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Thread: confused I guess

  1. #1
    jconti3's Avatar
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    confused I guess

    How does this comment from a fellow photog make sense?

    "You were at f3.5 here, which wouldnt give much less bokeh than f2.8

    The reason it is all in focus is that you are only at 18mm."

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    jconti3's Avatar
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    Re: confused I guess

    confused I guess
    this is the picture in question.
    My 40yr old peer, Matt doing a kick flip.
    D90
    1/1600
    ISO500
    18-105kit
    0flash

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    ktuli's Avatar
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    Re: confused I guess

    I'm pretty sure based on the short context given here, it is regarding DoF with regards to focal length. Depth of Field and Focal Length are related. A shorter focal length has much deeper DoF than a longer focal length. Focal distance also comes into that triangle as well.

    So on an 18mm lens, changing from f/3.5 to f/2.8 isn't really going to change the DoF as noticeably as say going from f/3.5 to f/16. Or the DoF change on a 180mm lens going from f/3.5 to f/2.8.

    I think... Is there more context here?

    Remember that f/# distinctions is actually an equation (where f stands for focal length) that specifies the actual aperture opening. So if we ignore the focusing distance piece, and just look at focal length and aperture, we get:

    18mm / 3.5 = 5.142857143 mm aperture opening
    18mm / 2.8 = 6.428571429 mm
    Difference in aperture opening is only 1.285714286mm

    180mm / 3.5 = 51.42857143 mm
    180mm / 2.8 = 64.28571429 mm
    Difference in aperture opening is 12.85714286mm

    So the affect on DoF or bokeh is 10 times less noticeable at 18mm compared to 180mm.

    At least that is how I've always understood it.

    - Bill
    Last edited by ktuli; 5th May 2011 at 06:22 PM.

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    jconti3's Avatar
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    Re: confused I guess

    So, I missed my focal point then and that is why the house is in such clear focus? 18mm @ 3.5 should of produced more bokeh here , right? Or am I still missing something.

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    Moderator Donald's Avatar
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    Re: confused I guess

    I have to confess at glazing over when the discussion gets technical. Me - I just do whatever works. But I know enough to know (if you see what I mean) that Bill has got it right in terms of concept. At 18mm you're going to have a pretty huge depth-of-field and there isn't going to be any noticeable difference between f2.8 and f3.5.

    Rob also commented, today, on depth-of-field and his very clear comments, here, may help as well.

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    Moderator Donald's Avatar
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    Re: confused I guess

    Quote Originally Posted by jconti3 View Post
    So, I missed my focal point then and that is why the house is in such clear focus? 18mm @ 3.5 should of produced more bokeh here , right? Or am I still missing something.
    Sorry - that's the other point I meant to mention.

    Your quote in the original post is confusing. I think it is more easily understood if it is worded as something like, 'At f.2.8 you wouldn't get much more bokeh than you would at f3.5'. Does that make more sense?

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    ktuli's Avatar
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    Re: confused I guess

    Quote Originally Posted by jconti3 View Post
    So, I missed my focal point then and that is why the house is in such clear focus? 18mm @ 3.5 should of produced more bokeh here , right? Or am I still missing something.
    No - you'll see almost no noticeable difference in bokeh between f/2.8 or f/3.5 at 18mm because 18mm inherently has a very large DoF (little bokeh) in the first place and the difference to the aperture opening at that focal length is minimal to begin with.

    In this case, to get the same photo with much more bokeh (less DoF), you would have switch to a longer focal length (perhaps the full 105mm), which would cause you to have to move back to be able to frame the shot the same way.

    - Bill

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    jconti3's Avatar
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    Re: confused I guess

    it just clicked.....thanks, Bill.

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    Moderator Dave Humphries's Avatar
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    Re: confused I guess

    Quote Originally Posted by ktuli View Post
    No - you'll see almost no noticeable difference in bokeh between f/2.8 or f/3.5 at 18mm because 18mm inherently has a very large DoF (little bokeh) in the first place and the difference to the aperture opening at that focal length is minimal to begin with.

    In this case, to get the same photo with much more bokeh (less DoF), you would have switch to a longer focal length (perhaps the full 105mm), which would cause you to have to move back to be able to frame the shot the same way.

    - Bill
    True Bill.

    But, in doing so John, although your friend would be better separated in terms of bokeh from the home behind, but because you have changed the perspective in moving further away, the home in the background will be much larger in relation to the your friend than it appears now. That may still be preferable, just making the point that it will change.

    There's a good demo of the effect of changing perspective in the dog picture here

    Cheers,
    Last edited by Dave Humphries; 5th May 2011 at 08:11 PM.

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    You cannot have more bokeh or less bokeh!

    You can have better bokeh, smoother bokeh, worse bokeh, ragged bokeh or any number of descriptors of bokeh but, you cannot have more bokeh, greater bokeh or less bokeh.

    Bokeh is a Japanese term which qualifies (not quantifies) the subjective look of the out of focus areas of an image.

    Depth of field is the object measurement of the distance in front and to the rear of the point focused on which is still in acceptable focus.

    Depth of field is quantifiable and is determined by focal length, f/stop, distance focused on and by the circle of confusion of the image you are working with.

    A very basic and simplistic explanation of circle of confusion is that since smaller images have to be enlarged more than larger images, they need to be sharper.

    Bokeh is so frequently misused and than it will probably eventually become accepted as another term for depth of field. However, I don't think that we are yet at that point in the development of photographic jargon.

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    Moderator Dave Humphries's Avatar
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    Re: You cannot have more bokeh or less bokeh!

    Quote Originally Posted by rpcrowe View Post
    You can have better bokeh, smoother bokeh, worse bokeh, ragged bokeh or any number of descriptors of bokeh but, you cannot have more bokeh, greater bokeh or less bokeh.
    True again Richard,

    I think we know what was meant by phrases like "more bokeh" (i.e. = less depth of field), but you are right to correct us.

    Normally I spot that kind of thing, but I missed it this time (in fact I even joined in)

    Oh, the shame ....

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    ktuli's Avatar
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    Re: You cannot have more bokeh or less bokeh!

    Yeah - I know the difference too, and you're right... I should not have continued to muddle the distinction like I did in my answer. I was trying to still keep it in the terms that John had presented his question, but perhaps that was not necessarily correct to do in this case.

    Sorry.

    - Bill

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    Re: You cannot have more bokeh or less bokeh!

    thank you all! this place is awesome!

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