Function
While taking a picture, the photographer may need to focus on one area in the picture frame. For example, when the photographer takes a portrait photo, they may want to make sure the object’s face fills the photo frame; and when taking a group photo they want to make sure everybody is in the photo frame. In such situations, the photographer can either physically move closer to the objects or use the camera’s zoom feature.
When using the zoom feature the camera enlarges that area to fit the full picture frame. There are two types of zoom – optical and digital.
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Optical zoom works by physically moving the camera’s lenses and changing the focal length. Optical zoom lenses physically extend to magnify the subject. A motor controls the lens movement according to the user’s command. When the user rotates a special button or presses the switch to the subject is either magnified or reduced in size.
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The user can take a photo of the subject from a greater distance and still get a clear, close-up image in a higher rating optical zoom camera. The concept of optical zoom is the same as the zoom on a non-digital camera. Optical zooms have adjustable focal length and every optical zoom lens has a range. By changing the focal length the photographer can make objects appear bigger and fit in the full photo frame. When satisfied with the zoom position he can shoot the photo by simply applying the shutter button.
Digital zoom is a technique to decrease or narrow down the apparent angle of view of an image. It is accomplished by cropping the image down to a centered area with the same aspect ratio as the original, and enlarging the result back up to the pixel dimensions of the original. This is done electronically, without any adjustment of the camera’s optics. Enlarging disturbs the original pixel layout of the image, which is captured by the camera’s image sensor. So it reduces the quality of the image. With digital zoom, photographers use built-in software in the camera to define a portion of the photo.
After selecting the portion, the software crops the rest of the photo and enlarges the selected area to fit the complete photo frame. The software calculates new values for the pixels that were cropped in order to result in a full frame photo. The main disadvantage of this digital process is that the enlarged photo quality is lower than the original photo taken.
For example, lets us assume that a photographer has a 8MP (2 megapixels) camera, and wants to zoom in 2X via digital zoom. To accomplish this zoom, the camera crops half of the photo and enlarges the other half to create a 2X zoom effect. In the process a 4MP area is discarded. The other 4MP area is enlarged in a process that copies every pixel once to generate a 8MP photo. Although the new photo seems to include 8MP it really includes only 4MP of information. This results in a photo with a quality equivalent to a 4MP photo.
Pros and cons
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Professional photographers prefer to use optical zoom because:
- It does not reduce the picture quality or resolution of the image.
- Optical zoom is not dependent on the resolution (megapixels)
Optical zoom capabilities can make a big the difference in the final photo. The higher the optical zoom, the photographer can take the photo of the subject from a greater distance and still can get a clear quality shot. In digital zoom, enlarging the “zoomed” area reduces the image resolution and the image quality. So using digital zoom is not a good idea. The same kind of enlarging, cropping can be done later, using photo editing softwares, like Adobe Photoshop. In fact using PC software is always the preferred method to built-in digital zoom since it allows the photographer to try different zoom sizes, different zoom areas and different zoom algorithms while not losing the original photo.
Optical zoom is expensive and difficult to implement, especially in smaller devices like phones. In such cases, digital zoom offers a good alternative. Digital zoom can also be used to augment the zooming in capabilities of optical zoom. e.g., one can achieve a 10X zoom via 2X optical and 5X digital zoom.
Zoom factor
While cameras may have a digital zoom of over 700x, cameras are generally restricted to a zoom of 25x. In terms of simple zooming capability, it may mean that digital zooms bring a picture closer or zoom closer but since they lose picture quality so they do not capture the picture well. So a 25x optical zoom may turn out better results than a 700x digital zoom also. When cameras are advertised, they have much much higher digital zoom than optical zoom.
References
- Wikipedia: Optical zoom
- Wikipedia: Digital zoom
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