Basics: Aperture and Depth of Field

By Jack Berglund

There are numerous books and websites explaining the ins and out of shutter speed and aperture.  I’ll explain them in a simple way that you can actually use and when you need to care about what.  Here we’ll cover aperture, in a follow-up post I’ll include shutter speed.

Depth of Field is Your Friend

One of the best things about upgrading to a Digital SLR from a point and shoot is being able to control your depth of field. Depth of field is how much of photo is in focus from the point your focusing on.To get a portrait with an in focus face and a nice blurry background, you want a shallow depth of field.

Large Aperture, Shallow Depth of Field

The main way to control your depth of field is to alter the size of the aperture (the hole through which the light passes).  Due to something called ‘physics’ a larger aperture means a shallower depth of field.  Aperture is measured in f stops.   There are two important things to know about an f-stop: 1) smaller numbers mean a bigger aperture and 2) I’ll tell you in a moment.

Small f number (e.g. f/2.8) = big aperture = shallow depth of field = blurry background = nice portrait

Maximizing a Blurry Background

Lenses with a large maximum aperture (a.k.a. ‘Fast Glass’) tend to be expensive and the general purpose telephoto lens you got with your first camera is unlikely to be super fast.  This reduces the ‘blurry background’ effect you can get.

All is not lost: Firstly any DSLR lens will be infinitely better in this regard than your point and shoot (its physics thing again, but cameras with small sensors like a point and shoot really struggle to give a shallow depth of field).

Secondly, you can emphasize the affect by either putting your subject a good distance from the background and/or by zooming in (depth of field is shallower at greater magnifications, all other things being equal).

Thirdly, you should get a 50mm f 1.8 which can be got from most manufacturers for around 100 bucks.  The large maximum aperture will let you really blur that background! [Note: the low end Nikons (D3000, D5000) can’t focus Nikon’s 50mm 1.8]

Small Aperture, Greater Depth of Field

Shooting a landscape is very different to a portrait.   Normally you want a lot of the image to be in focus, not a single object.  Making the aperture smaller will increase the depth of field and allow the whole image to be sharp.  A typical landscape shot would be taken at f/8 – f/11 with the focus about a third up from the bottom.

Bigger f number (e.g. f/11) = smaller aperture = greater depth of field = whole picture in focus = nice landscape

As with any rules of thumb, they were made to be broken.  Once you understand how to control the depth of field, you can use it as you would like.

Let there be light

Bigger apertures also allow more light into the camera.  This is useful when shooting at night or indoors without a flash when you may chose a big aperture (small f number) regardless of the depth of field implications to allow you to get a sharp image.  More on this in a future post.

Now Try It

To see the effects of aperture, try this exercise:

1) Pick a subject such as a flower in a garden in a well lit place (during daylight!) with a background in the distance.

2) Set your camera to Aperture priority (Av on a Canon, A on a Nikon).  Put yourself in control, leave the full auto mode behind!  In aperture priority mode, you can change the aperture and your camera will automatically set the shutter speed to maintain a correct exposure so you don’t need to worry about this for the moment.

3) Use the wheel on your camera (front wheel on most Nikons, Canon, erm, its either the front or the back) to change the aperture.  You’ll see the f-number change (either on the top of the camera or when you look through the lens).  Set your aperture to the lowest number it will go, e.g f/3.5

4) Focus on the object and take a picture

5) Now set your aperture to f/11

6) Take another picture the same way as the first

7) Pick up your camera and actually follow steps 1-6 rather than just read them

8 ) Either in the camera or on the computer, compare the two photos.  The effect should be quite striking.  In the first instance, the object is in focus with a blurry background.  In the second instance, the background will be more in focus.

One More Time

Smaller f-stop =>bigger aperture => shallower depth of field

Learning to control your depth of field is an important basic principal that will allow you take more interesting photos.  No discussion of aperture would be complete without covering exposure, shutter speed and ISO: more on this soon.  Oh, the second important thing about f-stops…you’ll have to read my next post.

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    • Julia
    • April 21st, 2010

    I like it!! I’m glad that you didn’t dedicate this email to me, even though I keep on asking you the same question over and over again :-). Maybe I should print out this and keep it with your camera. Can’t wait for your part on shutterspeed! xoxo

  1. Great write up again. You have a way of taking what can be quite confusing things and making them very simplistic for those that just wanna get the basic concept down.

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