Where is infinite focus?

Discussion in 'Photography Beginners' Forum' started by Simonch, Oct 2, 2007.

  1. Simonch

    Simonch TPF Noob!

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    Hey folks, time for a stupid question!! i have a 75-300 lens for my digital rebel, its the cheapest lens that CAnon do, and doesnt have any markings on the focus ring, where about is the infinite focus? or does this lens even hav such a thing?!
     
  2. EOS_JD

    EOS_JD TPF Noob!

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    Check out the depth of field calculators. Longer lenses do do lend themselves well to focusing on infinity. You need to focus on a subject many metres away in order to get the far limit to infinity. Depth of field is determined by focal length, subject distance and aperture.

    At 300mm f22 you would need to focus on a subject 206m away and the near limit of focus would be 103.9 away with the far limit being infinity.

    Using the calculator will provide you with more info on what you need.
     
  3. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    Just focus on something very far away...that's your infinity focus.

    Or, put it into manual focus mode and turn the ring all the way until distant objects are in focus.
     
  4. Simonch

    Simonch TPF Noob!

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    sounds complicated!! Mike, the reason im asking, is that i was tryin some night stuff, and was struggling to get it in focus, so couldnt really see what i was shooting, to focus it! tough times! lol.... i shall google the DOF calculator, and see what i make of it! thanks folks!
     
  5. WDodd

    WDodd TPF Noob!

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    Hey I have that lens and used it for fireworks in which case I used a larger aperture like F/13 etc. and focused on something in the distance (I used distant illumination) its a bit tricky but it worked out fine.
     
  6. Garbz

    Garbz No longer a newbie, moving up!

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    The problem with setting it manually is that some lenses focus further than infinity. May sound stupid but if one of the lense elements moves because of a drop or something it can spare you a trip to the shop.

    This is something you should try to figure out during the day. If your lens doesn't have markings for it use a white pen or something to make a line between the focusing ring and a stationary part of the lens. So in future you can go there straight away.
     
  7. Simonch

    Simonch TPF Noob!

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    yeah sounds like a plan!! thanks a lot guys! much appreciated!
     
  8. Mike_E

    Mike_E No longer a newbie, moving up!

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    Argh, I can't stand it anymore. It's 'second star to the right and straight on till morning.

    I am truly sorry. LOLOLOLOL
     
  9. Chris of Arabia

    Chris of Arabia Herding cats since 1988... Supporting Member

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    Surely, that's 'to infinity and beyond'....
     
  10. six-five-two

    six-five-two TPF Noob!

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    Remember, if you are shooting landscapes, set it to auto focus to infinity. Also do this at night. I accidently used auto focus one time and all my photos were blurry (during a few night snapshots)
     
  11. Hertz van Rental

    Hertz van Rental TPF Noob!

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    This is not really the best way to focus for landscapes.
    The depth of field of a lens extends both before and behind the point you focus on. If you focus on infinity then approximately half of the available depth of field falls beyond infinity and is wasted.
    If you focus on the hyperfocal distance then the depth of field behind this point goes to infinity (or near enough) and the depth of field in front falls much nearer the camera. This optimises your focussing and gets far more of the subject in focus.
    The hyperfocal distance is defined as the distance from the lens to the nearest point of the subject which is just acceptably sharp when the camera is focussed on infinity.
    The hyperfocal distance varies with focal length and f-number so you have to work it out for each f-number and each lens. It's worth the effort.

    (focal length)squared/f-number x diameter of the circle of confusion = hyperfocal distance.

    The diameter of the circle of confusion varies according to negative size and expected degree of enlargement.
    A reasonable figure for the c of c can be calculated:
    0.25mm/degree of enlargement.
    A 10x8 print from a 35mm neg has a magnification of x8. So the c of c would be 0.03mm.
    A figure of 0.02mm or 0.015mm would probably be the maximum for general purposes. This calculation should also be adequate for digital.
     
  12. Helen B

    Helen B TPF Noob!

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    Not everyone agrees that setting the lens at the hyperfocal distance is always the best way of shooting landscapes. There are many cases when biasing the depth of field towards infinity, or simply focusing on infinity, may be preferable.

    Depth of field is all about setting the limits of acceptable blur. If you set the lens to the hyperfocal distance the horizon will not be in sharp focus, but it will be out of focus - in fact it will be just on the limit of acceptable blur. Theoretically the blur will be undetectable to a human at the print magnification and viewing distance that was used for the DoF calculation. At closer viewing distances or greater magnifications it will look out of focus. It may even seem a little fuzzy to some people at the designed magnification, particularly if there are sharper parts of the picture. In many cases the horizon is the most important part of the picture to have sharp.

    There is a good case for setting the lens to the hyperfocal distance for an aperture two stops wider than the one in use. This means that the horizon will not be at the limit of acceptable blur, but should be safely below the limit. The actual aperture can be used for determining the near limit of depth of field because it is generally less critical than having the horizon razor sharp. Every case is different and should be judged on the particular circumstances.

    " (focal length)squared/f-number x diameter of the circle of confusion = hyperfocal distance."

    Do you mind if I rewrite that?

    h = f^2 / (NC)

    f is the focal length
    N is the f-Number (ie 2.8, 5.6, 16 etc, not 1/2.8, 1/5.6, 1/16 etc)
    C is the diameter of the c-o-c.
    The units should be consistent -ie everything in millimetres, or everything in metres, or everything in inches.

    Best,
    Helen
     

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