I sold my DSLR and in favor of an iPhone because I love to "travel light" and I also "travel light" when it comes to apps. That's why I use only a total of 4 iPhone camera apps. The built in camera app plus three more for special settings that don't work well with the stock camera app: long exposures, HDR and low light photography.
You may (or may not) be surprised that the stock camera app is my most frequently used camera app. It has focus and exposure lock, a super fast burst mode, allows me to adjust exposure, a rules of thirds grid, decent HDR, live filters and live photos, a self timer, it records video in 4K and produces excellent results in good light situations.
But this is also the limitation of the iPhone camera app. It requires a decent amount of light and does not work well in low light situations or at night, for shooting HDRs or capturing long exposures.
The stock iPhone camera app will get you quite nice photos in good light situations. But as soon as you want to take photos with your iPhone in low light situations or want to capture light trails of cars driving by at night you'll see the its limitations. Before you go and buy another camera app, I strongly recommend to check if you really need one.
Ask yourself what kind of scenes you shoot and what you need. Do you need manual ISO control for low noise photos? Do you want to shoot HDR? Do you want to take photos at night? Do you need manual focus control? Do you need priority modes like ISO or shutter priority mode? Do you want to take long exposures? Do you want to shoot photos in RAW format?
And then, depending on your needs, find a camera app that supports those functions. I tried dozens of camera apps and I only kept three that perfectly supplement the stock iPhone camera app for the kind of scenes I shoot which are low light and night photography, long exposures, and HDR.
Here's a screenshot of my camera app folder containing the four iPhone camera apps I use. Read on to see for which kinds of scenes I use those apps.
Low Light and night iphone Photography
ProCamera excels here and offers up to 4 different modes for low light photography. The four modes are Low Light, Low Light+, Low Light+ with Lux+ and Low Light HDR. Moreover, ProCamera also has a manual mode which I can use to set ISO and/or shutter speed manually. And since iOS10 is out, ProCamera can also shoot RAW.
Shoot HDR and 32 megapixel Photos
For HDRs and if you want to shoot photos up to 32 megapixel (yes, that's correct), turn to Hydra. The HDR effect is a little stronger than what you would get with an iPhone and works great indoors and in calm outdoor environments.
Shoot Long Exposures with your iPhone
Long exposures are the domain of Slow Shutter Cam. And when I say long, I mean like up to 30 seconds and even longer. That's great for shooting spinning ferris wheels or to get the smooth and silky touch of waterfalls. In the latest update, Slow Shutter Cam got an option to control ISO manually.
With iOS10 Apple introduced a way for iPhone camera app makers to capture photos in RAW image format. Shooting in RAW gives you the unedited, unfiltered data from your camera sensor. I use ProCamera App whenever I want to shoot in RAW
And here are the four camera apps I use at a glance. The selection may surprise you as it does not contain all of the well known apps that you'll read about on the web. But after trying lots of camera apps, those are the four that work best for me and for my travel photography. Those four apps allow me to shoot everything I want to shoot and they have survived many "app spring cleanings" on my phone. Those are the "fantastic 4" in no particular order.