Aperture and Depth of Field

Among the three elements of the exposure triangle, aperture is the one that comes up in discussions the most. Perhaps it is because it is the least understood. It certainly is the most complicated.

From the Nikon website:

Aperture refers to the opening of a lens’s diaphragm through which light passes. It is calibrated in f/stops and is generally written as numbers such as 1.4, 2, 2.8, 4, 5.6, 8, 11 and 16. The lower f/stops give more exposure because they represent the larger apertures, while the higher f/stops give less exposure because they represent smaller apertures. This may seem a little contradictory at first but will become clearer as you take pictures at varying f/stops.

The camera is, more or less, patterned after the human eye. Like our eye’s pupil, the camera’s aperture is that opening that lets light into the camera, which hits the sensor. The size of that opening is increased or decreased by blades in a lens, acting like an eye’s iris. The increase or decrease in that size is done by increments, which are the f/stops.

Computing for the actual dimensions or areas  of these f/stops and the amount of light they let through can be complicated. It is easier to regard them as proportions, wherein each f/stop lets half as much light in as the next larger one, and lets twice as much light in as the next smaller one.

It is difficult to talk about camera aperture without touching on depth of field.

I’ve seen a lot of photographers practically obsessed with it. If you are a beginner, you may have encountered this term asked yourself what it is.

Depth of field is the measure of the zone or distance over which any object in front of a lens will appear acceptably sharp. It lies both in front of and behind the plane of best focus, and is affected by three factors: lens aperture, lens focal length and image magnification (Ang, 2010).

Does it make sense now? If not, I’ll try to put it in less technical terms.

 

Suppose you’re focusing on something relatively up close, say the face of a friend, as you try to shoot his or her portrait. You will, of course, ensure that your camera takes a sharp picture of that face. When you go and review that picture to confirm a job well done, take a look at the background. In all likelihood, it’s going to be blurry, or at least noticeably less sharp than your friend’s face. That is a manifestation of depth of field.

Brisbane, Queensland, Australia (2009) ISO 100, f/5.6 , 1/60 sec
Brisbane, Queensland, Australia (2009)
ISO 100, f/5.6 , 1/60 sec

On the last day of my month-long trip to Australia, I watched this street performer doing all sorts of amazing feats. As you can see in the picture above, his upper body as well as the…. props… are more or less, sharp. But the background — a tree in the park, Victoria Bridge and the buildings on the other side of the Brisbane River — are all a blur. This means the performer and his props are inside the depth of field, while the rest are outside.

It’s not as commonly seen in pictures, but as defined by Tom Ang, there is also an area outside the depth of field in front of the subject.

ISO 100, f/4.5, 1/80 sec
ISO 100, f/4.5, 1/80 sec

The obvious question now is, how does one control depth of field? We go back to Ang’s definition. We will be concerned with three parameters: aperture, focal length, and image magnification.

If we take another look at the Exposure triangle, it says that if you have your camera set to a larger aperture, you get a shallower or thinner depth of field. Decreasing aperture, in turn, deepens the field. Your physical distance from the subject and how much you zoom in your lens will also affect depth of field.

Next logical question: How do you determine Depth of Field?

Well, you can compute for it mathematically! But I’m sure most of you would rather not. I know I wouldn’t, so we won’t get into that here.

Digital technology has given us the luxury of figuring things out through trial and error without the cost of having to go through rolls of film in the process. You are certainly welcome to eyeball it as you cycle through the aperture values allowed by your camera until you get the depth of field that you want. Eventually, you’ll get the hang of it.

Conversely, you can make use of any of the multitude of photography apps available these days.

DOF Calculator in Light Meter Tools for Android
DOF Calculator in Light Meter Tools for Android

Personally, I have not extensively tested the accuracy of these tools in measuring depth of field. But they sure make for useful guides and can help in eliminating much of the guesswork in a trial and error method.

I hope this lesson has given you a better idea on what depth of field is and how to manipulate it. So, go out there and have fun playing with it!

References:

Ang, T. (2010). Digital Photography: An Introduction 3rd edition. Dorling Kindersley Limited, London, UK.

Cambridge in Colour (n.d.). Camera exposure . Retrieved from http://www.cambridgeincolour.com/tutorials/camera-exposure.htm.

Mansurov, N. (2009). Understanding aperture – a beginner’s guide. Retrieved from https://photographylife.com/what-is-aperture-in-photography.

Nikon. (n.d.). Understanding maximum aperture. Retrieved from http://www.nikonusa.com/en/learn-and-explore/article/g3cu6o1r/understanding-maximum-aperture.html.