The new Pixel 4 has launched with some awesome photography chops that make owners of previous generations of the device jealous. While Google will eventually bring a few of these to its older phones, that's not the case for all and the Android community has already stepped up to fill the void. The most prominent Google Camera mod, GCam by cstark27, has been updated to bring several Pixel 4-exclusive features to older Pixels.
Astrophotography, the Pixel 4's most jaw-dropping photography skill, is the headliner of this mod update. It should work on both Pixel 1 and 2; all you need is switch to Night Sight and keep your phone extremely steady then tap the shutter button when the "Astrophotography on" toast message shows up. I tested it on my Pixel 2 XL and was able to take many 60-second exposure photos. If I had less light pollution around me, it should have gone higher.
It's a very cloudy night, so you'll need to look closely to see some stars toward the upper part.
Another backported feature is Super Res Zoom (which came with the Pixel 3, not Zoom+ which launched with the 4). I tested it against the regular Google Camera and saw a sharper image as a result when zoomed at up to 7x on my Pixel 2 XL.
Left: 4x zoom with Google Camera. Right: 4x zoom with GCam mod 7.2. Bottom: Same, cropped.
The mod also contains a lot of new settings and improvements, like focus distance and Super Res Zoom while in Night Sight, the Photobooth mode (look for the Auto timer in the selfie camera's settings), and synthetic fill flash to brighten up faces when shot against a bright background.
Left: Night Sight focus distance and "Astrophotography on" toast. Right: Photobooth for selfies.
* Enable Astro mode for Pixel 1 & 2
* Enable Focus options in Night Sight (along with "infinity" - may be same as far)
* Modded lib for Learned Depth in Portrait Mode (Pixel 2)
* Enable Super Res Zoom while zoomed or in Night Sight (no zoom required) - this is like Pixel 3 stock
* Enable auto timer (photobooth), it's in the selfie camera in the Timer options
* Enable synthetic fill flash for Pixel 2 and enable SFF in Camera mode for 3a
cstark27 says he tested these on his Pixel 2, as did I, but some of them should work on the first and third-gen Pixels as well. To try them out, you can grab GCam PXv3.0, which is based on Google Camera 7.2, from the official source or from APK Mirror.