Image forming apparatus

1. An image forming apparatus comprising:

a density detector that detects a density of a patch image formed on an image carrier over a plurality of places;

a determination unit that determines whether a plurality of detection results of the density detector are within a threshold; and

a change unit wherein

when the number of determination results, by the determination unit, that the density of the patch image is not within the threshold exceeds a predetermined number, the change unit changes (i) a detection timing at which the patch image is detected by the density detector or (ii) a formation position of the patch image on the image carrier.

2. The image forming apparatus according to claim 1 , wherein

when the number of the determination results, by the determination unit, that the density of the patch image is not within the threshold does not exceed the predetermined number, neither (i) the detection timing at which the patch image is detected by the density detector nor (ii) the formation position of the patch image on the image carrier is changed.

3. The image forming apparatus according to claim 2 , wherein

when the number of the determination results, by the determination unit, that the density of the patch image is not within the threshold does not exceed the predetermined number, the density of the patch image is calculated excluding detection results of the patch image for which the determination unit determines that the density of the patch image is not within the threshold.

4. The image forming apparatus according to claim 1 , wherein

the determination unit identifies a position of the patch image for which it is determined that the detection result of the density detector is not within the predetermined threshold, and

the change unit changes (i) the detection timing at which the patch image is detected by the density detector or (ii) the formation position of the patch image on the image carrier such that the patch image which is identified by the determination unit and for which it is determined that the detection result is not within the predetermined threshold is within a detection area of the density detector.

5. The image forming apparatus according to claim 1 , wherein

the determination unit identifies a position of the patch image for which it is determined that the detection result of the density detector is not within the predetermined threshold, and

the change unit performs change such that the patch image which is identified by the determination unit and for which it is determined that the detection result is not within the predetermined threshold is within a detection area of the density detector.

6. The image forming apparatus according to claim 1 , wherein the density detector detects the density of the patch image between a plurality of image areas formed by an image forming operation.

7. The image forming apparatus according to claim 6 , wherein the change unit changes the detection timing at which the patch image is detected by the density detector between the plurality of image areas.

8. The image forming apparatus according to claim 1 , wherein the density detector detects the density of the patch image before an image forming operation is started.

9. An image forming apparatus comprising:

density detecting means for detecting a density of a patch image formed on an image carrier over a plurality of places;

determining means for determining whether a plurality of detection results of the density detecting means are within a threshold; and

changing means for, when the number of determination results, by the determining, that the density of the patch image is not within the threshold exceeds a predetermined number, changing (i) a detection timing at which the patch image is detected by the density detecting means or (ii) a formation position of the patch image on the image carrier.