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Thread: The illusion of depth of field

  1. #1
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    The illusion of depth of field

    Thank you for your excellent article on Understanding Depth of Field. This web site is awesome.

    I'm a little confused a by the statement that with constant magnification, depth of field is virtually constant regardless of focal length. My confusion stems from the fact that in order to keep the same magnification you have to change your distance from the subject. You are changing two variables simultaneously. It seems you could just as easily argue that with constant magnification, depth of field is virtually constant regardless of distance to subject. (Maybe that's obvious but I've never heard it stated that way before.)

    I found another couple of excellent articles (http://toothwalker.org/optics/dof.html and http://www.scottbideauphotography.co...s-compression/) that point out that moving the camera position changes the perspective and so comparing a photo that was taken at a distance of 1 m with a focal length of 20 mm to another photo that was taken at a distance of 5 m with a focal length of 100 mm is really comparing two different photos.

    This got me thinking. It seems the whole concept of depth of field is based on:

    • Limitations in our ability to discern minute differences in focus.
    • The size of the out of focus object (circle of confusion.) If the object is larger we can more easily discern its "sharpness" or "bluriness."


    If that's the case then the illusion of depth of field is impacted by two or three things: aperture, magnification, and resolving power. Magnification happens in a lot of ways:

    • Increasing the focal length
    • Moving the camera closer to what is being photographed
    • Enlarging an image taken by a cropped sensor to the same size as an image taken by a full frame sensor
    • Post-production cropping and enlarging
    • Moving your eyes closer to a print
    • Etc.


    Each of these ways to magnify an object may have a different level of impact, but a generalization might be that any time you enlarge/magnify something you can see more detail. Increasing focal length (magnifying) while simultaneously moving the camera further away (de-magnifying) sort of cancels each other out, doesn't it? And it's no wonder that taking a small portion of the background from one image and enlarging it to compare with another "un-enlarged" background will result in almost identical perceived depth of field. In the end, smaller things always appear sharper.

    Further, anything that impacts our ability to resolve minute differences in focus should have an impact. The resolution of the lens, the resolution of the sensor, the resolution of a monitor or printer, the resolution of our eyes -- these all should impact the illusion of depth of field, right?

    I'm sure I'm glossing over a ton of detail but I'm trying to understand the basic concepts that I can leverage to impact my photography. Among these should be:

    • Only one plane in the photo is truly in focus
    • Aperture impacts perceived depth of field
    • Magnification or enlargement impacts perceived depth of field.
    • Viewing distance impacts perceived depth of field.
    • Depth of field is almost never spaced equally in front of and behind the focal plane


    Am I understanding this correctly?

    Thanks again for your great article and this wonderful web site.
    Last edited by mfreesto; 13th October 2012 at 02:53 PM.

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    Re: The illusion of depth of field

    Quote Originally Posted by mfreesto View Post
    [*]Only one plane in the photo is truly in focus
    I didn't read either of the two articles that you mentioned and I don't pretend to understand the science behind the definition of what is and is not in focus. However, you mentioned that you're trying to become a better photographer. To that end, I would argue that depth of field is called that because there is a range of focus that the typical human eye can detect in a photograph. Otherwise, it would be called only the plane of focus to emphasize that nothing else is in focus.

    So, even if there is only one plane of focus by scientific definition (I don't know if that's true), that really doesn't matter. That's because the viewers of our photographs can't distinguish the difference between that plane and the planes that are only perceived to be in focus.

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    Re: The illusion of depth of field

    Well stated. My confusion has centered around claims that say focal length has very little impact of depth of field. In reality, anything that magnifies or enlarges the subject will decrease the perceived depth of field. Therefore, as a photographer I can leverage that concept in lots of ways (increasing/decreasing focal length, increasing/decreasing distance to subject, enlarging/shrinking the final image, etc.) to isolate a subject or make more of photo appear "sharp." It's just my way of trying to get my head around the subject. Thanks for your comment.

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    Re: The illusion of depth of field

    Something about stretching/magnifying the circles of confusion in the background when you zoom, creating the illusion of depth in the photo, which is different than the depth of accepably sharp focus on/around your subject.

    Ouch. I sprained my brain just now.

