To prevent or lower chromatic aberration (colored fringes).....Astin said:IIRC, diff light colour travel at diff speed, so they wont focus at exactly the same point, the ED lens element is meant to compensate for the diff, so all colours will focus at the same point.
Bingo, Extra-low DispersionAstin said:IIRC, diff light colour travel at diff speed, so they wont focus at exactly the same point, the ED lens element is meant to compensate for the diff, so all colours will focus at the same point.
IIRC, the ED element is actually plastic, not really glass, I still prefer glass...espn said:Bingo, Extra-low Dispersion
IIRC, the ED element is glass leh :what:Astin said:IIRC, the ED element is actually plastic, not really glass, I still prefer glass...
Astin said:IIRC, the ED element is actually plastic, not really glass, I still prefer glass...
Maybe u are right, I only remember very vaguely reading from some Nikon articles that they cannot bend the glass so easily so they use plastic instead, forgotten whether they refer to ED element or aspherical element, something about physics of glass / air properties.johnyu said:I think you mixed up.
ED elements are glass.
But aspherical elements used in budget lenses are moulded (aka "plastic"). I think Tamron pioneers this technology. (But not all aspherical elements are plastic.)
Light regardless of colour travels at exactly the same speed actually. What varies is the length of the wave that is propogated or in otherwords it's frequency.Astin said:IIRC, diff light colour travel at diff speed, so they wont focus at exactly the same point, the ED lens element is meant to compensate for the diff, so all colours will focus at the same point.
Nikon don't bend lenses. Tey are rough cast to shape, then ground and polished using a variety of technologies depending on the lens element being produced. Aspherical elements used in Nikkor lenses are made from both glass and optical plastic and it depends on the lens (and element) as to the material being used.Astin said:Maybe u are right, I only remember very vaguely reading from some Nikon articles that they cannot bend the glass so easily so they use plastic instead, forgotten whether they refer to ED element or aspherical element, something about physics of glass / air properties.