my aperture is constant as i set it myself.I learnt that when you want to stitch photos together, these are the things you must follow:
a. Shoot in manual, set the aperture and shutter speed constant throughout your shots.
b. Use manual focus only.
c. Overlap at least 25% with next frame.
d. Use tripod.
e. Must shoot all of them within the short period of time.
I think your photos are shot with different exposures which explain the difference in tone. Are you sure the settings of aperture and shutter speed remain the same throughout the shot?
Your aperture constant doesn't mean the exposure will be constant because the shutter speed would vary according to the metering (I assume you shoot under Aperture Priority mode). So the procedure you need to follow is metering first under Av mode, remember the aperture and shutter speed, then set them in manual mode.my aperture is constant as i set it myself.
over-lap ard 10% only.
using a tripod.
shot under a min.