Twilight Factor Definition in Binoculars

By Rafael Chamón Cobos

Created Nov. 2008

Reviewed Aug. 2014

The following table shows twilight factors of some common binocular types according to the usual definition (Twilight factor 1).

A new column (Twilight factor 2) has been added with values corresponding to a new proposed definition. This definition takes into account a reference value of TF = 1 corresponding to the wide opened naked eye as an optical instrument, in which values of M = 1 and A = 7mm are assigned. 

With this definition, common binoculars get reasonable figures between 1 and about 20, being the value TF = 1 the reference value for the wide opened naked eye. Note that all binoculars having 7mm exit pupil get TF values equal to their magnification factor. This is a reasonable assumption: the more magnification, the more details are distinguished.