Digital Zoom

Vincent Bockaert, 123di.com

Optical zoom is the number of times the maximum focal length of a zoom lens is larger than the minimum focal length. Consumer and prosumer cameras often come also with a digital zoom, which we will discuss based on an example of a 5 megapixel prosumer camera.

A. Scene shot with a 31mm lens B. Scene shot with a 50mm lens
Changing the focal length from 31mm to 50mm (50/31=1.6X optical zoom) reduces the field of view. In image B, the sensor captures the red zone indicated in image A. In both cases the camera will store 5 megapixel of information into a 5 megapixel image.
C. 1.6X Digital Zoom
Cropped and saved as lower resolution
D. 1.6X Digital Zoom
Cropped and upsampled to full resolution
A 1.6X digital zoom will only use the information of a 1,600 x 1,200 crop and discard the rest (2,560/1.6=1,600 and 1,920/1.6=1,200). In image C, the camera has captured the same field of view as in image B but only uses 2 megapixel out of the 5 megapixel resolution! If the digital camera has the option to output 1,600 x 1,200 images, the crop will be saved as a 2 megapixel image. In most cases, the 1,600 x 1,200 crop will be upsampled to the full resolution of the camera as indicated in image D. No additional information is created in the process and the quality of image D is clearly lower than image B.

To Use Or Not to Use Digital Zoom

So what is the best thing to do? If your purpose is to capture the information shown in image B, using a lens with focal length of 50mm is of course the best option. If you only have a 31mm lens available (or in general, if you reached the maximum optical zoom and need to zoom in more) there are three things you can do:

  1. The recommended approach is to shoot image A with digital zoom OFF and crop it later the way you want it.
  2. If the 5 megapixel camera has the option to output 2 megapixel images, then shoot with 1.6X digital zoom ON. The 1,600 x 1,200 crop will be saved without resampling and 2 megapixel of info is efficiently stored onto a 2 megapixel image. You save card space compared to image A, but lose the ability to change the way you cropped. This is recommended if card space is critical and is equivalent to cropping in the camera.
  3. It is generally not recommended to shoot with 1.6X digital zoom ON and output it as a 5 megapixel image because you are combining the disadvantages of 1. (more card space) and 2. (lose cropping flexibility) without major benefits[1]. You are saving 2 megapixel of information (crop C) into a 5 megapixel upsampled image (D). Upsampling cannot create detail that was not captured by the lens. Image B (optical zoom) has more detail than image D (digital zoom).

Footnote

  1. (1) If for some reason your intention is to upsample and you are shooting in JPEG, one benefit of digital zoom is that the upsampling in the camera is done before JPEG compression. If you shoot A, crop the 1,600 x 1,200 area, and then upsample to 2,560 x 1,920 on your computer, you will magnify the JPEG compression artifacts and the upsampled image will look not as good as image D. Because not all digital zooms are created equally, you may want to verify the quality differences with your particular digital camera before using digital zoom for this purpose.
This article is written by Vincent Bockaert,
author of The 123 of digital imaging Interactive Learning Suite
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