DOF, focus point, the sweet spot??

Discussion in 'Photography Beginners' Forum' started by itsajeepthing, Dec 10, 2008.

  1. itsajeepthing

    itsajeepthing TPF Noob!

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    Good morning fellow photography junkies! :)

    I've been doing some reading, and I do feel that I am making some headway and starting to develop a better understanding of photography basics. I need some help wrapping my head around a few things though:

    1. First, I have a landscape scene that I want to shoot with a very deep DOF, so I'm going to stop down my lens as far as it will go. Where should my focus point be? Should I a) "aim" the focus point for 1/3 into the shot, or b) manually set the focus ring to "infinity" and ignore the viewfinder, or c) manually set the focus ring to lets say 2 feet and ignore the viewfinder? Which is the correct approach for maximum DOF in the shot? Since the viewfinder is only showing you what the shot looks like wide open, is it just a matter of practice and experience to get this right?

    2. I understand that there is a "sweet spot" for sharpness and clarity (in terms of arpeture size). Is this a known value that can be determined by the lens you are using, or this something you have to find on your own for each lens you use? Does changing the focal length of your lens change this value? It seems to me that you would want to keep your arpeture as close to this value as possible for sharp pictures, and then compromise as needed when DOF control is a major element of the shot you are taking. Am I on the right track here, or completely confused?

    thanks for your input!
     
  2. Steph

    Steph No longer a newbie, moving up!

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    Google 'hyperfocal distance' or 'hyperfocal focussing'. That should help.
     
  3. ksmattfish

    ksmattfish Now 100% DC - not as cool as I once was, but still

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    It would be better to check out a DOF calculator, and use a larger aperture if you can get away with it. I find that with my APS-C format cameras I can get 4ish' to infinity with a 17mm focal length lens set at f/5.6 or f/8. The smaller the format/pixel size the sooner aperture diffraction becomes an issue.

    http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm

    http://www.dofmaster.com/dofjs.html

    As posted above, you are probably looking for your hyperfocal distance. Once again consult a DOF calculator if your lenses don't have a DOF scale.

    This is old photo-geek lore from before the days of computer aided lens design when people were grinding lenses by hand. If you are using any half-way decent quality, modern lens and the whole range isn't pretty darn "sweet" then return it. Of course people still claim to be able to find sweet spots for lenses, but you would never see the difference in large prints. If you do see a difference (in real world prints, not at outrageous amounts of magnification), your lens is a lemon.
     
  4. sabbath999

    sabbath999 No longer a newbie, moving up!

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    I just wanted to emphasize how important it is to understand this concept, because ksmattfish is dead on the money.

    Diffraction is a real consideration, most especially with 1.5 or 1.6 crop sensors... and even more with the 2/3rds system.

    The reason I bring this up is that I shoot a lot of macro, and the "impulse" is to crank that aperture as small as you can to get max depth of field, but unless you watch it your image quality significantly starts to degrade. Macro shooting with small f/stops on a cropped sensor REQUIRES pristine glass.
     
  5. Turnerea

    Turnerea TPF Noob!

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    Thanks for those links- easy to read through... I think I understand the messages of the difraction limited case. However I'm a little confused at why it occurs so quickly- for example I plugged in the numbers for my camera, leaving the other default values of print size etc, and it said f/13 was the smallest aperture I should be using. That corresponds to a hyperfocal disatnce of ~34ft, so my DOF is limited to 17->inf.

    If I stop down further than this, am I just trading off image quality for DOF then?
     
  6. davebmck

    davebmck TPF Noob!

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    The OP is looking for the hyperfocal distance or point. One way to find this is to set your camera on manual focus, set the focus ring to infinity and then press your DOF preview button. The closest point in focus is the hyperfocal point. You can then focus your camera on that point and you will achieve the maximum DOF for that setting.

    If you don't have a DOF preview, you can take a picture with the camera focused at infinity and see if you can determine the closed point in focus by looking at the LCD.

    Note that this works for a given aperature and focal length setting. You will get a different hyperfocal distance with different settings.
     
  7. dxqcanada

    dxqcanada Been spending a lot of time on here!

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    A Film Camera shooter will tell you to look at your DOF markings on the lens.

    Most newer lenses do not have these markings anymore.

    Generally you will have a shallower DOF at closer distances ... therefore you should focus about 1/3 the distance into the shot.
    It is easy if you have a lens with the markings.
    http://www.kenrockwell.com/nikon/images1/28-ai-s/DSC_5128-28-ais-950.jpg

    The colored lines represent the DOF range corresponding to the aperture.
     

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