How to set eyedropper to sample image and not screen ?

Hi,

draw a green background 100s of pixels x 100s of pixels large, , on it draw a black square 51 x 51 pixels in size, set eye dropper to 51 x 51 pick current layer and zoom in and sample centre of square. we get black.

Now zoom out a lot then sample that square again, we dont get black.

its taken a 51x51 sample of the screen and the square is now only a few screen pixels wide.its also pulled in the green.

I want the colour picker to sample my image, not my screen, so whatever the zoom, it samples a 51x51pixel chunk from my image.

I think folk assume its sampling the image not the screen.

Instead its being polluted by adjacent colours. It doesnt give feedback on just what its sampling, no crawling ants square or similar to show you, I have to create a brush size 51x51 to see exactly where I am to sample, but then if I am zoomed in too far it wont sample 51pixels of my image but just a few, and zoomed out its sampling far more than 51px worth !

Where is the setting to have it sample  always 51px x 51px of the image, whatever the zoom ?

Likewise any other size setting such as 5x5 or 31 x 31 ?

Merlin