Ipa rondedzero yemarudzi akasiyana-siyana emalenzi
Context:Kugadzirwa Kwemagirazi Akapfava  Mazhinji emalensi ndiwo malenzi ane chimiro chedenderedzwa: nzvimbo dzawo mbiri zvikamu zvemativi edenderedzwa. Nzvimbo imwe neimwe inogona kuva yakanyunguduka (inobuda panze kubva pagirazi), yakanyunguduka (inodzikira mugirazi), kana kuti yakatwasuka (yakapfava). Mutsetse unobatanidza nzvimbo dzezvikamu zvinoumba nzvimbo dzomuraraungu unonzi axis yomuraraungu. Kazhinji akisi yemalensi inopfuura nepakati chaipo pemalensi, nekuda kwenzira yavanogadzirwa nayo. Magirazi anogona kuchekwa kana kuti kugaywa pashure pokugadzirwa kuti ave nechimiro chakasiyana kana kuti kukura. Axis yegirazi ipapo haigoni kupfuura nepakati chaipo regirazi.  Magirazi e toric kana kuti espheric-cylindrical ane nzvimbo dzine mativi maviri akasiana-siana okurereka mundege mbiri dzakatwasuka. Vane simba rinokosha rakasiana mumameridian akasiana-siana. Izvi zvinoumba lens astigmatic. Muenzaniso ndiwo magirazi anoshandiswa kugadzirisa astigmatism muziso romumwe munhu.
Aya anotevera ndiwo maumbirwo akasiyana-siyana emalensi: - Convex: Inotenderera kubva pakati pegirazi - Concave: Makona ari pakati pemalensi - Planar: Flat lens