Tools you might need next
Calculate fishing line strength and leader size for target species, technique, and drag setting based on line type and test rating.
Estimate fish weight from length and girth measurements using species-specific formulas when you do not have a scale on the water.
Calculate optimal trolling speed in mph and knots for target fish species, lure type, and desired lure running depth on lakes or oceans.
Focal ratio is focal length divided by clear aperture diameter. An f/5 scope has a focal length five times its aperture.
f/ratio = Focal Length / ApertureLight gathering depends on aperture area (diameter squared). Focal ratio affects surface brightness at the eyepiece: faster (lower f/) scopes produce brighter images at the same magnification.
Relative brightness ∝ 1 / f²Given aperture and desired f/ratio, multiply to find required focal length. Useful for designing imaging trains or selecting reducers/flattens.
Focal Length = Aperture × f/ratioUpdated: July 2026
203mm aperture × f/6 = 1218mm focal length. Moderate speed suitable for visual and beginner deep-sky photography with coma corrector.
80mm aperture at f/7.5 gives 600mm focal length. Slower ratio gives high contrast for planets; longer exposures needed for faint nebulae.
Same 130mm aperture: f/4.9 = 637mm FL vs f/6 = 780mm. The f/4.9 scope gathers the same total light but produces 1.5× brighter image per unit exposure time.
Light gathering depends on aperture area, not f/ratio alone. A 200mm f/10 gathers the same total light as 200mm f/4 — but the f/4 image is brighter at the same magnification.
Fast scopes often have more coma and require expensive correctors. For lunar and planetary work, moderate f/8–f/10 refractors and SCTs provide excellent contrast without extra optics.
Focal ratio (f/number) determines how "fast" or "slow" a telescope is, affecting image brightness, field of view, and exposure times for astrophotography. Calculate f/ratio from aperture and focal length to compare scopes and plan imaging setups.