Somehow or other, my iPhone 5s gets a lot of dust inside it, affecting both front and rear cameras. Following instructions on ifixit.com, I was able to get in and clean both out.
I found out that you should NOT use compressed air on the rear camera (probably on the front camera, too). While the image overall is better, there are now several noticeable smeared out darker blotches on the image. I am guessing they are due to dust on the sensor.