A
- Abbe
- Abbe number
- absorptance
- absorptive lens
- Active ophthalmic instrument
- addition power
- additive power trial case lens set
- Adjusting rail
- afocal lens
- aid microscope
- aid telescope
- alignment reference marking
- angular magnification
- antireflection coating
- apex
- apical angle
- arm
- as-worn power
- aspheric lens
- aspherical surface
- astigmatic difference
- astigmatic effect
- astigmatic lens power
- astigmatic power lens
- astronomical telescope
- atoric lens
- atoroidal surface
- auxiliary beam
- auxiliary lenses
astigmatic power lens
Lens bringing a paraxial pencil of parallel light to two separate line foci mutually at right angles and hence having vertex power in only the two principal meridians. NOTE1: One of these powers may be zero, with the corresponding line focusat infinity. NOTE2: Lenses referred to as cylindrical lenses, spherocylindrical lenses and toric lenses are all astigmatic-power lenses.