For finding the focal length of a convex lens by obtaining the image of a distant object, one should use as the object:

To determine the focal length of a convex lens, a distant object (at infinity) must be used so that the incident rays are parallel to the principal axis and converge at the principal focus ($F$). A 'well-lit distant tree' is far enough to satisfy the condition of being at infinity, and because it is well-illuminated, it reflects enough light to form a sharp, bright, and clearly visible image on the screen. 'Any distant tree' might not be well-lit, resulting in a faint and unclear image. A 'window grill' or a 'lighted candle at the other end of the table' are at finite distances, so their images will form beyond the focus (between $F$ and $2F$, or at $2F$), which would yield an incorrect focal length.