Lisa Bettany, a popular photographer and co-founder of Camera+ app for iOS, has done a comparison of all iPhone models to date (excluding 6 and 6s Plus). The camera comparison compares the different generation models in various light conditions, macros and more.
Lisa used her app Camera+ to compare the camera between 9 iPhone models. The cameras were tested for following: macro, backlit, backlit macro, daylight, portrait, sunset, low-light and low-light sunrise.
TL;DR: apart from the low light photography comparisons, the results say that iPhone 6s has the best camera. It improves on previous generations thanks to 50% more focus pixels, deep trench isolation, faster auto-focus and better color reproduction. For low light photos, it does aggressive noise reduction which results in ‘pixelation and ‘a decrease in sharpness’.
Our opinion: iPhone 6s camera is not a major upgrade as evident by the photos and does not seem like one of the features worth upgrading your iPhone for, unless you want 4K video recording. If you have an previous generation model, you are not losing anything on still photography by not upgrading.
Note: it has been observed that Live Photos decreases low light photo quality
Check out the complete comparison here. Lisa has also done similar reviews in the past. You can see her comparisons for iPhone 4s, iPhone 5 and iPhone 6. There’s also a handy infographic which compares the specs of all models: