Unknown button canon 500D

[Petejones]

I have a Canon EOS 500D/Rebel T1i

Does anyone know the purpose of this button just below the lens release, as it doesn't seem to do anything

 

[JackBarley]

Depth of field preview button.

 

[andyred]

As above

 

[Petejones]

How does it work? As it doesn't seem to change the viewfinder at all

 

[vetonbike]

Put into manuel mode at f22 focus on some thing a few feet away . Look through the viewfinder then press the button and see how the depth of field changes.

 

[Petejones]

Thanks for that all clear now