What is adaptivity?

The purpose of adaptivity (adaptive image sampling) is to make noise level uniform across the whole image. By focusing computing power on the most problematic areas it reduces the risk of situations where some parts of the image are noise-free, and some parts are still noisy even after many passes or long rendering time (such as shadows or glossy reflections). This, along with denoising (3ds Max | C4D), can greatly reduce the render times needed to produce a clean image.

The only available parameter affecting adaptivity calculations is "Adaptivity recalc. (passes)" which decides how often adaptivity is recalculated. The default value should always be used, and that is why this setting is hidden in the Development/experimental stuff rollout (3ds Max | C4D).

Disabling adaptivity makes it impossible to use the noise level limit.

How to use adaptivity?

Adaptivity is enabled by default and should never be disabled. There is simply no reason to ever disable it in production renders. However, if you really need to disable it (for example for testing purposes), you can do it in the Development/experimental stuff rollout (3ds Max | C4D). 

Note: you may notice it sometimes takes longer to finish a certain number of passes with adaptivity enabled, but you actually need less passes to achieve the same quality. For example, if a scene took 100 passes to clean up a shadowed area while other areas were noise-free after just 50 passes, with adaptive image sampling you may now need 75 passes to get a clean result in both the "easy" and the "problematic" areas. If you choose to render for the same 100 passes as before, you will find those 100 passes take slightly longer with adaptivity enabled, but you should not need that number of passes anymore. This results in an overall time saving – the best way to compare results to see the difference it makes is to limit your render by time rather than by a number of passes (3ds Max | C4D).

Examples