Ever wondered why a picture taken at say F3.5 on a non-SLR digital camera like the Fujifilm Finepix 602Zoom does not have the same DOF than a SLR camera like the Canon EOS 1D Mark II? In this article I'll explain what causes these differences.

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Above is a (bad) picture taken with the Fujifilm Finepix 602Zoom @ 1/11s, F3.6 (Av), ISO 200, 35mm equiv. f.l. = 35mm.

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Above is another (bad) picture taken with the Canon EOS 1D Mark II @ 1/6s, F3.5 (Av), ISO 100, 35mm equiv. f.l. = 35mm.

Compare the curled telephone cable between the two images. Can you see that the Fujifilm's picture has much more depth of field than the Canon? Even though (for all practical purposes) the three key factors determining DOF (aperture size, distance from subject and focal length) were identical, why is there still a difference?

The answer lies in physics, and going back to fundamentals.

Aperture The aperture is defined as the size of the opening of the lens allowing in light.

Focal Length The focal length of a lens is the distance between the middle of the lens element and the film or CCD (focal plane) of the camera.

F-Stop The f-stop value is defined as the ratio between the diameter of the aperture to the focal length of the lens.

Being armoured with these definitions, lets do a simple calculation. The real aperture size of the Fujifilm is given by:

           Focal length
F-Stop = ----------------
         Aperture Diameter

rewriting yields...

                      Focal length
Aperture Diameter = ----------------
                        F-stop
           7.8mm
       =  ------
           3.6
       =  2.16mm

and area by...

  Area = πR2
       = (3.14159)(2.16/2)2
       = 3.687mm2

The real aperture size of the Canon is given by:

           Focal length
F-Stop = ----------------
         Aperture Diameter

rewriting yields...

                      Focal length
Aperture Diameter = ----------------
                        F-stop
           28mm
       =  ------
           3.5
       =  8mm

and area by...

  Area = πR2
       = (3.14159)(8/2)2
       = 50.26mm2

Firstly, as an aside the aperture area is 13.6 times larger on the Canon than on the Fujifilm, allowing much more light to be recorded on its much larger (35mm/1.3) CMOS censor than the Fujifilm's (35mm/4.5) CCD sensor.

Secondly, to answer the mystery surrounding the DOF difference, consider the real focal length differences between the two cameras. The Fujifilm has a real focal length of 7.8mm (35mm - 35mm equiv.), and the Canon is 28mm (35mm - 35mm equiv.). A longer focal length decreases DOF, and a shorter focal length increases it. Comparing the real focal lengths, 7.8mm is much shorter than 28mm, which is why the Fujifilm has much more DOF than the Canon.

As a tip for all those cellphone-camera-cruisaders out there, a typical CCD diagnonal size on popular cellphone cameras is 2.619mm, with a focal length of 3.3mm and F2.8 aperture (fixed). This implies an aperture diameter of 1.179mm, or an area of 1.09mm². It is easy to see that even at F2.8, with a focal length of 3.3mm there is no way it can match the limited DOF the 7.8mm Fujifilm can yield, not even to mention the 28mm of the Canon.

You might be asking of what significance this whole article is, since having a huge DOF is a good thing, and a limited DOF bad. Right? However, as any professional photographer knows there are usually more scenario's in which you want to limit the DOF when doing portraiture or macro photography, than not. Having a SLR camera allows us to play with DOF almost the same as a person with a 35mm film camera can, whilst using a compact digital camera or cell phone camera denies us this freedom.