Raws li lub npe qhia, daim iav bifocal muaj ob lub luminosity. Feem ntau, nws yog siv los saib qhov kev ncua deb, xws li tsav tsheb thiab taug kev; Cov hauv qab no yog pom qhov ze luminosity, pom qhov ze, xws li nyeem ntawv, ua si hauv xov tooj ntawm tes thiab lwm yam. Thaum lub lens bifocal nyuam qhuav tawm los, nws tau raug suav tias yog Txoj Moo Zoo ntawm myopia + presbyopia, tshem tawm cov teeb meem ntawm kev xaiv thiab hnav, tab sis raws li tib neeg siv, nws pom tias qhov tsis txaus ntawm lub lens bifocal kuj muaj ntau.