Lens measures tail of rat in various periods (8)

'lens measures' is the definition.
(unit of refractive power for measuring lenses)

'tail of rat in various periods' is the wordplay.
'tail of' says to take the final letters.
'in' indicates putting letters inside.
'various' is an anagram indicator.
The final letter of 'rat' is 't'.
'periods' anagrammed gives 'diopres'.
't' put inside 'diopres' is 'DIOPTRES'.