Originally posted by joodiespost thank you brothers for the help. Say Marc, why are these raynox lenses achromatic? What happened to the color?
Achromatic means the glass is designed to reduce or eliminate Chromatic Aberations / Purple Fringing.
Natural light, as you know, is made up of a continuum of colors (think rainbow) whose photons travel in differing wavelengths. What we see is the light photons that are NOT absorbed by whatever we are looking at.
A lens contains a number of glass elements that, combined, bend and focus the light photon waves (semi) precisely in a circle on the film or sensor plane. An image is the reflection of the non-absorbed light photons from that object. The photons are "captured" on the "sensor" in a chemical or electronic process.
Since parts of the light (the different colors) have slightly different wavelengths, sometimes some of the individual light color waves are slightly separated, producing visible "color aberations" in the image. A common visible effect is a purple fringe around the edges of a subject when viewed against a high-contrast background, frequently found when photographing birds at telephoto lengths (Eek!
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This can be pronounced on digital sensors where film was more forgiving, and even more pronounced when you add another piece of glass to the front of a lens (a Close-Up filter).
An Achromatic Close-Up filter is specifically designed to compensate for this effect.
A = (something like) "Not" or "No" or "Anti"
Chromatic = (something like) "Color Aberations" (Chroma = color in Greek)
A-chromatic