1. An image forming apparatus comprising: an image bearing member; an image forming unit configured to form an image on the image bearing member; a measuring unit having a first sensor, a second sensor, and a shutter, and configured to measure a measurement image formed on the image bearing member, wherein the shutter protects the first sensor and the second sensor; a driving unit configured to be driven to move the shutter, wherein, in a state in which the shutter is at a first position, the shutter prohibits measurement of the measurement image by the first sensor and the second sensor and wherein, in a state in which the shutter is at a second position, the measurement image is measured by the first sensor and the second sensor; and a controller configured to: control the image forming unit to form the measurement image on the image bearing member, drive the driving unit to move the shutter to the second position, control the measurement unit to measure the measurement image, control whether or not to change an image forming condition of the image forming unit based on (i) a comparison result of a first output value corresponding to a measurement result of the first sensor and a first reference value and (ii) a comparison result of a second output value corresponding to a measurement result of the second sensor and a second reference value.