Does a prime give shallower DoF?

Discussion in 'Digital Photography' started by bananabar, Nov 15, 2008.

  1. bananabar macrumors regular

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    #1
    Would a 50mm prime at 2.8 give a shallower DoF than an 18-70mm set at 50mm at 2.8?

    If so, why??
     
  2. FrankieTDouglas macrumors 65816

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    #2
    Nope, although the quality of the image may be better in one or the other. That just depends on the glass.
     
  3. bananabar thread starter macrumors regular

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    #3
    That's what I thought. This photo was taken with exactly the same camera as I have (a D80) but with a 50mm. I could never hope to get this DoF with my 18-70. Does going from 2.8 to 1.8 really make that difference.

    For £80, I'm going to get one!
     
  4. svndmvn Guest

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    #4
    Yes, you won't regret it, that's 4 stops of difference,right? It is also supposed to be faster at focusing
     
  5. cube macrumors G5

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    #5
    Thats 1.33... stops of difference.
     
  6. bananabar thread starter macrumors regular

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    #6
    Does anyone have any example of, or know of where I can find examples of, the Nikkor 50mm f1.8 D AF?
     
  7. PCMacUser macrumors 68000

    PCMacUser

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    #7
  8. OreoCookie macrumors 68030

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    #8
    I doubt you have an 18-70 with an initial aperture of 2.8, Nikon's 18-70 has an initial aperture of around 5 at that focal length and 2.8 gives you a significantly shallower dof.
     
  9. svndmvn Guest

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    #9
    sorry,bad choice of terms,I meant there's 2.0 2.2 and 2.5 in between,therefore there should be a nice difference..
     
  10. compuwar macrumors 601

    compuwar

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    #10
    From Wikipedia:

     
  11. lucero1148 macrumors member

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    #11
    Full Stops are 2.0/2.8/4.0/5.6 etc so anything in between 2.0 to 2.8 are incremental to a full stop so I believe you're right the 1s time that it's a 1.33 stop difference from 1.8 to 2.8.

    Also a prime lens wide open normally will be optically superior to a zoom lens at the same focal length and f stop. This is especially true when it comes to chromatic aberrations and flare. I have a 85 F1.2 L lense and that is just an incredible piece of glass. Very sharp wide open and has superior contrast even when shooting backlighted subjects.
     
  12. bananabar thread starter macrumors regular

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    #12
    Thanks for posting copy from Wikipedia that doesn't answer the question asked.
     
  13. Abstract macrumors Penryn

    Abstract

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    #13
    Depth of field only depends on aperture, focal length, and the distance between you and the subject...... as stated in compuwar's post.

    Besides, no point being a smartass about it. Why the hell should we bother answering your question if you're a complete cock about it? :confused:
     
  14. seenew macrumors 68000

    seenew

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    #14
    It does answer your question. :| Don't be a jerk.
     
  15. ChrisA macrumors G4

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    #15
    If a 50mm lens is focused to 10 feet for DOF at f/2.8 is 1.29 feet. If you open the lens to f/1.4 the DOF goes to .65 feet.

    at 5 feet the DOFs are .16 feet (just under 2 inches) and .32

    Use this handy DOF calculator
    http://www.dofmaster.com/dofjs.html

    What? It answes the question is exact detail and is worded clearly and an addition provides enough information so you know why the answer is what it is.

    What you should have said was "I can't understand the Wiki's answer. Could you say it using smaller words?"
     
  16. pdxflint macrumors 68020

    pdxflint

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    #16
    Sorry to be redundant, since so many already responded to your post... but you should be appreciative that someone took the time to actually do the research you could have done, and then selected the appropriate text and quoted it here for you. If you don't see it that way, then next time you ask a question don't expect anyone to go out of their way, especially since you have access to the same information we all do. It's called courtesy. :cool:

    BTW: you can just be lazy and ask all these basic questions here, but keep in mind that lots of folks can give you misinformation without really knowing it. Quoting a solid (although sometimes unconfirmed) source like wikipedia is helpful for those who don't know where to look. There are tons of other places to research photographic technical questions. Start with Google, go from there.

    From my understanding, same f/stop at same focal length = same depth of field at same distance. Few, if any, non-pro zooms will give you 50mm at f/2.8 anyway, and the Nikon 18-70 won't come close.

    If you really want the very shallow depth of field and better performance wide open, primes lenses are really your only choice.
     
  17. FrankieTDouglas macrumors 65816

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    #17
    I thought that, too. But this past summer, I was having a discussion with someone. He said there are only two ways to control depth of field: aperture and distance. He explained that focal lengths do not alter the depth of field, they merely shrink it and push it further into the background (wide angles... giving the illusion of sharpness) or they enlarge and pull the background closer in (telephoto, zooming in to show the out of focus elements). He then proceeded to show three images, each shot at wide, normal, and telephoto. The background of the image, when zoomed in to equal viewing perspectives, had the same focal quality.

    Just ponder it. See if you agree or disagree. I'm thinking that a perceived change in dof could still be considered a change, even if it technically is not. But maybe not.
     
  18. mcnicks macrumors regular

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    #18
    Frankie, I suspect that the differences you saw in those photographs were down to psychology. Its common to say that telephoto lenses "compress" the perspective, bringing background and foreground closer together but in reality nothing of the sort happens. Its just an apparent by-product of the narrow field of view.
     
  19. compuwar macrumors 601

    compuwar

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    #19
    He's wrong- If you change distance so the subject occupies the same area, then you'll get nearly equivalent DoF, but there is a slight change (not normally big enough to be noticeable, but it's there so it's part of the equation.[1]) Longer telephotos distribute the DoF further forward, not shrink it (or it'd be affecting the DoF more.) However, the magnification of a longer lens will give a narrower DoF if you shoot from the same spot, as will a larger sensor from the same spot.

    If I shoot with my 400mm lens at a subject 100m away with an aperture of f/2.8 on a 1.5x crop factor body, I get 7.68M of total DoF from 96.307m to 103.988m. The same shot, from the same spot with a 35mm lens puts everything from 16.953m out in focus. However, if I take my 35mm lens in to 8m, then I get 7.633m of total DoF and roughly the same sized subject in the viewfinder.


    [1] It's a shallower change than you're likely to be able to see in a print, but focal length is part of the formulas:
    http://www.dofmaster.com/equations.html
     
  20. taylorwilsdon macrumors 68000

    taylorwilsdon

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    #20
    The point here is that the 18-70 can't shoot f/2.8, its at like f/4.5 wide open at 50mm so he's just making things up.

    If you take both lenses, have them at 50mm and shoot at the same aperture (for real) then they will have the same depth of field. That means the nifty would be at f/4.5 or f/5 or whatever the 18-70mm offers.
     

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