Canon EOS "Depth of field preview"

Discussion in 'Digital Photography' started by shady825, Dec 31, 2008.

  1. shady825 macrumors 68000

    shady825

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    #1
    Could someone please tell me what this does? (depth of field preview)

    I read in the manual that it says you can hold the button down and change the aperture and you will see the viewfinder darken or lighten. I see that and I understand that. I just dont know what this is "telling" me.

    So I guess my question is, what is the view finder showing me when its going darker or lighter. (if that makes sense)

    Thanks in advance!
     
  2. luminosity macrumors 65816

    luminosity

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    #2
    Look at the sharpness of the entire image and how it changes when you use the preview button.
     
  3. compuwar macrumors 601

    compuwar

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    #3
    Instead of showing you the scene at the largest aperture, it's showing you the scene stopped down so you can see the effect of stopping down (and therefore the depth of field you'll attain.) Unfortunately, as you stop down the viewfinder tends to not be bright enough to fully evaluate what's in focus. Since you can always chimp with a digital camera, DoF preview is pretty much an anachronism.
     
  4. hhlee macrumors 6502

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    #4
    i always go by the aperture setting on the lens rather than the dof viewer. the larger the number, the more of the scene is in focus.
     
  5. AlaskaMoose macrumors 65816

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    #5
    A lot of cameras, including film cameras, have such a preview button. To understand what it does, set four of five bright objects in a row at an angle to the lens. For example, set the lens to f/2.8 or so, stand five or more AA battery cells, one behind another on a straight line, focus the lens manually on one battery near the front, and press the preview button. If you do his properly, you will see some of the battery images blur at the front, as well as the background.

    I use this feature quite often when using telephotos at wide apertures for close-ups of flowers and such, since I want as much as the flower's body to be in focus, while the background is blurry. Once you have figured the right aperture of the lens for the task, then you will rarely need to use the preview button for that lens and type of photography. If it's dark out, then this feature is not very useful.
     
  6. shady825 thread starter macrumors 68000

    shady825

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  7. compuwar macrumors 601

    compuwar

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    #7
    That doesn't tell you *where* the focus is- without a DoF calculator you can't control what's in focus and what isn't, the DoF preview, doing calculations or chimping on the LCD will let you know where your focus starts and ends. "More in focus" or "less in focus" isn't nearly as useful as "I want those particular elements of the picture in focus."
     
  8. wheelhot macrumors 68000

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    #8
    Well here is how I remember what does the DOF do and how it helps me: I just remember, (DoF button pressed) The darker the viewfinder, the clearer the image (as in no blur at the sides), but since its darker, the shutter speed will get slower. Hope this helps :rolleyes:
     
  9. Scarlet Fever macrumors 68040

    Scarlet Fever

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    #9
    The best way to see what the button does is to point it at a window with a thin curtain on it and an object in the background. Put the camera into Av mode, focus on the object in the background, hold the DoF button down, and bump the aperture up. You'll notice the curtain becomes sharper as you step the aperture up.
     
  10. shady825 thread starter macrumors 68000

    shady825

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    #10
    Oh wow. I miss understood then.. I thought the brighter the view finder, the clearer the image will be. Ok, I was backwards.. Thank you!!
     
  11. wheelhot macrumors 68000

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    #11
    No problem. I had the same problem when I was starting to learn about photography especially understanding the relation between shutter speed and aperture.

    Anyway I wonder, what is the use of DoF preview button in real life usage? Pressing it will only make you see the viewfinder darker, how does this help in getting a good picture?
     
  12. chrono1081 macrumors 604

    chrono1081

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    #12
    When you look through the viewfinder, you will always see the scene with the widest aperature the lens has available. For example:

    If you have an f2.8lens, when you look through the viewfinder you see f2.8. Now, if you change the aperature to say, f8, and look through the viewfinder again, you still see everything in the viewfinder at f2.8 until you press the depth of field preview button, then the aperature closes, showing you what the scene will look like at f8. (Also making your viewfinder darker in the process since the aperature is letting less light in.)

    When you press the shutter, whatever the aperature is set at is what the lens stops down to to take the picture. Hope this clarifies things :)
     
  13. anubis macrumors 6502a

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    #13
    Another way to think about it is that closing the iris blocks the unfocused light. So you're left with less light, but the light that remains is the focused light from the various parts of the field.
     
  14. shady825 thread starter macrumors 68000

    shady825

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    #14
    Ok I think I get it now! Thanks everyone!
     
  15. pprior macrumors 65816

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    #15
    That last explanation is the correct one - you have to look closely and unless you have a fast lens 2.8 aperture or faster you might not see much difference in most shots because most of the frame will be in focus.

    One caveat is that with the standard EOS focus screen, you cannot see a difference below F2.8, so if you own fast primes like the 85L/1.2 you won't see any difference below a 2.8 aperture (i.e. 1.2 will look the same as 2.8). To solve that problem you need a different focus screen, which also makes manual focus with such wide open apertures a lot easier.
     

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