Scanning optical apparatus

A multibeam scanning optical apparatus comprising:

• a light source unit for emitting a plurality of light beams;
• a light deflector for deflecting and reflecting the light beams;
• and
• an imaging optical system for imaging the light beams deflected by said light deflector on a surface to be scanned,
• wherein said imaging optical system has a plurality of lens surfaces, and
• wherein curvatures of the lens surfaces in a sub-scan direction change continuously from the on-axis toward the off-axis along a main scanning direction so that Fmin/Fmax≧0.9 is satisfied, where Fmax is a maximum value of a F number of the light beam incident on the surface to be scanned with respect to the sub-scan direction, and Fmin is a minimum value of a F number of the light beam incident on the surface to be scanned with respect to the sub-scan direction.

A multibeam scanning optical apparatus comprising:

• a light source unit for emitting a plurality of light beams;
• a light deflector for deflecting and reflecting the light beams;
• and
• an imaging optical system for imaging the light beams deflected by said light deflector on a surface to be scanned,
• wherein said imaging optical system has a plurality of lens surfaces, and
• wherein curvatures of the lens surfaces in a sub-scan direction change continuously from the on-axis toward the off-axis along a main scanning direction so that a pitch unevenness due to a curve of the scanning line is not more than 1/10 of a beam pitch of the scanning line in the sub-scan direction.