Why Digital Zoom May Not Be Completely Evil After All

If there’s one rule that I’ve drummed into people over the last decade it’s that digital zoom in smartphone cameras is a no-no. You gain nothing and, if anything, actually degrade the image of what you’re trying to capture. And I bet that you’ve seen some horrendous examples of digital zoom in action in the past, with little more than VGA resolution images blockily upsampled to 5 megapixels because the user ‘wanted to get closer’.

Which is why I find myself, somewhat shockingly, pulling a slight about turn on the subject of digital zoom. Don’t get me wrong, it can still produce ugly results in the worst cases but, used wisely, it can help rather than hinder.

Here then is everything you ever wanted to know about when it’s OK to use the digital zoom built into every smartphone camera.


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Anyone who knows me, of course, will be waiting for me to start evangelising the lossless digital zoom in the Nokia 808, running Symbian. But that’s a special case, with a camera that works in a totally different way — this feature is about the ‘normal’ cameras built into ‘normal’ Android-powered smartphones.

Digital zoom, as the term suggests, is a way of appearing to ‘zoom in’ on a subject without needing fancy lenses or optics. On-screen, everything viewed gets dramatically larger and there’s little clue that you’re not really zooming at all. Later on, when looking at saved images more closely, you’ll notice that ‘zoomed’ photos are a lot less crisp than usual and, comparing details with images taken without zooming, you’ll notice little difference. In other words, zooming in hasn’t actually gained you any detail at all.

Or has it? Under certain conditions, I now contend that there are some advantages to using digital zoom, here tested extensively on a typical high end Android smartphone, the Samsung Galaxy S III.

The Good and Bad of How Digital Zoom Actually Works

Think about how digital zoom works, shown diagrammatically below. As you ‘zoom in’ on your smartphone, all you’re effectively doing is cropping in to a smaller part of the image being projected onto the phone camera’s sensor. This cropped part is then shown larger on your screen — in the (bad) old days, the ‘pixels’ would just be shown larger, so you’d zoom in by 2x and a pixel would then be shown on-screen (and saved) as a block of four pixels in the JPG image. In other words, everything would just get blocky.

Digital zoom

Note how the amount of sensor used decreases as digital zoom (in red) is applied

Now, don’t get too excited, the diagram shows that the laws of physics are absolute here. Zoom in by a factor of (say) two and you’re only using half the sensor — in fact, a quarter, in terms of surface area. So, as per the traditional wisdom, you’re still not gaining any more detail as you zoom in, you’re just using less and less of the sensor.

Think now about what modern phone camera electronics and software do in order to present and save the zoomed image. You may be zoomed in by 2x or 3x or 4x, but the Android camera software has been set to output (say) five megapixels. As a result, in a fast and modern phone, intelligence is used to ‘interpolate’ between the data from individual pixels, i.e. imaginary pixels are inserted in between, with values such that they marry up nicely to the real data. The result is that, although you don’t really gain more data, the finished image can look slightly smoother. Slightly. In practice, on average, you usually lose as much image quality as you gain and digital zoom is therefore not worth bothering with.

But this is where I twist things slightly, and don’t worry, I have real life example images to back my thoughts up. There are in fact four reasons why you should consider using digital zoom when appropriate.

1. Convenience and visibility

Most obviously, when you zoom in, you can see the subject more clearly on the screen of your phone, meaning that you can time a shot to get the right moment/expression, as needed. If, as with me, your eyesight is merely ‘OK’ and not 20:20, then your phone camera may be able to ‘see’ a subject better than you can, and digital zoom can help here.

2. More Accurate Exposure

The camera in your Android smartphone is, of course, fully digital. The exposure of each photo is automatically calculated by looking at what’s in the viewfinder and making adjustments for capture speed according to how bright the overall scene is. This works out fine for most subjects, but what if you’re trying to capture something in the shade amidst a bright foreground, or vice-versa, a very bright subject in a darker overall scene? In the former case, the subject would be far too dark, in the latter, far too bright.

Garage scene

Typical scene, sunny day, aiming at detail in shade

Happily, by using the much-maligned digital zoom to re-frame the image around your subject, the camera software and electronics recalculate exposure for what’s now in the frame. This means more accurate shots in terms of light levels and (usually) visible detail.

Comparison

Left: original detail in shade, Right: detail in shade after using digital zoom to ‘crop’ in

3. More Accurate Focus

In the same way as for exposure, by digitally zooming in, you can explicitly tap on a distant subject more accurately. So instead of the camera software guessing a focus point, you’re able to zoom in to some extent and surgically pick the exact subject to focus on (i.e. maximise contrast for).

4. Helpful Pixel Interpolation in Less than Perfect Conditions

Think about the difference anti-aliasing made to your use of computers fifteen years or so ago (if this is before your time then you’re excused). All of a sudden, jagged, pixellated fonts on computer monitors became smoothed out, prettier and easy to read, by the simple expedient of using the pixels around (typically) black and white pixels to show various shades of grey, in such an intelligent way that, when looked at from a distance, the jagged fonts appeared higher resolution and smoother than they really were.

Something similar can happen, under certain circumstances, when looking at interpolated detail in a digitally zoomed image.

Now, in bright light (e.g. in the sun), there’s usually such clear cut, hard and fast detail available in the image that any zooming and interpolation is a fool’s errand — nothing is really gained, as shown below: 

Comparison

Looking at detail with and without digital zoom in bright sunlight. Zero gain, as discussed in the text

However, when conditions are slightly tougher, at the point where you’d typically start to see digital noise and artefacts in a camera phone image, the interpolation mentioned above in a modern smartphone camera, aided by a fast processor and plenty of RAM, can help, by trying to improve sharpness and fill in ‘detail’ that’s in danger of getting lost. Here are a couple of examples:

Comparison

Cropped detail from an indoor gloomy scene, showing the difference between that in a native full-res image and that in the digitally zoomed version.

Comparison

Tight crop of detail from a photograph of my office desk – this was some scribble on a post-it note. See the difference in clarity, focus and detail between shooting the wider scene (left) and the digitally zoomed version (right)

So Digital Zoom is Good to Go, Right?

Buoyed up by the four points above, you may be tempted to head out and use digital zoom on every photo. Don’t. I’m specifically picking out use cases and examples to show that digital zoom is not all bad. In general, you’ll get better results by not using it all.

And, if you do decide to experiment with digital zoom, go easy on it. Go to 2x and see if that does the job for you in terms of re-framing, exposure and focus, don’t just slide the zoom control (or use buttons, if appropriate) all the way to 4x or 6x (i.e. maximum) ‘just because you can’.

As in so many things in life, less is more. But at least I’ve hopefully dispelled some of the bad press that digital zoom has acquired over the years.


  • Gary Gotham

    Even to a photo muppet like me the logic for the use cases presented made sense. Thanks for enlightening me. Less is more. Got it. :-)

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  • https://stacksocial.com/ Stefan Wrobel

    Wow, great stuff! I was always firmly anti-digital-zoom but I have to say I haven’t re-evaluated in years. Thanks for doing the work for me :)

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