What is the difference between digital zoom vs optical zoom for an imaging system? This is a question that some users ask; while all experts agree that optical zoom is much better and more effective, some may have a hard time explaining why.

Imaging systems (thermal, visual, or other spectral areas) provide visual information on distant targets, beyond what the eye can see. To gather enough information to perform imaging tasks such as detection, recognition, and identification, the imager must provide an appropriate field-of-view to allow enough magnification so that the user can see the necessary details. To increase magnification (make the image appear bigger), the field-of-view (FoV) must be decreased. Here’s where some confusion begins to take place.

Optical Zoom

Optical zoom systems move the optical elements to change the focal length of the imaging system; longer focal length means more magnification, which equates to the ability to image a target at a greater range. Optical zoom systems can be either stepped zoom (where there are fixed, discrete zoom levels) or continuous zoom, in which the focal length can be continuously adjusted to whatever setting the user wants. Step-zoom systems are simpler, from an optical design standpoint; some years back, these were common in the thermal imaging world. Continuous zoom designs have been the standard in visual imaging for many years and have become much more common in thermal imaging since around 2010. Today, nearly all variable-focal length thermal imaging systems use continuous zoom capabilities.

A key attribute of an optical zoom system is that the image created by the optics utilizes the entire sensor array, regardless of the zoom setting. This means that the optical resolution (the ability of the system to discern small details) increases as the FoV is narrowed. This is where the optical zoom system differs from digital zoom.

Digital Zoom

In a digital zoom system, the pixels in a section (usually the center) of an image are expanded, though the optics maintain the same focal length. The pixels in the expanded area are simply replicated, then smoothed using advanced image processing algorithms. This zoomed in area appears larger on the screen, but the effective array is smaller. The new image is only using a subset of the pixels from the original image, this digitally zoomed image contains less information than the original image. This is in sharp contrast to an optically zoomed image, which uses all available pixels to form a more resolved image.

Notice how the image shows less detail as the zoom level increases, Figure 1. At high zoom levels, the image begins to show pixelated images because the system is trying to form an image with less information (smaller number of pixels that are being expanded).

Although these effects always happen with digital zoom, not all digital zoom algorithms are the same. In Figure 1, notice that the zoom levels (1X, 2X, 4X, and 8X) are all powers of 2. This is because decreasing the image size is computationally easier if it’s done by powers of 2. Also, the algorithm used to smooth this image is called ‘bi-linear’1 smoothing, where simple averaging of pixels around it are used to smooth the image. Stepped digital zoom (by powers of 2) and bi-linear smoothing are simple algorithms that can be done on low power processors and are therefore common on many thermal imagers. The markedly lower quality (beyond 4X) is why most digital zoom systems don’t go beyond 4X, or sometimes even beyond 2X.

With all of this being the case, is there any use for digital zoom? The answer can be yes, in some limited circumstances. The main circumstance when digital zoom can be of legitimate use is when viewing an image on a very small monitor. If the monitor (or portion of a monitor, in the case of a windowed screen) uses less pixels than the actual image does, then the user can digitally zoom up to that point with no real loss of resolution. While this is not frequently the case, it can happen. For those cases when it does happen, though, the quality of how the digital zoom is performed can make a significant difference.

Better quality, continuous digital zoom (sometimes called fractional zoom) can be performed, but it requires higher power on-board imager processors. This allows the user to digitally zoom to any arbitrary level, not just levels that are powers of 2. IEC Infrared Systems utilizes continuous digital zoom, and pairs it with a more sophisticated type of smoothing, called a Lanczos algorithm, which yields a much smoother digital zoom, and even allows it to be usable at higher digital zoom levels, Figure 2. Note that even up to 12X level, the general quality is preserved to a greater degree than that seen at 8X in the bi-linear method.

Summary

To summarize, optical zoom is always superior to digital zoom, because optical zoom uses all the pixels in the imaging array, not just a subset. This preserves the imaging system’s resolving capabilities, even in the narrower FoV. Digital zooming discards information from the image, in that is simply magnifies a subset of pixels, and discards the remaining pixels. Digital zooming can have some use, in limited cases where the viewing window is quite small. Digital zoom capability remains popular with many users, despite its limitations. Therefore, for both the cases of small viewing areas and for general user interest, the more sophisticated continuous zoom, with Lanczos smoothing, as performed by IEC Infrared Systems imagers, provides a better, more usable digitally zoomed image.

¹ Some systems use ‘bi-cubic’ smoothing systems, which are slightly better but generally similar to ‘bi-linear smoothing schemes.