Scribbling with light…

Photography for dummies :)

Focal Length

Let’s start with some definitions first. The diagram below explains various terminology that are used in photography. Let’s break it down.

Focal Length: The technical definition of focal length says it’s the distance between the optical center of lens and the focal point. The focal point of the lens is the point where all the rays parallel to the lens converge. Putting in layman language, it’s just a number they put on the lens. The bigger the number, the longer the lens (and heavier). The lenses are categorized based on it’s focal length but before we go into it, I want to define the field of view.

Field of view: Now this is an important concept as it affects the picture you are taking. The field of view describes the extent of a given scene that is imaged by a camera. It can be given as horizontal,

vertical or diagonal field of view. The field of view depends upon the focal length of your camera. The rule of thumb being: longer the focal length of lens, smaller is the field of view. A handy calculator to calculate the field of view for a given focal length can be found here. An example to explain the concept further:

Focal Length and Field of View

Focal Length and Field of View explained on an image taken from 28mm lens.

Focal Length

Types of lenses:

So the field of view reduces as focal length increases. Since our field of view is restricted, as the focal length increases we can see things at distance clearly (as rest of the image is cropped) . Based on the focal length or field of view, lenses are divided in following categories:

  1. Less than 21 mm. Extreme wide angle. Most of the point-and-shoot cameras fall in this category. These lens are useful in taking pictures of buildings or architectural sites.
  2. 21-35 mm. Wide angle. Useful for taking landscapes pictures.
  3. 35-70 mm. Normal. Useful for taking street or documentary pictures.
  4. 70-135 mm. Medium Telephoto. Useful for taking portraits.
  5. 135-300 mm. Telephoto. Useful for taking sports, wildlife and bird shots.
  6. More than 300 mm. Super Telephot. Useful for shooting birds. (Bird watcher special :) )

All these focal lengths are “35 mm equivalent”. If you are wondering about the term “35 mm equivalent”, you need to learn about the concept of focal length multiplier. This is a particularly important topic, if you are considering to buy a lens for your DSLR.

Focal Length Multiplier

In order to understand this concept, let’s start with the concept of image recording in DSLR. A digital camera uses sensors to record an image. It’s equivalent of film in an analog camera. So we all know (I am assuming), when the camera shutter opens, the light enters the light sensitive area of camera. This area (that contains either film or sensors) records the intensity of each component of light (Red, Green and Blue) which combine together to form the image. Now the size of this light sensitive area changes from camera to camera for DSLR.

For a 35 mm film camera this light sensitive area is 36mm X 24mm. However for DSLRs this area varies. Low-end to mid-range SLRs use sensors between 24mm X 16mm and 18mm X 14mm. Professional DSLRs use larger sensors between 29mm X 19mm and 36mm X 24mm. A DSLR with sensor 36mm X 24mm is called a full frame DSLR and the focal length for these cameras are same as for analog cameras. As far as I understand, the lenses can be interchanged between a full frame DSLR and an analog camera as they have same light sensitive areas.

Ok, so what happens when the camera has a smaller sensor? The camera can not record the entire image or the image is “cropped” compared to 35mm film. The sensor captures only the middle portion of the image recorded by a 35mm film. This phenomenon is known as “Cropped Field of View”.

A 35mm camera uses a longer lens in order to achieve the same field of view as DSLR with smaller sensor. The question is how much longer the 35 mm lens should be in order to get the same field of view as the DSLR. In other words we need a conversion factor to calculate the equivalent 35mm focal length for the lens used by a DSLR. This conversion factor is known as focal length multiplier. It is obtained as the ratio of the diagonal of 35mm film (43.3mm) divided by the diagonal of the sensor. I know it’s a bit confusing so let’s try out a little exercise.

The diagonal of a 36mm X 24mm film is 43.3mm. The diagonal of a DSLR with sensor 24mm X 16mm is 28.8mm. The focal length multiplier for the lens is 1.5x (=43.3/28.8). Hence a 17mm lens for the DSLR is equivalent to 25.5mm for 35mm camera. In other words, a 17mm lens for the DSLR will have same field of view as 25.5 mm lens for 35mm camera.

Focal Length and Depth of Field

I already covered realtion between the aperture and focal length in a previous post. In that post, I also mentioned Depth of Field (DOF) changes with aperture. However, I didn’t mention that depth of field is inversely propotional to the focal length of the lens. So the smaller the focal length the greater the depth of field. Another factor that affects the depth of field is the distance from the subject. It is directly propotional to the DOF, ie, a subject at a greater distance will have greater depth of field than a close-up subject. I will try to elaborate on these points further with some examples in next post.

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November 23, 2008 - Posted by | Basics |

4 Comments »

  1. [...] I discussed earlier, the focal length of the lens decides the field of view (extent of view imaged by the camera). [...]

    Pingback by Depth of Field « Scribbling with light… | December 1, 2008 | Reply

  2. [...] one more thing before I sign off. As I discussed earlier, the size of image sensor varies from DSLR to DSLR. A full-frame DSLR has sensor of size 36 mm X 24 [...]

    Pingback by ISO Speed « Scribbling with light… | February 21, 2009 | Reply

  3. [...] one more thing before I sign off. As I discussed earlier, the size of image sensor varies from DSLR to DSLR. A full-frame DSLR has sensor of size 36 mm X 24 [...]

    Pingback by Scribbling with light… | February 21, 2009 | Reply

  4. a wonderful post indeed! explained so well!

    Comment by magiceye | February 4, 2010 | Reply


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