Saturday, March 22, 2014


Most of us have heard the stand-by idiom regarding f/stops and depth-of-field (DOF): For more depth-of-field, use a larger number f/stop (e.g. f/16, f/22, etc). That is true, and I follow that idiom myself. I always tell my students that when you want a lot of depth-of-field and are not sure what f/stop to use, when in doubt, f/22. And that is also true. I would rather get more DOF than I need, than not enough.

However, did you know that it is not only the f/stop used that gives us more depth of field? Among other factors, the size of the opening, or diaphragm in our lenses also gives us more, or less, depth-of-field. For example, wide angle lenses, by design, have smaller openings, at any equivalent f/stop setting, than non wide angle lenses.

Let's look at a comparison. These two photographs were taken from the exact same spot (about 3 feet from the closest subject), same perspective, and with the same f/stop (f/11). The only difference was focal length. One photo was made at a 17mm focal length setting; the other at 70mm. Now, keep in mind, the comparison was only 17 vs. 70. As you will see, a variance of only 53 mm can make a big difference.    

F/11 @70mm. Note the blurred background

 f/11 @17mm. Note the depth-of-field coverage.
(I cropped the wide angle shot to show the difference)

So, what does this mean in actual in-the-field application? For me, I do use the old stand-by rule of thumb. However, if I am in a situation where I want a lot of depth of field AND a fast enough shutter speed to get what I want (assuming I'm shooting at 17mm), I might sacrifice a little f/stop (say f/16 or f/18 vs. f/22), in order to get a faster shutter speed, knowing that I am still going to get the depth-of-field I need. By doing so, I will also get a faster shutter speed on top of that, without having to increase my ISO. I can have my cake and eat it too!! Where does that saying come from anyway?