Aspherical 18-70??

[jben_net]

Hi All,

Have been reading till my eyes fall out trying to get up to speed with the technical side of photography. It's nowhere near as fun as the creative side!

Anyway..... Was reading about the noct lenses and how they are aspherical (meaning the blades are curved to help simulate a circle more accuratley.

Checked ou my 85 1.4 to see if it had this- it doesn't (not that I really expected it to)

Then was milling over my first lens I got with my d70 - the 18-70 DX - and what do you know - it says that it's aspherical! Looked down the barrel and opened the aperture wide and it does actually have it! And on such a cheap lens??

I'm guessing its not precision ground but I was still happy to see it.

So, my question is. Why would the 18-70 have it and the 85 1.4 not have it?

I'm probably labouring the point but I'm still curious.

James

[wendellt]

i think the 85mm has 9 apertur eblades s oit doesn't need rounded edges

anyway the quality of the bokeh doesn't soley depend on rounded diaphrams
even the 85 f1.4 creates defined halo rims around the blurry circles you get when you shoot lights at night at widest aperture this is not desirable as good quality bokeh out of focus rendered areas shoudl produce smooth edges

i read somewhere it is part of the nikon lens design that creates these defined edges on light circles,, i have learned to live with it the bokeh is still creamy when you photograph other things

i knwo the canon equivallent renders out smooth edges on the light circles
but for now i'll stick with the nikon

[huynhie]

Hi All,

Have been reading till my eyes fall out trying to get up to speed with the technical side of photography. It's nowhere near as fun as the creative side!

Anyway..... Was reading about the noct lenses and how they are aspherical (meaning the blades are curved to help simulate a circle more accuratley.

Checked ou my 85 1.4 to see if it had this- it doesn't (not that I really expected it to)

Then was milling over my first lens I got with my d70 - the 18-70 DX - and what do you know - it says that it's aspherical! Looked down the barrel and opened the aperture wide and it does actually have it! And on such a cheap lens??

I'm guessing its not precision ground but I was still happy to see it.

So, my question is. Why would the 18-70 have it and the 85 1.4 not have it?

I'm probably labouring the point but I'm still curious.

James

I think you may have your definition down incorrectly

here is the Nikon lens website

[losfp]

I was always under the impression that the term referred to the shape of the lens element(s)... Exactly what it does, more of a mystery

and I'm still not quite sure why the 85/1.4 wouldn't have it. Just a classic design that don't need no fancy new-fangled shapes?

[jben_net]

for the record here's the Nikon website description (thanks for the link Huynhie)

Aspherical lens elements
Nikon introduced the first photographic lens with aspherical lens elements in 1968. What sets them apart? Aspherical lenses virtually eliminate the problem of coma and other types of lens aberration — even when used at the widest aperture. They are particularly useful in correcting the distortion in wideangle lenses. In addition, use of aspherical lenses contributes to a lighter and smaller lens design.
Nikon employs three types of aspherical lens elements. Precision-ground aspherical lens elements are the finest expression of lens-crafting art, demanding extremely rigorous production standards. Hybrid lenses are made of a special plastic molded onto optical glass. Molded glass aspherical lenses are manufactured by molding a unique type of optical glass using a special metal die technique.


I'm thinking what wendel said about it having 9 blades would be the reason why the 85 doesn't need it.

Maybe the 18-70 employs the Hybrid lenses technique for its Aspherical blades?

Interesting about the defined halo rings - i'll have to go and review some of my photos taken at night with the 85

Thanks for the input guys.

[moz]

I suspect you're a bit confused. Aspherical glass and aperture blades are very different things. Aperture blades are curved (semi-circular) so that as they open and close the total aperture stays more nearly circular. More blades makes this easier.

Aspherical glass reduces optical abberations, but is more expensive to grind (it's easier to make a mechanical grinding system to produce semi-spherical mirrors or lenses). With cast plastic this is irrelevant (you just need to make one aspherical mold), but cast elements are less precise. Some makers have whizzy ways to put plastic onto spherical glass elements to get some of the effect, but it's not perfect.

[Glen]

Maybe the 18-70 employs the Hybrid lenses technique for its Aspherical blades?

J ben, I think you have missed the point aspherical refers to the lens elements ie glass, some of which can be made of glass and plastic. An aspherical front element can be clearly seen in the famous Noct 58 1.2 or the famous Sigma 12-24


Aperture blades, a seperate part of the lens, can be rounded, the current AF85 1.4 features rounded blades as does the 18-70