The sharpness of image formed by a simple lens telescope depends on:
- Radius of aperture
- Dispersion of lens
- Focal length of lenses
- Spherical aberration
Radius of aperture controls the light which is entering. If the aperture is narrow the resulting image on the image plane is sharp. In contrast if the aperture is wide the resulting image has sharp focus only for rays of certain focal length.
Dispersion of lens: Lower the dispersion caused by lens shaper is the image.
Focal length: This only determines the position where image is formed, not the sharpness of the image formed.
Spherical aberration: It is a defect which causes the rays near the edges to refract more compared to the rays which are near the axis of the lens.