Digital or optical zoom lens?

What is better for me, digital or optical zoom lens? - Aamir

A lens, which is designed in such a way that the focal length of that lens can be changed, is a zoom lens. Focal length is the distance between the film plane (digital image sensor) and the noddle point of the lens. (read about lens)

This noddle point is center point of a single lens element. With combining more than one elements in a lens, the effective noddle point can be shifted beyond its actual physical position. In extreme wide angle, the noddle point is much closer to film plane, though the lens is not that close to it. On the other side, in extreme tele lens, (say 1200 mm) the noddle point is far outside the lens, as actually the lens is not two meter long.

In case of optical zoom, this noddle point is shifting within the range of the lens. So if the lens is 18 to 55 mm zoom, then the noddle point moves between this range whenever we zoom in or zoom out. This is achieved by providing floating lens element in a lens. Through precision machining, and mechanism, the floating element moves and shift the noddle point of the lens.

When we change the focal length of a lens to tele from wide angle, two things happen. The subject at distance appears closer and is enlarged. Secondly, the perspective of an image is compressed. Both these effects will not degrade the sharpness of picture and we will get crisp image when we zoom in.

Now consider that we are on analog, and after capturing an image, we further enlarge it while removing a print and then we crop and recompose it to get an an effect of extreme tele lens, can we get the same sharp image quality?

Well, the obvious answer is NO. Same thing apply in digital format. If we enlarge an image in a photo editing software, we are loosing the quality and sharpness of the image.

Digital zoom is doing the same thing with the in built software of the camera. So the result is not a true zoom but just an enlargement. That is why optical zoom is better than digital zoom.

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