Optical glass, precision press-molding preform, optical element and process for producing the same

An optical glass comprising, by cationic %,

• 20 to 50% of B3+,
• 0 to 10% of Si4+,
• 5 to 35% of La3+,
• 0 to 10% of Gd3+,
• 0 to 10% of Y3+,
• 0 to 10% of Yb3+,
• 0 to 4% of Ti4+,
• 1 to 30% of Nb5+,
• 0.5 to 15% of Ta5+,
• 0 to 5% of W6+,
• 0 to 10% of Zr4+,
• 11 to 40% of Zn2+,
• 0 to 10% of Mg2+,
• 0 to 10% of Ca2+,
• 0 to 10% of Sr2+,
• 0 to 10% of Ba2+,
• 0 to 10% of Li+,
• 0 to 10% of Na+,
• 0 to 10% of K+,
• 0 to 10% of Te4+,
• 0 to 10% of Ge4+,
• 0 to 10% of Bi3+, and
• 0 to 10% of Al3+,
• the total content of B3+ and Si4+ (B3++Si4+) being 20 to 50%,
• the total content of La3+, Gd3+ and Y3+ (La3++Gd3++Y3+) being 5 to 35%,
• the cationic ratio of ((B3++Si4+)/(La3++Gd3++Y3+)) being from 1 to 5,
• the total content of Ti4+, Nb5+, Ta5+ and W6+ (Ti4++Nb5++Ta5++W6+) being 10 to 35%,
• the cationic ratio of ((Nb5++Ta5+)/(Ti4++Nb5++Ta5++W6+)) being from 0.7 to 1,
• the cationic ratio of ((B3++Si4+)/(Ti4++Nb5++Ta5++W6+)) being from 0.5 to 4,
• the cationic ratio of ((La3++Gd3++Y3+)/(Ti4++Nb5++Ta5++W6+)) being from 0.2 to 3,
• the cationic ratio of the content of Zn2+ to the total content of Zn2+, Mg2+, Ca2+, Sr2+ and Ba2+ (Zn2+/(Zn2++Mg2++Ca2++Sr2++Ba2+) being from 0.8 to 1,
• the optical glass having a refractive index nd of 1.89 or more and an Abbe's number νd of 27 to 37.