Method and apparatus for multi-beam optical scanning capable of effectively adjusting a scanning line pitch

An image forming apparatus, comprising:

• a plurality of photosensitive members each having a photosensitive surface;
• a plurality of development mechanisms each configured to develop an image with a different color toner;
• and
• an optical scanning apparatus which comprises:
• a plurality of light sources;
• a plurality of coupling lenses;
• and
• an aperture having at least one slit,
• wherein numbers of the plurality of light sources and the plurality of coupling lenses are equal to each other and said apparatus satisfies an inequality
• 12.7<(D×β0×m0×dpi)/(2×NA)<38.1
• and an equation
• β0=(β+−β−)/(n−1),
• wherein D represents a width of said at least one slit of the aperture, β0 represents a beam open angle, m0 represents a total lateral magnification ratio, dpi represents a number of dots per inch, NA represents a numerical aperture, β+ represents a positive open angle, β− represents a negative open angle, and n represents said numbers,
• wherein the optical scanning apparatus forms latent images for development in different colors on the photosensitive surfaces of the plurality of photosensitive members and the latent images for the different colors are developed into visual images with the different color toners by the plurality of development mechanisms.