Depth of Field question (pic included)

Discussion in 'Photography Beginners' Forum' started by DScience, Apr 25, 2009.

  1. DScience

    DScience No longer a newbie, moving up!

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    I am soon going to get the Nikon 35mm f/1.8G AF-S DX Wide Angle Auto Focus, and I want to get your opinions on how different the DOF would be with this lens compared to the one I used to take the following pic. The lens that I took this pic with is the Nikon 18-55mm lens in my sig. What i'm curious is how much more blurry the out of focus stuff will be compared to this pic. Thanks for your input.

    [​IMG]
     
  2. Sarah23

    Sarah23 TPF Noob!

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    a 1.8 will have a quite a bit more bokeh. Not only do you need to take into account your aperture, but your focal length and distance between you and your subject and your subject and the background will all be factors.
     
  3. DScience

    DScience No longer a newbie, moving up!

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    Thank you. I have been doing a lot of research, and reading the book 'Understanding Exposure'. Can I convince you to elaborate more on the second part of your statement, in regards to the focal length and distance between me and the subject and these affects on 'bokeh'. I would really appreciate that.
     
  4. Sarah23

    Sarah23 TPF Noob!

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    Basically, the closer you are to your subject, the more shalow depth of field you are going to get.

    Say you are shooting at 70mm at 2.8. If you are farther back, you are going to have a but more in focus, then if you were just doing a headshot, where your focus will start to fall off quicker.

    As for the background, the farther away something is from your subject, the more blurry it will be at a given aperture.

    Let me see if I can dig up some examples....
     
  5. ANDS!

    ANDS! No longer a newbie, moving up!

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    "BOkeh" is not automatic. Bokeh refers to how out of focus light is rendered. So if there are no reflective surfaces or no stray light bouncing around your shot, you will be "bokeh"-less no matter how small your aperture is.

    Subject Distance/Focal length is related to depth of field. The longer your focal length, and the longer the distance between your subject and the background, will determine how out of focus the rest of your scene is. This is why you can use a telephoto to achieve some pretty impressive defocusing in your scene. With that 35MM you'd probably have to be slightly "close" to your subject to throw the background out of focus, or move your subject farther away from the background. There is no exhaustive chart covering every focal length, subject to background distance and aperture to let you know where to position each. That would certainly be an interesting website, but man alive would it be tedious (and expensive).
     
  6. Sarah23

    Sarah23 TPF Noob!

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    OK, example...

    A shot at 3.2, farther away, there is a lot of her in focus...probably all of her. Even a little bit behind her is sitll in focus, and then everything else is blurred...

    [​IMG]

    Another shot, still at 3.2, and a focal length that is pretty close to the one above. See how the depth of field is more shallow? Even her far eye is getting to be soft. Its because I am closer to her, rather then pulled back, and at the same aperture.

    [​IMG]

    There is an equation for all of this...but I have no idea what it is. There are DOF calculators on the web, where you input your aperture and focal length and distance, and it will give you an idea of how shallow your depth of field will be (I think they use centimeters?)
     
  7. Dwig

    Dwig TPF Noob!

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    BTW, this lens is by no stretch of the imagination a wide angle lens. It is in fact a slightly long normal. It is a DX lens so arguing that a 35mm focal length would be wide angle on 35mm film / FX (actually its barely one, if one at all) is irrevalent. On the DX format, wide angle doesn't beging untill you get shorter than 24mm.

    Also, the first post that mentions "bokeh" uses the term incorrectly. Bokeh, in photographic jargon using the spelling that includes the ending "h", refers not to the presence of out of focus blur but, instead, to the quality of that blur. Bokeh is either good or bad, it is not present or absent. When speaking of the presence or absence of blur you use the term "blur". Only when you are refering to whether the blur has pleasing qualities or not do you use the term "bokeh".

    Back to the OP's original question: it can't be answered with the information presented. You fail to state the focal length used (somewhere between 18 and 55, but where?) and what f/stop was used. The picture posted has had the EXIF data stripped so we can't retrieve the info there.
     
  8. DScience

    DScience No longer a newbie, moving up!

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    Thank you everyone, here is more info.

    It was at focal length 82mm, f5.6..It would not let me decrease the f stop anymore at that focal length. I was shooting in manual.
     
  9. Samanax

    Samanax TPF Noob!

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  10. Dwig

    Dwig TPF Noob!

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    You took the picture at 55mm not 82mm. You've misread the EXIF data. An 18-55mm lens can't be used at 82mm, period. You've appearently read the "35mm equivalent focal lenght" field instead of the actual focal length field.

    The shorter focal length of the 35mm would, in general, yield more DOF at any given f/stop, but the substantially wider f/1.8 aperture available would, if used would yield very shallow DOF, less than the results from a 55mm @ f/5.6.
     
  11. DScience

    DScience No longer a newbie, moving up!

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    LOL you are right! I definitely misread that
     
  12. DScience

    DScience No longer a newbie, moving up!

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