Aperture: in depth

In a previous post, we talked about the function of the lens aperture; control over the volume of light that strikes the sensor.  A wider aperture will result in a brighter image compared to a narrower aperture.  Well, if a wider aperture results in a brighter image why don't we always use this, right?  There's a very good reason for this and it's called depth of field.

What is depth of field?

Depth of field refers to the area of an image that stays in focus from foreground to background.  The coffee cups we saw in the last post provide good examples of depth of field.  The images in the selection are all focused at the same point but shot using different apertures.  The shot using the wider aperture (smaller f-number) results in an image that is sharply in focus around the focus point and out of focus elsewhere (cup in background is blurry).  As you make the aperture narrower (larger f-number) the distance from foreground to background that stays in focus increases (cup in background is sharper).  

It's important to note that depth of field is not symmetric around the focal point.  In general, only one-third of the depth of field falls in front of the focal point.  However, when the distance to your subject gets really close this tends to get more or less symmetrical; half in front and half behind.

The series of images below illustrates the concept of depth of field in detail.  Noah agreed to be our subject to shoot these samples.  These images were shot with the camera on a tripod and each image was shot using a different aperture value, going from f/2.8 to f/16.0 in full stop increments.  Remember a smaller f-number means a wider aperture, so f/2.8 is wider that f/16.0.

The focus point in all these images is the bird on Noah's shoulder so Noah will be in focus in all our images.  To see the increase in depth of field pay attention to the patterned foreground and background in the images.  You will observe that as the aperture becomes narrower larger areas of the image start to become focused.  At f/2.8 the background is a haze with no discernible detail.  At f/8.0 we start to see the good detail in the background and foreground and at f/16.0 almost everything in the image is sharply in focus.  Click on the images to view in high resolution and to check out the EXIF data.

Noah at f/2.8

Noah at f/4.0

Noah at f/5.6

Noah at f/8.0

Noah at f/11.0

Noah at f/16.0

This simple concept when used correctly can be a great composition tool. As the photographer you decide if you want your subject's surroundings to contribute to the image and select the appropriate aperture.

Besides aperture there are two other factors that contribute to depth of field; distance to the subject and focal length.

Distance to Subject

Increasing the distance to the subject will increase the depth of field in an image.  This means that even using a very wide aperture you can get large areas of the image in focus by simply focusing at point that is far away.  And conversely, if your subject is very close (cms away) even a narrow aperture will keep most of the image out of focus.

Back to Noah again for a quick demonstration.  The series of images below were all shot at f/5.6.  The only variable between the images is the distance to the subject.  By simply varying the distance to the subject we can add depth of field to an image. However, increasing the subject distance will make your subject look smaller, a point you should consider when composing your image.

Subject distance 1.5 feet

Subject distance 2 feet

Subject distance 3 feet

Focal Length 

The third factor that contributes to depth of field is the focal length of the lens. At identical aperture setting a telephoto lens will produce shallower depth of field than a wide-angle lens. Most literature I've read specifically talk about telephoto and wide-angle lenses but in my experiments I was able to observe this even by simply varying the focal length on zoom lens.

The images below were shot at 15mm and 50mm respectively and I've tried to maintain the size of the subject. You'll observe several differences between the two images, mostly because they were shot a different focal lengths but for this exercise the only thing to note is that at 15mm the background is noticeably sharper than in the image shot at 50mm.  And keep in mind that at 15mm the subject distance is also shorter.  So, the focal length definitely has an impact on the depth of field. The motion blur in the second image is due to my carelessness.  I'll update this as soon as I can.  

 

Aperture is a fairly simple control to understand and use but when applied appropriately it can serve as a great compositional tool.

Please post any thoughts or questions in the comments section below.

Recommeded Reading:
http://toothwalker.org/optics/dof.html