Use a variety of techniques to bring the ability to shoot stars with Google Pixel 4 phones.
Google has made a further improvement on the Google Pixel 4‘s Night Sight mode on Pixel 4 and introduced the Astrophotography astrophotography mode that can take amazing photos of the stars on a mobile phone. Now Google is in a special article Explains the secrets behind astrophotography.
Taking photos with a starry sky as the background has always been the exclusive domain of high-end DSLRs, and it is usually accompanied by professional equatorial mounts or photo stacking software to eliminate long-exposed star trails.
In order to increase the amount of light, astrophotography must rely on a long shutter. In a case where the environment is sufficiently dark, the shutter of the Pixel 4 can be as long as 4 minutes. But this is not a continuous 4 minute, but 15 independent photos, each no longer than 16 seconds.
This is to prevent the deviation of the bright points (stars) from being too large when the photos are superimposed, so that the stars maintain a point shape, rather than dragging a line. As for the 4-minute duration, it is a limitation defined by the general user’s patience and uses the environment. It is not that the mobile phone can no longer take longer exposures.
The next thing the Pixel 4 has to deal with is the noise generated by the photoreceptor itself after long exposure. Because the structure inside the CMOS chip is always imperfect, the current generated by it will also fluctuate slightly, which is not uniform. This slight difference is not obvious under short exposures, but under long exposures, they will make some parts of the picture appear particularly bright and form “hot spots”.
Google Pixel 4 knows how to compare multiple photos, find these hot spots that don’t move and replace them with the average of the surrounding pixels.
How to frame in such a low light source is another challenge. In this regard, Google Pixel 4 takes advantage of the previous feature of a unit photo every 16 seconds. After each photo is taken, it will be displayed on the screen. Although it is still too dark compared with the final product, it is enough for the photographer to adjust the angle and position of the framing, and then take a photo again. You don’t have to wait until all 4 minutes are over to see the result.
On the other hand, when the screen is too dark to auto-focus before shooting, the Pixel 4 will also use the time just pressing the shutter button to take two auto-focus photos that can last up to one second as the basis for contrasting auto-focus. If you can’t see anything even this way, the astrophotography mode will automatically switch the focus distance to infinity.
Finally, Google Pixel 4 ’s astrophotography mode also has a built-in “Sky Judgment” AI system that can find out which pixels are the sky and which are not. This allows the Google Pixel 4 to adjust the brightness and contrast against the sky, highlighting features such as clouds of the Milky Way, but without changing the foreground ground.
The end result is that at night with no clouds, no moon, and no light damage, you can shoot a very amazing starry sky with your mobile phone alone, even beyond the range that human eyes can see. If there is a little moonlight, Google Pixel 4 can even shoot foreground landscapes with accurate colors.
The only problem is that when the moon and the stars appear in the picture at the same time, because the brightness contrast between the two is too large, the stars cannot be photographed. However, Google has expressed “stay tuned” at Pixel 4’s previous press conference.