The filters used  in narrowband imaging typically have bandpasses in the range of 3-5nm.  The most popular filters are made by Astrodon and Custom Scientific. 

Older Custom Scientific narrowband filters had a bandpass of 3nm.  Current filters typically have a bandpass of 4.5nm, while the popular Astrodon H-alpha filter 

has a 6nm bandpass.  The bandpass was expanded to allow the use of the popular faster-focal-ratio scopes such as the Takahashi and TeleVue refractors, etc.  

At fast focal ratios, narrowband filters shift off band, meaning they shift away from the wavelength they are designed to capture. 

The effect is a significant decrease in sensitivity.  A wider bandpass allows the required emission line to remain within the filter's highest transmission zone even

if it has shifted slightly.  For very fast systems, an even wider bandpass is required.  For telescopes faster than f/4, such as the Takahashi Epsilon astrographs 

(f/3 to f/3.8) and Celestron HyperStar-equipped SCTs (f/1.9), a 10nm bandpass filter is required.