On the other hand, the eye-glass must not be of such small focal length that the image appears indistinct and contorted, or dull for want of light.
The rapidity of a lens depends mainly on its aperture and its focal length.
The speed of a photographic lens depends on the ratio of the effective aperture to the focal length.
To overcome this obstacle to clear vision, and also to secure magnification, the focal lengths of the instruments were greatly extended.
The aperture and focal length on the camera are set according to need.
The procedure was applied to the 3 m focal length off-axis parabola.
The focal length of the evolved system is 0.992298 and the sum of the distances is 0.09881 ($1/10).
