In fact, it is essential that the observer who would rightly apply the powers of his telescope, or fairly test its performance, should understand in what respect an image formed by an object-glass or object-mirror differs from a real object.
The peculiarities to be noted are the _curvature_, _indistinctness_, and _false colouring_ of the image.
The curvature of the image is the least important of the three defects named--a fortunate circumstance, since this defect admits neither of remedy nor modification.

The image of a distant object, instead of lying in a plane, that is, forming what is technically called a _flat field_, forms part of a spherical surface whose centre is at the centre of the object-glass.

Hence the centre of the field of view is somewhat nearer to the eye than are the outer parts of the field.

The amount of curvature clearly depends on the extent of the field of view, and therefore is not great in powerful telescopes.

Thus, if we suppose that the angular extent of the field is about half a degree (a large or low-power field), the centre is nearer than the boundary of the field by about 1-320th part only of the field's diameter.
The indistinctness of the image is partly due to the obliquity of the pencils which form parts of the image, and partly to what is termed _spherical aberration_.