Depth of Field and F-Stops: What They Mean

December 29, 2013  •  2 Comments

Chances are you have a camera.  Whether it's a smartphone camera, a compact point-and-shoot camera or a camera that takes different lenses, most of us have had the satisfaction of pressing the shutter, hearing the camera "click" and then promptly checking the shot on the LCD.  I'm also willing to bet you've seen images, particularly portrait shots, you absolutely love that you try to duplicate with your camera, but can't.  A great portrait shot has a distinct look, doesn't it?  The background is blurred and the subject seems to "pop".  Have you wondered why your shots may not be giving you that unique portrait look?  Maybe not, but as part of my new photography education series on this blog, I'm going to delve into and try to explain some basic photography principles that hopefully will shed light on different aspects of photography you may not have thought about.  For this inaugural photography education series post, I thought I'd talk about "depth of field" and "f-stops" and what these mean to your photography. 

If you've ever looked at the lens on your camera, you've probably noticed a bunch of numbers that may not make a whole lot of sense.  One number usually relates to the focal length of your lens and is often followed by "mm" for millimeters.  This number essentially indicates whether or not you can zoom with your lens and if so, how much.  You may even have a lens that has one focal length, a prime lens.  The second number is usually preceded by an "f" followed by a number, like "f/3.5" or "f/5.6".  This number (actually, usually a range of numbers, like f/3.5-5.6) indicates a lens' "speed".  Before I explain, let me give you Wikipedia's technical definition of an "f-stop" or "f-number" as they refer to it: "In optics, the f-number (sometimes called focal ratio, f-ratio, f-stop, or relative aperture[1]) of an optical system is the ratio of the lens's focal length to the diameter of the entrance pupil.[2] It is a dimensionless number that is a quantitative measure of lens speed, and an important concept in photography."

Confused?  The two most important pieces of this definition, in my opinion, are "aperture" and "lens speed".  Both have compelling effects on how an image is captured.  First, "f-stops": the smaller the f-stop number the faster an image can be taken.  In other words, f-stops directly affect the shutter speed (again, how fast an image can be taken).  I used the word "smaller" two sentences ago, but this is actually incorrect.  If you think of f-stops as a fraction, with "1" as the numerator and the f-stop number as the denominator, you'll see what I mean.  Let's use the numbers above (f/3.5-5.6).  The fraction 1/3.5 is actually larger than 1/5.6, right?  Just like 1/2 is larger than 1/8.  No one can deny that 2 is smaller than 8.  In terms of f-stops, however, the opposite is true; f/2 is actually a larger f-stop than f/8.  Again, think fractions and the idea becomes clearer.  Interestingly, if you were to set your camera's f-stop to f/22 (think 1/22 - small), for example, and you were to look down the center of the front of the lens, you would see a small hole created by the lens' diaphragm blades, or aperture blades.  Conversely, if you were to set the f-stop to f/1.8 the hole would be much bigger.  This hole or opening is what is referred to as the aperture and it explains why when you shoot at a larger f-stop (f/1.8, for example) you will get a faster shutter speed compared to trying to take the same image at f/3.5.  Shooting with a larger f-stop allows the lens to gather more light quicker, which results in a faster shot.  If your lens' f-stop rating is f/3.5-5.6, the largest f-stop you can use is f/3.5 when the lens is zoomed out to its widest setting.  You may have heard the phrase "shot wide open", which means that you are shooting with your lens' largest f-stop.  Another common phrase you may hear is "stop it down".  This refers to choosing a smaller f-stop to increase the depth of field in your shot, which I'll demonstrate below.

We've covered f-stops and aperture.  So what do they have to do with depth of field?  Wikipedia defines depth of field (commonly referred to as DOF) as "the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image."  Great.  How do f-stops fit in then?  The larger your f-stop the SHALLOWER your depth of field.  Therefore, the smaller your f-stop the GREATER your depth of field.  Below is a series of 4 images shot using my "fastest" lens, Canon's EF 85mm f/1.8 lens.  This is a classic portrait lens that renders out-of-focus areas buttery smooth while allowing the in-focus areas to pop.  Starting from the left, I've shot the Santa statuette "wide open" (very large f-stop = f/1.8) and I've slowly "stopped down" (using a smaller f-stop) the lens for each consecutive image.  Each image is labeled with the f-stop used.  Notice how the background (the out-of-focus areas) become more in focus as the f-stop number increases (smaller f-stop).

Notice the blur, or bokeh, in the first image on the left where I used f/1.8.  Bokeh refers to the aesthetics of the out-of-focus area of an image.  The Canon EF 85mm f/1.8 lens excels in this area.  The "balls" of light may be distracting, but I chose this background (our Christmas tree) so you could really see how the depth of field changes as I stop down the lens (make the f-stop smaller).  In this next set of four images, continue to notice how the tree, its ornaments and the lights become even clearer and more in focus as I further stop the lens down. 

By f/16 a totally different phenomenon is occurring: each Christmas light is rendered with a star burst quality, a function of the lens and its aperture blades.  Notice how by f/22 (a really small f-stop and a corresponding small aperture - hole - created by the aperture blades) much of the tree, the ornaments and the lights are more in focus.  You can even begin to make out some texture in the walls.  What do you think happened to the shutter speed (the speed at which an image is captured) as I stopped the lens down?  If you guessed that the shutter speed got longer, you're right!  Remember, as you stop a lens down (choose a smaller f-stop) you increase your shutter speed. 

If you've made it this far, you must have really been curious about depth of field (DOF) and f-stops or you have nothing better to do.  Thanks for making it this far!  Here are some quick takeaways:

1. If you're looking for that signature portrait image look (out-of-focus background with your subject popping), look into buying a "fast" prime lens.  Most manufacturers, even third party ones, make relatively cheap prime lenses that serve well as portrait lenses.  As a Canon shooter, the EF 50mm f/1.8 II comes to mind.

2. If you want to just stick with what you have and still get decent portrait shots, use your zoom lens' maximum focal length to achieve a better out-of-focus background.

3. Depth of field is most critical in landscape shots when you want everything in the image to be as in focus as possible.  Stop your lens down!  Depending on the "depth" of the scene, consider anything from f/10 - f/22.

4. Get out of Auto mode on your camera!  While this function usually takes the frustration out of picture-taking, it prohibits your creativity and prevents you from learning some pretty cool things about photography.  Most cameras (except for smartphone cameras, maybe) have modes that allow you to select your own f-stop, which is the setting I use 100% of the time.  This mode is typically called "aperture priority".  A similar setting can also be found for shutter speed, a mode which allows you to determine how fast you want each image to be captured. 

Please don't hesitate to contact me with any questions about this topic or any others you're curious about.  I can best be reached at justinpetersonphotography@gmail.com

Happy New Year!

 


Comments

2.Colorado Mountain Images
Mere,

Thanks for your feedback! Look for a blog entry shortly on shutter speed and how it affects your image taking.
1.Meredith Childs
Fantastic article, Justin. Your clear writing style made this difficult topic easier for me to understand. Very well-written and explained. Keep these types of articles coming!
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