Fast f/4 to f/5 focal ratios are generally
best for lower power wide field observing and deep space photography.
Slow f/11 to f/15 focal ratios are usually better suited to higher power
lunar, planetary, and binary star observing and high power photography.
Medium f/6 to f/10 focal ratios work well with either.
system can photograph a nebula or other faint extended deep space object
in one-fourth the time of an f/10 system, but the image will be only
one-half as large. Point sources, such as stars, are recorded based on
the aperture, however, rather than the focal ratio – so that the larger
the aperture, the fainter the star you can see or photograph, no matter
what the focal ratio.