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    Re: The illusion of depth of field

    Quote Originally Posted by mfreesto View Post
    . . . but a generalization might be that any time you enlarge/magnify something you can see more detail . . .
    Hi Matt and welcome to the forum.

    The trouble with generalizations is that they are supposed to cover all cases of something without limits. And images and scenes both have limits. Images easily disprove the generality - at a certain limit of magnification all you can see is pixels. On modern cameras, that limit is about 5um at an optical magnification of 1:1 (please let's not get into MTF's, Nyquist and such). At a more normal 1:5 (flower shot, e.g.) 25um which is quarter of a mm - quite significant.

    Even scenery has limits. Due to photon scattering, you will not be able to see a ant on a leaf 10 miles away, no matter how big a telescope is used.

    Don't take this to heart, I can be a little picky at times . . .

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    Re: The illusion of depth of field

    Thanks, Ted. Makes sense. If I understand what you're saying, it's better to magnify optically than digitally if you can help it.

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    Re: The illusion of depth of field

    Quote Originally Posted by mfreesto View Post
    Well stated. My confusion has centered around claims that say focal length has very little impact of depth of field. In reality, anything that magnifies or enlarges the subject will decrease the perceived depth of field. Therefore, as a photographer I can leverage that concept in lots of ways (increasing/decreasing focal length, increasing/decreasing distance to subject, enlarging/shrinking the final image, etc.) to isolate a subject or make more of photo appear "sharp." It's just my way of trying to get my head around the subject. Thanks for your comment.
    I saw this demonstrated once, but it was some time ago so let me see if I can recall how it works.

    The discussion was centered around wide angle lenses, and the idea that using a wide angle lens gives greater depth of field. This is often demonstrated with wide angle shots where the foreground object is very large, with distant background objects still in clear focus (say, a flower on a lawn and a building behind).

    the author of the article demonstrated that, were one to move into a position where a telephoto lens gave the flower the same degree of prominence, that is where the flower was the same size as in the wide angle shot, then the building in the background would also be in focus for the telephoto shot.

    The big difference is that there would be much, much more of the building showing in the wide angle shot; and one would be much farther away from the flower in the telephoto shot.

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    Re: The illusion of depth of field

    My confusion has centered around claims that say focal length has very little impact of depth of field.
    I think John's post gets to the key issue. There are really two entirely different questions:

    1. You stand in one place, focus on something a set distance away, and change lenses. Under those circumstances, shorter focal lengths yield greater DOF.

    2. You switch focal lengths and then move so that the subject in focus is the same size as it was with the first lens. The dept of field stays approximately equal.

    The second is nicely illustrated in the toothwalker article you noted, which I think is one of the two or three best I have ever found on this topic.

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    Re: The illusion of depth of field

    Quote Originally Posted by mfreesto View Post
    Thanks, Ted. Makes sense. If I understand what you're saying, it's better to magnify optically than digitally if you can help it.
    I am still a newbie, But i can safely state that optical magnification is far superior to digital magnification.
    Digital magnification essentially enlarges the pixels, nothing more.
    It is the same as taking a picture found online & zooming in using software (all that happens is the pixels are exponentially enlarged)
    Last edited by groovesection; 21st October 2012 at 07:20 PM. Reason: unacceptable spelling

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    Re: The illusion of depth of field

    Since I believed what I was told when I started and have only once used digital zoom I rely on optical zoom. That one time that I did use digital zoom was to compare it with cropping in editing and I couldn't see much difference. Maybe you can
    The illusion of depth of field
    Taken with my old Nikon 5700 back in July 2005 [5Mp]
    Last edited by jcuknz; 21st October 2012 at 08:33 PM.

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    Re: The illusion of depth of field

    Quote Originally Posted by jcuknz View Post
    Since I believed what I was told when I started and have only once used digital zoom I rely on optical zoom. That one time that I did use digital zoom was to compare it with cropping in editing and I couldn't see much difference. Maybe you can

    Taken with my old Nikon 5700 back in July 2005 [5Mp]
    Frankly your result is exactly what I got when I ran a similar "experiment" on my old Canon Powershot S40. The two results should be pretty well identical, other than perhaps any in-camera processing or PP work,

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