Understanding Depth of Field

Depth of Field seems to be a confusing topic for a lot of amateur and novice photographers. Granted, every photographer understands it to some degree, but rarely to the detail to explain it. Depth of Field (DOF) is a term to define the range in a photograph that appears to be in focus. That range is defined by distance from the lens to the farthest point out. Now, most know that the larger the aperture the less the depth of field and the smaller the aperture the greater.

aperturerange

Before we can start on depth of field, we need to define the Circle of Confusion. Your eyeballs have a predefined ability to see fine detail which is 1 minute of arc. This means, that with perfect eyesight you can see 1/16mm in size in perfect (or near perfect) conditions. In other words, if you place two dots next to each other smaller than 1/16mm they will appear as one dot to the naked eye. However, most photography manufactures have set their parameters on around 1/6mm.

Speaking of photography manufactures, the generally accepted for 35mm format (and it’s digital equivalent) that the CoF is 0.0333. How they got to this is they assume a typical enlargement is a 5×7 print or 5 times from negative to print. So take 0.1667 (1/6th of a mm) / 5 and you get 0.0333.

Now that you understand Circle of Confusion, we can get into how this ties into Depth of Field. A simplified definition is that DoF is the range in which objects both behind and in front of the subject will appear sharp within the limits of the set CoF. Note: there are many DoF calculators out there for your smart phone that will help you do all this math. Take a look in your app store for one. Most have a list of cameras with all the numbers plugged in. You select your camera, your lens, desired aperture, and how far the subject is away from the focal plane, and it will let you know what will be in focus and what will not in distance terms. Also, if you are fortunate enough to have used older 35mm film cameras and lenses you may have noticed lines on the lens associated with distance and f/stop. This was a manual calculator for DoF!

Now onto a subject most new photographers are not even aware of, primarily because on anything but pro or high end lens the focus distance isn’t even labeled anymore. What I am referring to is the Hyperfocal Distance. Simply defined, the hyperfocal distance is the closest point at a set aperture at which everything behind it will fall into focus. If you are into photographing landscape, this is a critical step — if you have the time. This is the point in which you will manually set your focus so that everything from that point out is sharp. Example: if you are using an 18mm lens on a Nikon D4 and set to f8, the hyperfocal distance is 4′ 5.86″. This means that everything from 2′ 8.10″ (half the hyperfocal distance) to infinity will be in focus. The key here is knowing the hyperfocal distance, setting your lens manually to that, and making sure anything you want in focus is at least half the hyperfocal distance.

 

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