What is "35mm equivalent focal length?"

What does the 35mm mean in "35mm Camera"?

Let's get something out of the way.  Lots of things on a camera are measured in millimeters.  For 35mm cameras, there are two measurements that are sometimes confused with the focal length.

First, the 35 in "35mm camera" doesn't refer to the focal length or even to the size of the image on the film.  It is the width of the film including the sprockets.  35mm film is more properly called 135 format film and has these dimensions:

It can be confusing when a 35mm camera is described as having a 35mm lens.  Does this mean that the lens has a 35mm focal length (very possible) or that the writer is being specific about the camera, not the lens?  Sometimes it's hard to be sure.

The second measurement that is sometimes confused with the focal length is the diameter of the filter ring.  This is the threaded ring on the front of the lens barrel that accepts  Filter rings come in various sizes like 49mm, 52mm, 58mm.  This size is dictated by the diameter of the lens barrel and is independent of the lens focal length.

What makes a lens "normal"?

A "normal" lens produces a field of view that is similar to human vision.  The diagonal of the image plane is typically used as the reference for field of view determination.  A "normal" lens has a focal length that is approximately equal to the diagonal measurement of the image plane.  When the focal length is equal to the diagonal measurement it produces a diagonal field of view of approximately 53 deg which is similar to human vision.

For 35mm film (24mm by 36mm), the diagonal measurement is 43.27mm.   50mm is close to this measurement so it is considered a "normal" lens.  A 50mm lens produces a 47 deg diagonal field of view which is close to 53 deg.

What about digital cameras?

The CCD arrays on digital cameras are typically much smaller than the imaging area of 35mm film.  This means that the focal length that produces a "normal" field of view is much smaller than 50mm.

Unlike 35mm film which is the same regardless of what camera you use, the size of the CCD array varies from camera to camera.  This poses a problem.  You cannot know whether a focal length is "normal", wide angle or telephoto without knowing the exact size of the CCD array.  Unfortunately, even knowing the exact size of the CCD array may not help because sometimes the imaging area doesn't even cover the entire array!

Because many people are familiar with focal lengths of lenses for 35mm cameras, the digital camera manufacturers choose to describe the focal length of their cameras by reference to the focal length that would produce a similar field of view on a 35mm camera.

Consider, for example, a digital camera with a CCD array measuring 8.10mm by 6.08mm.  The diagonal measurement of this CCD array would be 10.13mm.  A lens with a focal length of 11.7mm would produce a diagonal field of view of 47deg, the same as a 50mm lens on a camera.  So for this camera, an 11.7mm lens would be described as having a 35mm equivalent focal length of 50mm.

By describing the lenses this way, the digital camera companies appeal to users' familiarity with 35mm camera equipment.

[from What is "35mm equivalent focal length?"  http://www.panoramafactory.com/equiv35/equiv35.html#What makes a lens "normal" ]

焦距(相当于35mm相机)是什么?

目前数码相机的成像器件面积都小于普通的135胶卷（即35mm胶卷相机）的面积，所以同样的视角时其镜头焦距相对于35MM相机要短，说到其镜头焦距时常不会涉及到其实际的物理焦距，而说与其视角相当的35mm（国内的135）相机的镜头焦距，也就是说，其“镜头的视角相当于XX”。 35mm胶片的尺寸是36 x 24mm，也就是我们平时在照相机馆中看到的最为普遍的那种胶卷，由于35mm焦长的广泛使用，因此它成为了一种标尺，就像我们用米或者公斤来度衡长度和重量一样，35mm成为我们判断镜头视野度的一种标注。例如，28mm 焦长可以实现广角拍摄，35mm焦长就是标准视角，50mm镜头是最接近人眼自然视角的，而380mm镜头就属于超望远视角，可捕捉远方的景物。