The exception in English to using "affect" as a noun is a fairly straightforward difference between "affect" and "effect," as far as exceptions go. There is really only one usage in which "affect" should be used as a noun instead of a verb.
The exception to using "affect" as a noun affects a narrow segment of options used when describing, speaking or writing about emotions or mood. Knowing this exception can help you know when to use "affect" as producing change and when to use it as a noun that represents an object.
For example, the noun "affect" becomes the right word to describe "an emotional response or impact."
This often is the case in psychology or psychiatry when the noun "affect" refers to an emotional response — for example, the gestures, body language, vocal tone or facial expressions that we produce as the consequence of specific emotions.
The result of using "affect" as a noun is that this "affect" — usually referred to as "flat affect" — also commonly is used to describe a lack of response.
The following sentences are examples of using the noun "affect" as an object word:
• She did not have an emotional response to the sad movie; her affect was flat.
• His accent was an affect because he was only pretending to be French.
While knowing the difference between "affect" and "effect" when writing or speaking can take a bit of getting used to, knowing when to deploy the right word — and what the exceptions to the right word are — will help you make more sense of the English language.