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16 Oct 2013

Few days ago, a friend came to me with question on his food dishes photo that is taken with his EFS18-135mm lens. His question is "How come there is so much bokeh in the picture taken when I'm already using F11? I thought using a small aperture will give shallow depth of field? Is my lens faulty?".
After looking at the pictures, I told him that there is nothing wrong with the lens as it is the depth of field (DOF) based on the focal length, aperture and distance from the subject that causes it. I ended up spending the next 2 hours explaining to him in details.

I don't blame him for such questions as he has only started playing with DSLR for less than a year and I myself had similar questions many years back when I started with DSLR. In fact many others whom have owned DSLR for years may still have the same question.

What is Depth of Field (DOF)?

It is the distance from the front (also known as Near Limit Distance) to the back (also known as Far Limit Distance) of the focused object that will appear to be sharp in focus. This range of distance is dependent on the aperture used, the focal length, the distance from the subject and the sensor type of the camera.

DOF Increases with Smaller Aperture (i.e. Big F-Number)

The smaller the aperture (i.e. bigger the F-number), the larger the depth of field. As there are 2 other parameters that will affect DOF, so don't ever have the misconception that a very small aperture would always make the picture sharp from front to back.

DOF Increases with Smaller Focal Length

The technical explanation of focal length of a lens is the distance from the optical center of the lens to the camera's image plane on the sensor. There are more in-depth explanation relating to focal length on angle of view and field of view which I will not explain here as it could probably lead to more confusion - just google if you are interested to find out more.

For simplicity, focal length is typically the "mm" of the lens you are using. So if you are using 50mm prime lens, your focal length is 50mm. If you are usually a 70-200mm zoom lens and you are focusing at 120mm, the focal length is 120mm.

So, assuming that the distance from the subject and aperture remain constant, a shorter focal length (e.g. 24mm would produce greater DOF as compared with a longer focal length (e.g. 200mm).

DOF Increases with Distance from Subject

Assuming both Aperture and Focal Length is held constant, the farther the subject you are focusing, the larger the depth of field.

Type of Camera Sensor Affects DOF

Whether the camera is a full-frame of cropped body affects the DOF. Given the 3 parameters discussed above being held constant, a cropped body will produce a shallower/smaller DOF as compared to a full-frame body.

Note: In the case of macro photography (i.e. shooting using macro lens or mode), the DOF is affected only by the aperture and the magnification ratio. DOF decreases with a higher magnification ratio.

DOF Calculator

So how do to compute the DOF once the parameters are known. Simple - just download a DOF Calculator mobile application and enter the values for the parameters. The DOF Calculator will compute and provide values on the Near Limit Distance, Far Limit Distance, Depth of Field (DOF) and Hyperfocal Distance.

(Note: I will discuss Hyperfocal Distance in the next post so as not to confuse the topic on DOF.)

To Conclude

So going back to my friend's question on why the shallow DOF ... According to him, he has zoomed his lens to ~80mm on his EOS550D so that he can have a close-up of the food dishes, and shooting at 30-degree angle from a distance of about 50cm @ F11.

From these parameters, it gives only a depth of field of about 1.5cm. which explains why the rest of the picture outside this zone is blurry (in order words, huge bokeh). So the solutions for him to get a picture with deeper DOF are:
  • Shooting from a more elevated angle so that the surface would appear to be "flatter" and thus more surface will fall within the DOF zone.
  • Use a smaller focal length (i.e. wider angle).
  • Move slightly farther away from the subject.
End of day, understanding the DOF concept will enable better control over whether a shallow or deep DOF composition is desired for the picture.