Depth Of Field

In one of my photography tutorial, I was explaining to my student how depth-of-field works. Aperture as part of the exposure triangle, helps control the amount of light coming in by adjusting the size of the opening. By changing the size of the opening, you are also indirectly changing the depth-of-field.

Shot taken at f/5.6, focal length 155mm. The background is visibly blurred even at relatively small opening. (shooting for Ody's Cafe)

Shot taken at f/5.6, focal length 155mm. The background is visibly blurred even at relatively small opening. (shooting for Ody’s Cafe)

Depth-of-field is always dependent on the size of the opening. If your intention is to reduce the depth-of-field, meaning blur background and sharp subject, your objective is to increase the opening. The bigger the opening, the lesser the depth-of-field and vice versa.

However, you probably tried shooting a flower at f/5.6 using a kit lens and unexpectedly got a good blur in the background. You wonder why even at a relatively small opening, the background was blurred. What actually affects depth-of-field and how to control it?

If you look closely, an aperture when it’s written it starts with an “f ” like f/5.6. “f ” stands for focal lengths and slash means divided by. If depth-of-field is dependent by the size of the opening then a longer focal length would have less depth-of-field than have a shorter focal length with the same aperture.

A 50mm focal length at f/5.6 is equivalent to 8.93mm. This refers to the circumference of the opening while a 200mm focal length at f/5.6 is equivalent to 35.71mm. This means that even having the same aperture, the size of the opening differs. In this case, the 200mm focal length at f/5.6 will give you lesser depth of field than the 50mm given the same aperture value.

If you plan to buy a 16-35 2.8 lens, don’t expect a blur background. At 16mm focal length set to 2.8, your opening is 5.71mm in circumference. It’s relatively small to blur the background. However, having a bigger opening on a wider lens allows you to capture more ambient light for a natural looking shot but keeps a good level of sharpness all through out.

Another argument arises if you divide the focal length over the aperture value. At 50mm, 5.6, the opening is bigger than at 16mm/2.8. Does this means that since it has smaller circumference with the 16mm, you have to compensate for the lesser light coming in? The answer is no. The wider lens gets more light because it covers a wider view which compensate for the smaller opening.

Photography is as technical as the laws of physics when it comes to light. I’ve known demure fashion photographers that you would never expect for them to understand this deep but they do. The challenges that photography poses to you in every instance is never ending. That’s what I love about photography.

Keep on shooting everyone!

Text and photos by Albert Pedrosa

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