Non-APO lenses splits the white light and disperse them to produce color fringes. And this is exactly what is meant by CA/Chromatic aberrations. This is because the colors red, green, blue are bent to different extent hitting different focal plane which causes degradation in sharpness as well as producing CA.
Mr Thomas M Back is deemed to be a famous optical lens designer and as per him the definition of apochromatic is as follows:
“An objective in which the wave aberrations do not exceed 1/4 wave optical path difference (OPD) in the spectral range from C (6563A - red) to F (4861A - blue), while the g wavelength (4358A - violet) is 1/2 wave OPD or better, has three widely spaced zero color crossings and is corrected for coma.”
Most telephoto zoom lenses employ ED glasses to provide apochromatic like performance. These are low dispersion glasses to bring all the colors on the same focal plane. The more the combination of ED glasses the more accurate all the colors brought to the same focal plane or the less focal shifts and thereby increases the sharpness.
Mr Thomas M Back is deemed to be a famous optical lens designer and as per him the definition of apochromatic is as follows:
“An objective in which the wave aberrations do not exceed 1/4 wave optical path difference (OPD) in the spectral range from C (6563A - red) to F (4861A - blue), while the g wavelength (4358A - violet) is 1/2 wave OPD or better, has three widely spaced zero color crossings and is corrected for coma.”
Most telephoto zoom lenses employ ED glasses to provide apochromatic like performance. These are low dispersion glasses to bring all the colors on the same focal plane. The more the combination of ED glasses the more accurate all the colors brought to the same focal plane or the less focal shifts and thereby increases the sharpness.
Apochromats with ED glasses are less stable with temperature than apochromats with conventional glasses and so the focal length of these lenses changes with temperature.
Nikor 18-200mm VR the best commercial lens in market is built with 2 ED elements and thus more prone to CAs on bright sunny day. Nikor 70-200mm VRII employs 7 ED glass apart from providing better build quality/ sharpness reduces colour fringes to maximum extent. Sigma lenses will be marked with APO for having apochromatic features.
Do consider these factor while buying glasses.
Nikor 18-200mm VR the best commercial lens in market is built with 2 ED elements and thus more prone to CAs on bright sunny day. Nikor 70-200mm VRII employs 7 ED glass apart from providing better build quality/ sharpness reduces colour fringes to maximum extent. Sigma lenses will be marked with APO for having apochromatic features.
Do consider these factor while buying glasses.
