2010.03.31

iPhone Plus Laser Equals DoF Magic

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    One of the essential controls in photography is depth of field (DoF).  Sometimes you want everything in focus (e.g. landscapes) and sometimes you want to isolate a subject against an out-of-focus background (e.g. portraits).  While the general rule is to stop down the aperture to maximize depth of field and open up the aperture to minimize depth of field, in fact aperture, focus distance, and focal length all interrelate based on optical science.  So, while the stop down/open up rule of thumb is helpful, it’s a crude approach.

    A practical use of depth of field is for landscapes. For instance, you might want to extend focus from a rock in the foreground back to the horizon.  Presuming you desire a certain focal length (70mm) and focus distance (18ft to the rock), your only variable is aperture.  What aperture should be used to yield this result?  Since a tiny aperture requires longer exposures and can yield less crisp images versus a larger aperture, you don’t want to stop down more than is required(!)

    The equations to figure all this stuff are complicated and something you don’t want to do on a calculator.  Did I mention that every camera has its own special constant, based on the size and resolution of the sensor or film in the camera?  It’s called the circle of confusion (CoC).  Does your head hurt yet?

    Fortunately, there’s an app for that.  Hand held computers such as the iPhone allow you to completely understand DOF for any shot, simply by wheeling in key variables including your camera/CoC pair (e.g. Canon 5D/0.03mm).  The quality of apps vary; I’ve tried a half dozen on the iPhone and recommend DoF ($2 from the App Store). Once you’ve selected your camera (one time, for  CoC), just dial in focal length and aperture along with your focus distance and it calculates eight key parameters on the fly:  DoF total, DoF in front, DoF behind, CoC, Hyperfocal, DoF far-limit, DoF near limit, and focus distance.

    So where does the laser come in?  The laser provides precise distance measurements, especially on distances beyond 10 yards when it becomes difficult to precisely “eyeball” a distance.  Is a foreground subject 35 meters or 45 meters away?  I used to guess and often guessed wrong.  By adding a small and modestly priced laser rangefinder to my field kit, I can id key distances with confidence.  Golfers and hunters both use rangefinders, and most sporting goods stores sell them.  I purchased one at the low end (a Bushnell for $150) and it’s been all I need.

    Being able to understand your hyperfocal and other DoF parameters for any given combination of variables is sweet and makes what used to be hard, easy.

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