Optical imaging system including seven lenses of +−++−+−, +−+−++− or +−+−+−− refractive powers

An optical imaging system, comprising:

• a first lens, a second lens, a third lens, a fourth lens, a fifth lens, a sixth lens and a seventh lens, sequentially from an object side to an image side along an optical axis,
• wherein,
• the first lens and the third lens each has a positive refractive power;
• the second lens, the fourth lens, the fifth lens, the sixth lens, and the seventh lens each has a positive refractive power or a negative refractive power;
• an object-side surface of the first lens is a convex surface and an image-side surface of the first lens is a concave surface;
• an image-side surface of the second lens is a concave surface;
• an object-side surface of the third lens is a convex surface;
• an object-side surface of the seventh lens is a convex surface and an image-side surface of the seventh lens is a concave surface;
• the optical imaging system satisfies:
• f/EPD≤1.5;
• −1.5<(f2−f4)/(f2+f4)<−0.5;
• and
• 1<CT7/CT5×TAN(HFOV)<2,
• where f is a total effective focal length of the optical imaging system, EPD is an entrance pupil diameter of the optical imaging system, f2 is an effective focal length of the second lens, and f4 is an effective focal length of the fourth lens, CT7 is a center thickness of the seventh lens on the optical axis, CT5 is a center thickness of the fifth lens on the optical axis, and HFOV is half of a maximal field-of-view of the optical imaging system.