Digital Zoom - A brief discussion with recommendations



This article was co-authored by Terry Bibo and Mike Boesen.  It appeared in the April 2006 issue of the PC Users Group magazine Sixteen Bits.

In the early days of only optical lenses on cameras we had to carry around a bag of various focal length lenses to meet different needs - wide angle, telephoto, close-up. Ponderous!  Then we had the luxury of zoom lenses that encompassed focal lengths from wide angle to quite powerful telephoto in the one lens. A further development embraced macro photography, so the one lens was then capable of close-ups of insects or flowers through wide angle to telephoto. At the same time lens construction advanced to reduce the physical size and weight of the lens by an astonishing degree. Approaching photographic Nirvana.

Then came digital cameras, becoming increasingly smaller and lighter in weight. And with them came the concept of digital zoom to reduce the need for the physical components of telephoto lenses. That feature could be accomplished by digital manipulation of the image within the camera, in the same way that we resize pictures using digital image editing software on our PCs.  Now my 3.0 megapixel old Olympus with an 8x optical zoom and a 3x digital zoom claims to give me a seamless zoom to 24x. But is this really an acceptable alternative to optical magnification?  In a word - "No".  However, in some situations, for some people optical zoom may be useful. 

Cameras achieve optical zoom by moving lenses to increase the magnification.  What happens is that the image that is directed onto the full area of the sensor in the camera is a magnified part of the image that would appear if there were no zoom.  An unzoomed image occupies all the full area of the sensor;  the zoomed image also occupies the full area of the sensor and is simply a magnified smaller part of the real life image. 

In most, if not all digital cameras that have both optical and digital zoom, the digital zoom is implemented only after the full extent of the optical zoom has been reached.  However, to keep things simple, in the hypothetical example of optical versus digital zoom that follows, I have compared what happens with a 3x optical by itself and a 3x digital zoom by itself. 

Optical zoom example

In my 3 MP (megapixel) camera I can set various resolutions for my picture, from 640x480 pixels to 1984x1488 pixels (the maximum for a 3 MP camera). For convenience, lets see what happens if I get my camera to record an image of a bird in a tree, with a resolution of 1600x1200 pixels. 

An unzoomed image encompassing the whole tree will produce an image of 1,920,000 pixels (1600x1200).  Let's say we do a 3x optical zoom to achieve an image comprising just the bird, the size of the captured image will still be 1,920,000 pixels.  However, the area captured in this image will be only 1/9th of the area for the unzoomed image of the whole tree. 

If we compare the bird part of both images, the resolution (i.e. fineness of detail captured) for the optically zoomed bird image will be much greater than for the bird portion of the unzoomed image: the bird in the unzoomed image will be represented by 213,333 pixels of information; the bird in the 3x zoomed image will be represented by 1,920,000 pixels of information.  

Interpolated digital zoom example

An image captured via digital zoom is quite different.  There are two types of digital zoom.  The most common form of digital zoom involves image "interpolation" and is discussed here first.  The second type is called "smart zoom" and is discussed later.  For our example, if we still want the bird as a 1600x1200 pixel image, and were to do a 3x digital zoom, the camera would create a crop comprising 1/9th the 1,920,000 pixel image of the whole tree - that part of the image that contains just the bird.  That crop contains only 213,333 pixels of information.  The 213,333 pixels in the crop would then be used to generate a 1600x1200 image containing 1,920,000 pixels.  The extra 170,666,666 pixels are generated by the camera's software using a process called "interpolation".  

If you were to compare those images of the bird captured using optical zoom and digital zoom, the optical zoom image would provide a more accurate depiction of what the bird really looks like.  The digital zoom image would be a less accurate depiction because the interpolation process has created (fudged up) a whole lot of pixels.  In some circumstances, the quality of such an image may be quite adequate.  For example, it may be quite OK in creating a 6 x 4 print.  Loosely equating pixels on a monitor to dots per inch on paper, we can see that a 1600x1200 pixel image will print at 5.3 x 4.0 inches at 300 dpi - or near enough to the standard 6 x 4 postcard sized output that can be produced from one of those ubiquitous print kiosk machines.  If you don't intend to produce anything larger than postcard size photos, then even 100 dots per inch can be acceptable. And the resolution of an interpolated image may be quite adequate for viewing on many PC monitors or inserting into HTML pages for the Internet, or as an attachment to an email.  However, if you plan to generate larger prints - say, 8 x 10 size - portions of the print may appear fuzzy and with peculiar colouring. 

In contrast, a 3x optically zoomed image of the same resolution as an interpolated image created through digital zoom will display a more accurate representation of the actual object.  When viewed on a high quality PC monitor, or if printed as a largish sized picture, the result should be much clearer, realistic and without interpolation artefacts. 

If you do decide to use a camera's digital zoom feature to capture an image, one way to create a higher quality result is to use a higher resolution.  For my camera, for instance, I could generate pictures with a resolution of 1984x1488.  In that case my digitally zoomed cropped digital image would have contained more detail at 328,021 pixels.  So cameras with a 5 or 7 megapixel image sensor provide more capacity for exploiting digital zoom.  However, the sizes of images saved are enormous and would severely restrict the number of photos you could fit on any memory card.  Not worth the effort in my opinion. But if you are planning on printing big pictures you can surely use all the pixels you have available.

A straight comparison of digital and optical zoom is presented at:

Smart Zoom

There is at least one different way of undertaking digital zoom:  Sony has a digital zoom function that it calls "smart zoom".  This achieves an in-camera digital zoom that does not involve interpolation with its associated artefacts and loss of original information.  What it does is to save as a non-interpolated image a smaller part of the full image.  So for instance, if the camera's sensor is 5 MP, the image that is actually saved may be the 3 MP central portion of that full image.  What would be saved is an image smaller than the maximum available but with the same image quality. 

More information about smart zoom, and a fuller explanation of digital zoom in general are available in these articles: ,

And if you are really interested in a very technical exposition with examples:

Bottom line

If you intend to process a digital image yourself using any of the excellent digital imaging applications that are available such as Photoshop or Paintshop Pro, keep in mind that you would be able to use the application to undertake the cropping and interpolation process that the camera undertakes in making a digitally zoomed image.  In doing that work yourself you may be able to achieve a better result than the camera does.  For example, you have the option of selecting exactly what part of the full area is to be selected as the crop, so as to get a good composition.  In addition, if you are going to make other enhancements involving interpolation (e.g. sharpening) you may obtain a better result, because you can manipulate the original information that was captured rather than undertaking interpolations on an already interpolated image. 

Even smart digital zoom is still only simulating the net result of an optical zoom by saving a crop of part of a full-size image.  If you intend to edit and manipulate your digital images in an image editing application, then you can achieve the same or better outcome through in-application cropping of the full size image.  Again, you may be able to achieve a better outcome because you can locate the cropping borders so that an ideal composition is achieved.  With an image saved with smart zoom, anything outside the borders of the "zoomed" image is not saved.

If you have the software and skills required to undertake editing of digital images yourself, my view is that you will gain nothing through use of interpolated digital zoom or smart digital zoom.  In fact, you may be able to achieve a better outcome through your own editing of an image that has not been generated via digital zoom. 

However, if are not ever going to do any image editing yourself and/or you are only going to get small prints made, and/or all you want are small images to be used in web pages or email attachments, digital zoom can be useful, especially "smart zoom". 

Terry Bibo & Mike Boesen