|5x and 10x loupes used in examining film|
image by Voxphoto (Image rights)
A loupe is a small, mounted magnifying glass; photographers often use these to examine negatives.
A magnifier is sometimes fitted into a viewfinder hood (e.g. in TLRs) to enlarge the viewfinder image, as an aid to focusing.
|Folding rectangular loupe|
in an Exa 1a waist-level finder
image by Uwe Kulick (Image rights)