Use Different Camera Apps to Improve Photo Quality
The Nexus 5 takes decent, average pictures. Key facets such as focus, exposure, and white balance are generally accurate. Of the three, exposure is most apt to be off a bit. The Nexus 5 has a hard time compensating for background light sources, for example, unless you think to turn on HDR mode first. Indoor shots are full of grain in many instances, but good use of the flash can help offset that. If you just point and shoot, you're never going to get anything other than average photos. If you take some time to adjust the settings, you'll be more likely to get better images. One thing I can say about the stock app: it is the fastest of these three to launch and the fastest to focus/shoot images. Sometimes speed really matters.
In general, I was quite pleased with all the images I captured using the FV-5 app. Images were in focus, well-exposed, and had proper white balance. It is a little slow to focus sometimes, and that can affect the outcome depending on the subject. For still lifes, it doesn't matter, but for action shots it might. Colors were bright and looked accurate. Speaking in broad terms, FV-5 produces slightly better results than the stock camera app from Google - even when used in just regular auto mode.
The benefit of FV-5 is that it really opens the door to creativity. I mean, the option to take a 30-second exposure is just bad-ass if you know how to work an exposure like that. (See my 2-second exposures of a moving subway car below.) Throw in stuff like bracketing, different metering modes, and adjustable interval shots, and you can really have some fun. The biggest surprise to me was macro mode. With the stock Google camera, the N5 is terrible at focusing closely. With FV-5, I was able to position the N5 much closer to the subject and still walk away with an in-focus image. Keep in mind, the optics didn't change. This clearly demonstrates the limits of the stock camera app and how using a different camera app can make a real difference.
Camera ZOOM FX
This app is a different animal. It combines useful shooting tools that surpass the stock camera and also includes some fun effects for producing artistic shots. When used as a regular camera in auto mode, the images were a hair sharper and had slightly better white balance to my eyes when compared to the Google app. Keep in mind, however, that I was only able to see these differences with the images blown up across my 27-inch monitor. The macro mode worked, but not as effectively as with the FV-5 app. I was able to get closer than with the stock app, but not as close as FV-5.
The wide range of scenes gives you a much higher chance of scoring accurate lighting in what are otherwise troublesome environments (parties, fireworks, etc.). You have to be smart enough to change these settings ahead of time, though, and the clumsy UI makes that harder than it should be. ZOOM FX doesn't offer as much control over the basics of the shooting experience as FV-5, but the built-in effects can make picture taking far more fun than the stock app.