Aspheric lens

DrGed

DrGed

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Ged
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My scant knowledge of Greek leads me to believe than in an aspheric lens something isn't spherical. Is it the shape of the glass?

Also, what is the advantage in using one and why do they cost so much?
 
DrGed said:
My scant knowledge of Greek leads me to believe than in an aspheric lens something isn't spherical. Is it the shape of the glass?

Also, what is the advantage in using one and why do they cost so much?
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Correct. They're used to improve off-axis performance, often replacing two or more conventional elements. Most are glass, either moulded or computer-ground, and some are hybrids with a plastic aspherical surface bonded to glass.

They're expensive because the conventional (ie cheaper) grinding and polishing process always produces spherical surfaces. Asphericals are much more difficult.
 
Thanks, Richard.

It occurred to me, after I'd posted my question, that I could use Google. So perhaps I ought to apologise for being lazy!

I looked it up and, not for the first time recently , was taken back to my A level physics days in the early seventies. I must admit I don't recall ever being told about the off-axis performance issues of spherical lenses but I am intruiged as to why this phenomenom exists in the first place.

Does this mean wide apertures can be less sharp than narrow ones?
 
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DrGed said:
Thanks, Richard.

It occurred to me, after I'd posted my question, that I could use Google. So perhaps I ought to apologise for being lazy!

I looked it up and, not for the first time recently , was taken back to my A level physics days in the early seventies. I must admit I don't recall ever being told about the off-axis performance issues of spherical lenses but I am intruiged as to why this phenomenom exists in the first place.

Does this mean wide apertures can be less sharp than narrow ones?
Click to expand...

Heck yes. Controlling off-axis aberrations is the major challenge and we wouldn't have the big apertures, wide-angles and high zoom ratios around today without asphericals, combined with high refractive index glass with low dispersion.

Not sure when asphericals came into general use, around 1980s sounds about right.
 
Thanks for the replies.

So, am I right in thinking many DSLR's, possibly even my own entry level model, have ashperical lenses? The reason I asked this originally was that one of the adverts at the bottom of the page was for an aspherical lens (50mm f1.4, I think) and the price was about £3k. This led me to believe that they were "high end specialist" lenses, if you know what I mean.
 
DrGed said:
Thanks for the replies.

So, am I right in thinking many DSLR's, possibly even my own entry level model, have ashperical lenses? The reason I asked this originally was that one of the adverts at the bottom of the page was for an aspherical lens (50mm f1.4, I think) and the price was about £3k. This led me to believe that they were "high end specialist" lenses, if you know what I mean.
Click to expand...

Some 50s have asphericals, though Canon 50/1.4 doesn't. Kit zoom 18-55 IS Mk2 does.
 
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