1. An image forming apparatus comprising: a first image forming unit configured to form toner images based on image data on a plurality of first image carriers; a second image carrier configured to move at a desired speed and on which the toner images formed on the first image carriers by the image forming unit are transferred; a second image forming unit configured to transfer the toner images transferred on the second image carrier onto a transfer medium that is moved at the desired speed; a test pattern generating unit configured to generate a test pattern group with a desired length in a moving direction of the second image carrier; and an adjusting unit configured to adjust an image formation condition for the first image forming unit using the test pattern group based on a relationship between the desired length and a length of an image area in which a print image based on the image data is formed in a sub-scanning direction corresponding to the moving direction of the second image carrier.