Image forming apparatus

An image forming apparatus comprising:

• a belt conveyor device including:
• an endless conveying belt;
• a plurality of support rollers configured to rotatably support the conveying belt;
• and a drive portion configured to rotate one of the plurality of support rollers, and configured to convey a sheet on the conveying belt;
• a rotation amount detection portion configured to detect a rotation amount of one of the plurality of support rollers;
• a printing device configured to form an image on the sheet by discharging a developer onto the sheet on the conveying belt;
• an image reading portion including:
• a light emitting portion configured to emit detection light to a target area extending along a surface of the conveying belt;
• and a plurality of photoelectric conversion elements arranged along a width direction intersecting a sheet conveying direction and each configured to detect a light amount of reflected light of the detection light from the target area, and configured to output a plurality of pixel data items representing the detected light amounts of the plurality of photoelectric conversion elements;
• and
• an adjustment portion configured to adjust a control parameter of the printing device based on the plurality of pixel data items obtained by the image reading portion, wherein
• the control portion includes:
• a first length deriving portion configured to derive, based on the plurality of pixel data items obtained by the image reading portion when a rectangular specific sheet is conveyed by the belt conveyor device with one specific side of the specific sheet along the width direction, a pair of lateral edge positions that are both end positions of the sheet in the width direction, and derive a length of the specific side of the specific sheet based on a derivation result of the pair of lateral edge positions;
• a passing period identification portion configured to identify a sheet passing period from a starting point at which a leading edge of the specific sheet reaches a facing position facing the image reading portion to an ending point at which a trailing edge of the specific sheet reaches the facing position, based on the plurality of pixel data items obtained by the image reading portion when the specific sheet is conveyed by the belt conveyor device in an orientation in which the specific side of the specific sheet is along the sheet conveying direction;
• a second length deriving portion configured to derive a length of the specific side of the specific sheet based on a rotation amount detected by the rotation amount detection portion during the sheet passing period;
• and
• a timing correction portion configured to correct a discharge timing parameter used for controlling a timing of the discharge of the developer by the printing device in accordance with a difference between a derivation result of the first length deriving portion and a derivation result of the second length deriving portion.