I guess you've heard of this amazing online photo community/application called flickr
. With the plentiful of various interest groups, the amount of fun one can get from just viewing the millions and millions of photos is never ending! Having spent quite an amount of time on flickr myself, I often come across things like "38 mm to 115 mm equivalent focal length" or "Focal length 7mm" which just leaves me dumbfounded. What do these terms actually mean?
This is what I've learnt today:
35mm is actually the width of the film (called 135-format) used by "35mm cameras". Most photographers are used to the focal length talk of the 35mm cameras.
The focal length is the distance between the aperture, and the image sensor. For all 35mm cameras, the focal lengths are all the same; Because only 1 type of film is used (the 35mm one.)
However for digital cameras, not all focal lengths are the same. Digital cameras use digital image sensors, like CCD or CMOS. And their sizes, are often smaller than 135-format film, can vary from different manufacturers and even models of the same brand. Therefore, for each particular digital camera, they can have their own specific focal length.
Now, at a certain focal length, say 25mm, the 35mm camera can take shot (of this apple) from a certain field of view, say 45 degrees.
As the digital camera's CCD or CMOS image sensors are of a different size with the 35mm camera's image sensor, to obtain a similar field of view of the apple, 45 degrees, the focal length of the digital camera will be different. In this case, the focal length of the digital camera is 20mm.
In short, there is a "35mm equivalent" because of the many different sizes of the image sensors of digital cameras, it is more convenient to express focal lengths in terms of "35mm equivalent" for easy comparison of cameras from different manufacturers.
Phew! What a long post! Happy Holidays!