Understanding Aperture & Depth of Field

Since many people have switched from point & shoot cameras to DSLRs (digital single-lens reflex), I think it’d be good to discuss what exactly AV (Canon) or A (Nikon) modes are. Hopefully this little post will help some of you move away from shooting on full auto mode because there is little difference between full auto on a DSLR and shooting on a point & shoot.

Aperture is the size of the opening that lets light into the lens and onto the digital sensor. The odd thing about aperture is that it’s kind of backwards… an aperture of 16 is intuitively ‘more’ than an aperture of 1.4. So ‘more’ should = more light… right? Well no… these aperture values are called f-stops and they are written inversely (lens focal length/aperture diameter). Basically, you can think of  an aperture value of 1.4 as 1/1.4 and a value of 16 as 1/16; as you can see, 1/16 is less than 1/1.4 which means the opening of the hole is much smaller and lets in significantly less light.

So why does the size of the hole matter? If you’re thinking that it shouldn’t matter, then you’re half right. Yes, a slow shutter speed + small opening = fast shutter speed + large opening. However, the other half of the story is that the depth of field is extremely different. A large aperture will yield a narrow depth of focus (also called focus spread) whereas a small aperture will yield a wide depth of focus (see Depth of Focus and Aperture Visualization).

At f/1.4, only the bunny is in focus and both the background and foreground are blurred out. At f/16, the bunny, background and foreground are all in focus. Click for a larger view.

As you can see in the pictures, shooting at f/1.4 yields a background that is distinctly out of focus and makes the subject stand out in the picture. However, shooting the picture at f/16 renders the background elements in detail. In some instances (landscapes), having everything in focus can be an advantage, but for many applications, it is desirable to blur (this blur is called ‘bokeh‘) out the background elements so as to isolate the subject; having a background that is too ‘busy’ most certainly detracts from the attention that should be placed on the main subject.

Of course, there are other elements at play here as well, including lens focal length and the distance between the camera-subject distance and the subject-background distance.

The best way to figure it all out is to experiment and move away from the full auto mode!