F Ratio or Speed
want could this mean?
By Greg Walton MPAS/ASV
In astrophotography we are
usually pushing the camera and lens to the limit, running
high ISO settings for long periods of time. We very rarely
stop a lens down; in the diagram below all Lenses are the
same Diameter with different F ratios.
The bigger the patch of sky the more light it contains and
the brighter the picture will be.
I hear you say what is an F ratio? It is how
many times the lens diameter, can fit into the focal length
of a lens. See scale on left. The boxes at the bottom of the
page are a comparison of the amount of sky that can be seen
with a curtain lens F ratio.