I would have thought the effect of earth curvature was mainly due to the fact that the image was distorted because the camera wasn't pointed exactly at the horizon. If it was pointed slightly above the horizon, you would expect to see apparent curvature as seen. Look at photographs of tall buildings taken horizontally from the ground- they look as if they are leaning over. I think there would be a slight effect of curvature if the camera was pointed exactly at the horizon because the horizon is slightly curved, though in the field of view of a camera through an aircraft window, not enough to be identifiable.
I have quite clearly identied curvature by eye at cruise altitude. You need a clear horizon, and high altitude shows this best, but on a really clear day I would have thought it would be apparent from 'medium' altitudes upwards.