Controlling Depth of Field

In response to a recent and common question, how to create photos with only a very small plane of focus…

The term for the plane of focus is called depth of field.  Depth of field (DOF) is the range in which the objects in the photo appear in focus.  When you have a small DOF, then sometimes the out-of-focus background will start blurring together, which is called bokeh.  Bokeh is usually very appealing because 1) selective focus gives the viewer a sense of what the key subject in the photo is and 2) it can look darn cool 🙂

So, jumping directly to the question, how do you create a photo with thin DOF?  There are four factors that affects DOF – (if I’m missing one, please feel free to add in the comments, thanks!) aperture, focal length, focusing distance, sensor size.  In the examples below, assume everything else is equal except for the factors specified.

1. Aperture size: aperture is the size of the opening in the lens, represented by f-number (or sometimes called f-stop).  The lower the f-number, the larger the opening, the smaller the depth of field.  An example, compare f/2 to f/8.  f/2 is a lower number, has a larger opening, and smaller depth of field compared to f/8.  For the curious, the f-number is calculated by the focal length divided by the diameter of the opening.  So, the f-number is always a proportional relationship no matter which lens you are using.

2. Focal length: this one most of us know.  Wide angle, telephoto lens, zoom out, zoom in – it’s the number on our lens that says 18-55mm, or 100mm fixed focal length, etc.  How does focal length affect DOF?  The wider the angle (lower number), the larger the depth of field.  An example, if you take a photo using a 100mm lens at f/2.8 vs 35mm at f/2.8, the 100mm lens will give a smaller depth of field.

3. Focusing distance: literally, how far away you are from what you’re focused on.  The relationship is: the closer you are, the smaller your depth of field.  So if you take a photo focused on a subject 3 feet away, and then used the exact same specifications focused on a subject 10 feet away, the first photo (3 feet) will have a smaller DOF compared to the second photo (10 feet).  Or being incredibly extreme, f/16 focused on something 50 yards away will give you a DOF of 20 miles, but f/16 in macro distances (inches) will give a DOF smaller than the little folds that hold cash in your wallet after buying all this camera gear.  Yea, THAT small!!  Just exaggerating of course, but you get the idea.

4. Sensor/film size: this is the factor that most people don’t realize.    The size of your capture medium (electronic sensor or film) makes a difference in your DOF.  I can’t explain why, but it has something to do with optics, vector path of light bending into the sensor…yea, it’s physics, that’s why I don’t get it o_O.  The larger the capture medium, the smaller the depth of field, when you correct for focal length.  The order of sensor sizes from small to big with more common cameras are: point and shoot > crop factor DSLR (which are most of them) > 35mm film = full frame DSLR > medium format film > large format film.  So, this relationship is inverse: the smaller the sensor, the larger the depth of field, and vice versa.  So for example, let’s say you take a photo with a crop sensor DSLR, 35mm lens, f/2, subject is 2 feet away.  Now use a full frame DSLR, 50mm lens (correcting for focal length discrepancy between crop sensor) all the same other factors, then your DOF is smaller.

This is why, for example, point and shoot cameras have a very difficult time creating photos with small DOF and any bokeh at all, much less nice bokeh.  You have almost all the factors against you: smallest sensor, most built in lenses usually have wider angles, and relatively smaller apertures.

And back to the original question of how to create small DOF – well, it really depends.  It depends on what lens you use, how far or close you are to your subject, which camera you use, and what aperture you set it at.  In a FB reply earlier, one person said using f/2.8 or larger will give nice selective focus and creamy bokeh.  With the factors mentioned above, that’s true only part of the time, but not all of the time.  If you use a 20mm lens at f/2.8 on a crop sensor DSLR focused on something 15 feet away, you won’t be able to achieve the small DOF and creamy bokeh (sorry dude, not picking on you!).  But if you use a 200mm lens in that setting, you will.  Actually, that was a bit tricky because you pretty much will never get creamy bokeh with wide angle lenses.  You might be able to have selective focus and bokeh if you’re clever, but wide angle lenses are notoriously bad for creating nice creamy bokeh.

Play around on this website, try it out:

http://www.dofmaster.com/dofjs.html

Hope this answers some of your questions!  Feel free to reply if something doesn’t make sense, and I’ll try to explain it better.

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