where N is an f-number
and t is a shutter speed .
The ratio of focal length to diameter of the mirror is typically referred to as the f-number
. Another ratio of importance is the solar concentration ratio: the ratio of the diameter of the mirror to the diameter of the target (the cooking pot or kettle).
Optical simulations have shown that the mirrors would need to be at least 12 millimeters in diameter in order to produce a zoom objective with a sufficient f-number
With most fixed-focus lenses, it is not possible to change the lens f-number
(F/#) in low-light conditions or with small pixel sizes when trying to obtain high-resolution images.
), or the eye's ability to admit light, was calculated using values for focal length (F) and the diameter of the retina (d), measured from the extremities of the rhabdomeres in each species (Opell & Ware 1987).
Extended integration on chip up to 15 field periods gives high sensitivity for low-light and higher F-number
zoom lenses, while-the optional remote control and on-screen display make for convenient set up in the field.
The optical speed or f-number
(f) of the pinhole lens is important for the successful implementation of a covert camera system.
It is easily re-imaged due to the long working distance and slow optical F-number
The aperture or f-number
, also called f-stop, is equal to the distance between the lens and the focal point divided by the effective diameter of the lens.
It is similar to an F-number
on a photographic lens.