1. An image forming apparatus comprising: an image processor configured to correct image data based on a correction condition; an image former configured to form an image based on the corrected image data, using developer; an intermediate transfer member on which the image former forms a measurement image; a sensor configured to measure the measurement image on the intermediate transfer member; a converter configured to convert a measurement result of the measurement image by the sensor, based on a conversion condition; a gamma generator configured to generate the correction condition based on the measurement result converted by the convertor; a controller configured to control the image former to form a first test image based on test image data, control the sensor to measure the first test image on the intermediate transfer member, and control the image former to form a second test image, based on test image data, on a sheet; a receptor configured to receive a user instruction based on a result of a user having compared a sample image and the second test image formed on the sheet; and a conversion condition generator configured to generate the conversion condition based on the user instruction and the measurement result of the first test image by the sensor, wherein the sample image includes a plurality of sample images arranged along a predetermined direction, the plurality of sample images is formed by using a developer of a predetermined color, the plurality of sample images includes a first sample image, a second sample image, a third sample image and a fourth sample image, the first sample image and the second sample image are adjacent to each other in the predetermined direction, and the third sample image and the fourth sample image are adjacent to each other in the predetermined direction, density of the first sample image is higher than density of the second sample image, density of the second sample image is higher than density of the third sample image, density of the third sample image is higher than density of the fourth sample image, a color difference between the first sample image and the second sample image is smaller than a threshold value, and a color difference between the third sample image and the fourth sample image is larger than the threshold value.