petrochemist
TPF junkie!
- Joined
- Mar 9, 2014
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"What's your focal Length??? "
I went and checked....it's roughly the length from the sensor to the front lens glass.
But in many cases it's nowhere near this.
My 10mm focal length fish-eye would probably be over 100mm from front of lens to sensor (>45mm to the mounting flange).
My 600mm mirror lens is around 150mm.
These are extreme cases, but it doesn't even hold true for most old SLR 50mm primes (nearer 75mm).
Wide angle lenses typically the measurement is more than the focal length (retro focus design) with the focal length being less than the distance from back of the lens to sensor.
Telephotos are technically lenses that the length is less than the focal length (not all long focal length lens are telephotos)