Depth of Field for Beginners

Ahh depth of field… aka DOF. This term strikes fear into the hearts of young aspiring photographers everywhere, both film and digital. They know it deals with optics and math - two things that most creative people are not really into. My first instinct is to say that depth of field is not that complicated… but then again I had to take an optics class in college which made me want to stab my eyes out with metal darkroom tongs… and then pour stop bath in them. I believe this class had a very deceptive name, along the lines of “Principles of Photography” or something like that. It was, in fact, all math. Ewww.

But as usual I digress, so let us get back to the task at hand - which is understanding the glorious principles of depth of field and how they apply to your film photography. Let me just say right now that I am about to do a basic overview “for dummies” style, so please do not write to me claiming that I did not explain such and such complicated principle. You can grab a copy of Ansel Adams’ “The Camera” for that. Here we go:

In über-simple laymen’s terms, depth of field refers to the part of your photograph that is in focus. If all or most of your photograph is in focus, you have a deep depth of field (also called deep focus.) If only a part of your photograph is in focus, you have a shallow depth of field (also called shallow focus and selective focus.) And that’s what depth of field is. Seriously.

The tricky part is figuring out how your aperture relates to your depth of field, and your beloved (or hated) exposure triangle. What is an exposure triangle, you say? You best be reading my Exposure 101, I answer. Several factors affect depth of field, including your distance to your subject, the focal length of your lens, your selected aperture (f-stop) and the format you are shooting. This means that a photo taken with a 50mm lens at f/1.8 from the same distance will not have the same depth of field when taken with a 35mm camera and 4x5 field camera.

A general rule to guide you: the smaller the f/stop number (so the larger the opening), the shallower the depth of field. F/1.2 has a shallower depth of field than f/1.8, which has a shallower depth of field than f/2.8 and so on. F/5.6 and F/8 tend to give medium focus, depending on your distance from the subject (and the format you shoot, of course.) If this confuses you, have a look at What is aperture/f-stop?.
Side-by-side examples:

{Selective Focus: F/2.8 - This is pretty shallow, but not to the point where it creates a complete bokeh effect and the background is indistinguishable. Both of these were shot at F/2.8 with a 50mm lens in 35mm.}

{Deep Focus: F/16. -These two, on the other hand, have deep focus - meaning that the foreground and background are in focus. Both were shot at F/16, but the left image is medium format and the right is 35mm.}

{Shallow and medium side-by-side: The background in the left shot is completely blurry with zero detail. It was shot at f/1.8, approximately 12 feet from the subject with an 80mm portrait lens on 35mm film. The right shot has a blurry background, but you can still tell what it is. It was shot at f/8, approximately three feet from the subject with a 50mm lens on medium format film.}

F/32 is most commonly the highest number on lenses that don’t cost a bajillion dollars, but you can definitely come across field cameras with an f/64. In fact, in the early 1930s, a bunch of photographers (including Ansel Adams) got together to form Group F/64. Their principal belief was that photographs should be  perfectly exposed, profoundly sharp and completely in focus (in contrast to the Pictorialist era, for the History of Photo buffs.) An aperture of f/64 was the best way to achieve this, as far as they were concerned.

Some of you may be saying, “Hey, but f/32 really doesn’t let a lot of light in….” No, it doesn’t. This is where mastering your exposure knowledge truly helps you create the photograph you want. If you absolutely have to shoot 100 ISO and need a very deep DOF, you’ll have to lower your shutter speed. If you want to use a specific shutter speed at f/32, you’ll have to pick a film with a high enough ISO.  For those who shoot digital, this doesn’t prove as much of a constraint, considering you can change the ISO. For my beloved kittens who shoot film, your ISO is your ISO and you can’t change it. Even if you decide to push or pull to fit the situation, you still have to shoot at that ISO for the entire roll. For more on that, please check out  What is ISO? in the Beginner’s Guide to Film Photography.
All of this information can seem confusing, but your lens actually tells you the depth of field if you really look at it:

See how it’s on F/2.8? And there’s a little white diamond on the middle ring? And more numbers on the third ring? Voila your DOF indicators. We can understand that the manufacturer says that this lens at F/2.8 has a DOF range of 1.5 to 2 meters, or 5 to 7 feet. Meaning that anything in between that range will be in focus. The manufacterer is most often, but not always, right. (Side Note: Mastery of using these numbers to focus without looking is known as “Zone Focusing” in fancy photographer talk.)
Many SLR film cameras have a depth of field preview button; it’s usually located on the front near the button to release the lens or the self-timer. When you hold the depth of field preview button and look through the viewfinder, you’ll notice it is significantly darker but accurately displays your complete depth of field. For a great explanation of this button, check out Ken Rockwell’s The Depth of Field Preview Button.
Let’s sum up the major points:

• Depth of field refers to the areas of the photograph in focus.

• Small f-stop numbers produce shallow depth of field, or selective focus. This is when the background is blurry. Great for portraits.

• Medium f-stop numbers produce a medium depth of field, still with selective focus, but with significantly more definition in the out-of-focus areas. Good for portraits and specific landscapes. 

• Large f-stop numbers produce a deep depth of field, meaning the foreground and background are in focus. Ideal for landscapes.

If you want to get more in depth on depth of field (sorry, couldn’t resist), I highly recommend Understanding Depth of Field in Photography from Cambridge in Color. They’ve got loads of fancy diagrams to confuse you ;)

Article from: "I Still Shoot Film" Blog