weird discovery with the Dust Alert function

[iceshu]

I was curious and took a photo of a white sheet of paper at f22. I saw three spots of dust at the bottom of the image.

I used the Dust Alert feature to test my findings. I shot the same piece of paper and the three spots of dust appeared at the TOP of the image, in the exact same position! Is there something weird going on or am i being silly?


Also, does the Dust Alert feature reveal dust in lens? Is it able to differentiate between sensor and lens dust?

 

[Gengh]

Nothing's wrong, the image formed by the lens on the sensor is actually inverted, upside down and left-to-right. When the raw or jpg file is presented to you, it has already been flipped by the camera back to the correct orientation. So dust at the top of the sensor will appear at the bottom of your picture. Or looking at it the other way, the sky is usually imaged at the bottom of the sensor and the ground on the top of the sensor.

Dust in the lens has to be really huge to show up clearly in the picture, and even if it does show up, the contrast is going to be far lower than dust sitting directly on the sensor. So my guess is dust alert will never pick up dust in the lens.