A Bit More In Depth with Depth of Field

Cherry Tomatoes (shallow focus)

Depth of Field is what looks like it is in focus in your shot. A camera can only focus on one specific point at a time.This is called a focal point, and it is the one place where a point will render as a point on your photograph. All the objects closer or farther then that point will render as a blur in the shape of your camera's aperture. (This blur is sometimes referred to as a circle of confusion). The Depth of Field is the area where those blurs will be so small, that they will still appear as a point to the viewer. A shallow focus, or narrow Depth of Field, (as illustrated in the photo to the left) will direct your viewers eyes right where you want them by leaving little else in the shot in focus. This is very useful when photographing an object, product or anything else that you want to be the center off attention in you photograph.

Depth of Field is affected by many factors, most of which are easily controllable. Since the out of focus area is simply a blur in the shape of your aperture, changing your f-stop will change your Depth of Field. A smaller iris (higher f-stop) will have greater Depth of Field, A wider iris (lower f-stop) will have a shallower focus. You can compensate for the different f-stops by using a different shutter speed, changing your ISO, adding or removing Neutral Density filters, or adjusting the light on your subject.

          300mm                  75mm (enlarged)      75mm (full frame)

Focal length is also a large determining factor in Depth of Field. The wider the lens you use, the greater the Depth of Field will be as long as you stay focused on the same distance away from you. (ie a 24mm lens at f5.6 will have a greater Depth of Field then a 70mm lens at f5.6 if they are both focused to 10' away). The photo on the left shows how the perceived focus increases as you "zoom out".  The background foliage is far sharper in the wider shot.

The San Fernando Valley (deep focus)

The distance from you to your subject will also play an import role in Depth of Field. If you are focused on an object 10' away, and then walk closer and (using the same focal length) refocus on the same object at 4' away your depth of field will have decreased. The picture to the right is a very wide shot focused to infinity with a relatively tight aperture (f/8). This creates a very wide depth of field (also called deep focus). Compared to the photograph of the tomatoes where 1 inch threw things way out of focus, this photo seems to hold sharp for miles.

The last factor for depth of field that I am going to discuss here is acquisition size. Whether it is the size of your piece of film, or the size of your digital sensor, the smaller the size, the greater the Depth of Field. It is because of this, that your cell phone camera, with its wide lens and tiny sensor, takes picture that seem like everything is in focus. Standard 35mm still pictures use an actual frame size of 24mm x 36mm. The Canon 5D , and many other "full frame" digital cameras have a digital sensor that is also 24mm x 36mm. 35mm motion picture film uses a frame size of 13.5mm x 24mm which is closer to the advantix C still film format which measures 14.9mm x 22.3mm. This is also the sensor size on a Canon 7D (making it a better choice to match a 35mm motion picture look). An Iphone 4s uses a sensor that is 4.54 mm x 3.42 mm. Far smaller then even a Kodak Disc camera that used an 8mm x 11mm frame size. That is why focus is rarely an issue with a cell phone camera. the tiny sensor size, combined with the usual wide angle lens makes for a very large Depth of Field.

*There is a nice Depth of Field calculator here
*There are Depth of Field equations here
*visit my website at daviddefino.com