F1.4) lets in more light and the DoF is smaller therefore more will be out of focus especially the background...

    Also that the closer you are to the subject there more the background will be blurred.

    But what about when it comes to focal length?

    It sounds like a 100mm will have a deeper blur compared to a 50mm if the aperature is the same.

    Hmm what does the number mean anyway?
    Does F/1.4 let in more like on a certain lens than any other lens with F/1.4? such as different brands n models n such?

    Thanks!
     
  2. Garbz

    Garbz No longer a newbie, moving up!

    Joined:
    Oct 26, 2003
    Messages:
    9,713
    Likes Received:
    203
    Location:
    Brisbane, Australia
    Can others edit my Photos:
    Photos NOT OK to edit
    You're missing the relationship. The relation ship is between the perspective of the subject and the focal length.

    If I photography a street sign from 1m away with a wide angle lens at f/2.8 the supposed church in at the end of the street may be identifiable, but very small.

    If I do the same thing from 20m away with a telephoto lens the background's blur retains the same relationship because of the aperture and the distance to the subject, but because of the focal length of the lens the background subject appears much bigger, and thus blurrier because of this relationship.

    That was a very simplified explanation. But unfortunately I can't find any resource on the net which explains it as well as in one of my books.
     
  3. JerryPH

    JerryPH No longer a newbie, moving up!

    Joined:
    Oct 14, 2007
    Messages:
    6,111
    Likes Received:
    15
    Location:
    Montreal, QC, Canada
    Can others edit my Photos:
    Photos NOT OK to edit
    It is possible to blur the background with a huge aperture (let's say F/1.4 at 30mm), and have the camera very close to the subject (about 1-2 feet, and blur starts in a very shot distance... like centimeters)...
    [​IMG]

    ... or you can have a higher focal length (200mm), and smaller aperture (bigger numerically like perhaps F/7), and still get a blurred background...
    [​IMG]


    At F/1.4 it is the same amount of light at all focal lengths.

    I think what you are confusing is light sensitivity and DOF.

    Yes, a lens that is faster has a shallower DOF, but bokeh (that background blur) is all about ratios.

    To get a blurred background, your distance from the camera to the subject has to be closer than the distance from the subject to what is behind it.

    You can get closer to the subject by walking up to them, or zooming in on them with a telephoto. You then adjust how much blur you want by changing apertures. The smaller the aperture (numerically bigger), the less blur.

    It is all about the numbers and ratios!
     
  4. Village Idiot

    Village Idiot No longer a newbie, moving up!

    Joined:
    Mar 20, 2008
    Messages:
    7,274
    Likes Received:
    406
    Location:
    Shepherdsturd, WV / Almost, MD
    Can others edit my Photos:
    Photos NOT OK to edit
    Actually, DOF does change with focal length. A 50mm lens at f/4 will have less DOF then a 200mm lens at f/4. There's even DOF calculators online that will show you the DOF of certain focal lengths with certain apertures.
     
  5. prodigy2k7

    prodigy2k7 No longer a newbie, moving up!

    Joined:
    Apr 22, 2008
    Messages:
    1,665
    Likes Received:
    22
    Location:
    California, USA
    Can others edit my Photos:
    Photos OK to edit
    thanks everyone :)
     

Share This Page

Search tags for this page

dof different focal lengths