Depth of Field: Basics

Today is a big day. Depth of field is a major--yet somehow under-appreciated--aspect of photography that, if mastered, can lead to a major difference in any type of photo genre. I learned some key things to remember (and constantly consider) as I'm setting up and taking my shots.

What I Learned

• Depth of field controls what is and isn't in focus
• Focus distance is the center of the depth of field (that you want to photograph)
• Represented on the lens (in meters and/or feet)

• The closer the focus distance or the wider the aperture, the more shallow the depth of field

• The farther the focus distance or the smaller the aperture, the greater the depth of field

Story

I woke up with lingering cold symptoms, so I decided to linger in bed for a while before trying to be at all productive. After video chatting with my parents, I decided that I should try to make an effort at doing something with my day off, so I made my way to the common room of the hostel at which I was staying. I really wanted to do photography stuff. I didn't bring a camera because that wasn't the point of the trip and because I'm a fool, so I sat reading a fashion magazine until someone appeared.

Eventually, Omi turned up with his Nikon D800; I'd wanted to affix my af-s Nikkor 50mm f/1.4g to it ever since the lens came into my possession. For practice, of course. Sure. Something like that.

Anyway. I gingerly transplanted my 50mm lens onto the D800 and twisted on an f/1000 close-up filter I'd bought a week prior at the Nakano camera junk shop. Y'know, for funsies. Sitting in front of the nondescript table of the hostel common room, took a photo of the first available object: my breakfast.

Ugly Banana

Thankfully, I'd gotten the camera to focus on exactly what I wanted (first try!), and, boy, does that bokeh look sexy! Never did I think that I'd be able to take such marvelous shots. I took a few more photos, experimenting with the aperture, and I noticed that nothing much changed, only the section of the banana that was in focus. So I did the first thing I could think to do, that is, ask Omi questions. And after a full day of pondering, I figured out a few ways to make sense of everything for myself.

Explanation

As is commonly known, the aperture controls the depth of field, but what does "depth of field" actually mean? In the simplest terms possible, the depth of field (DoF) controls how far into the frame (the Z-axis, if you will) the camera can focus. The closer the focus distance is to the camera, the smaller the DoF; the farther the focus distance, the greater the DoF.

But what does this mean practically, particularly when we're taking close-up shots, when the DoF is generally short anyway (say, f/4 and lower)?

• Notice that the focus is at the top (far side) of the map, rather than in the middle or bottom (closer to the camera)

Nikon D800; f/2, ISO 200, 1/125

• Focus is in the middle of the map 

Nikon D800; f/2, ISO 200, 1/125; close up filter (f/500) to artificially shorten focal distance

• Focus is at the bottom of the map

Nikon D800; f/2, ISO 200, 1/125; close up filter (f/1000) to artificially shorten focal distance

Usually this isn't the case, of course. The above examples show a flat image on a flat plane, rather than 3D subjects in a multidimensional plane. Still, the Z-axis is read only in terms of height (and sometimes width), like a single-point linear perspective landscape.

Image Credit: DC Design

As you adjust the DoF, the imaginary Z-axis (according to the camera sensor) "hits" the right height/width on the 2D plane on which the subject resides--or the right telegraph pole in the diagram above--and the subject becomes crystal clear. If the intended subject isn't crystal, it's because the Z-axis is hitting the wrong plane/pole. But this is easily fixed with a few tiny twists of the aperture ring.