lens design

Hi, I'm looking for some expert knowledge on lens design. I've come to understand that most lens elements use a spherical surface, this being the easiest to grind. Yet I know from my studies of physics that a spherical surface on a lens causes considerable chromatic aberrations.

I've assumed that the so-called aspherical lens elements depart from a spherical shape in order to reduce chromatic aberrations in a compound lens. From physics, I recall that a parabolic shape generates the sharpest focus of light rays. So what shape is an 'aspherical' lens surface following?

Thanks.
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jamesza

james laubscher said:

Hi, I'm looking for some expert knowledge on lens design. I've come to understand that most lens elements use a spherical surface, this being the easiest to grind. Yet I know from my studies of physics that a spherical surface on a lens causes considerable chromatic aberrations.

Click to expand...

The chromatic aberrations can be cancelled by using additional and complimentary spherical elements made of glass with a different refractive index. Unfortunately, it takes multiple elements to achieve high levels of correction for all colours, and for all the other lens aberrations there are.

Aspherical elements reduce the number of glasses needed to make a full compound lens of good quality. Aspherics go back to the Vikings of 10th Century, but more modern lens designs have relied heavily on computer modelling, and new transparent materials.. low dispersion glass, Fluorite, and special optical plastics.

james laubscher said:

I've assumed that the so-called aspherical lens elements depart from a spherical shape in order to reduce chromatic aberrations in a compound lens. From physics, I recall that a parabolic shape generates the sharpest focus of light rays. So what shape is an 'aspherical' lens surface following?

Click to expand...

Now don't tell me you have not heard of Wikipedia... [??] You could have Googled for Wikipedia...

http://en.wikipedia.org/wiki/Aspheric_lens
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Regards,
Baz

Well, I'll see your Cher, and your Streisand... and I'll raise you an Alice Babs!

james laubscher said:

Hi, I'm looking for some expert knowledge on lens design. I've come to understand that most lens elements use a spherical surface, this being the easiest to grind. Yet I know from my studies of physics that a spherical surface on a lens causes considerable chromatic aberrations.

Click to expand...

No, spherical surfaces cause spherical aberration. Dispersion (the property of materials that causes the index of refraction to vary with wavelength) causes chromatic aberration.

james laubscher said:

I've assumed that the so-called aspherical lens elements depart from a spherical shape in order to reduce chromatic aberrations in a compound lens. From physics, I recall that a parabolic shape generates the sharpest focus of light rays. So what shape is an 'aspherical' lens surface following?

Click to expand...

Chromatic aberration is reduced by combining glasses with different dispersion characteristics such that the changes in refractive index with wavelength compensate each other.

Your comment about parabolas is correct only for mirrors. A paraboloidal surface focuses all axial rays (the ones parallel to the optical axis) to a single point. But only axial rays. Paraboloids have horrible fields of view because anything off-axis shows terrible coma. For similar reasons, they also lose sharpness if you focus them at anything but infinity. But mirrors don't have any chromatic aberration; that's why Newton invented the reflecting telescope.

Aspheric surfaces are used in camera lenses to improve image quality, generally by reducing spherical aberration, astigmatism and coma. The actual surfaces that need to be generated are not as simple as conic sections because you have to balance the effect of the asphere with all the other glass. So you (or your lens design program) end up with some complex polynomial figure of revolution that corrects what you want while messing up other things the least.

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Leonard Migliore

james laubscher said:

Hi, I'm looking for some expert knowledge on lens design. I've come to understand that most lens elements use a spherical surface, this being the easiest to grind. Yet I know from my studies of physics that a spherical surface on a lens causes considerable chromatic aberrations.

Click to expand...

Spherical lens surfaces do not generate chromatic aberrations, they generate spherical aberrations. Chromatic aberrations are caused by the different amounts different colours (wave lengths) of light are bent by the lens element. They are minimised by using two glass lens elements with different refractive indexes to "cancel out" the worst of the chromatic aberrations.

james laubscher said:

I've assumed that the so-called aspherical lens elements depart from a spherical shape in order to reduce chromatic aberrations in a compound lens. From physics, I recall that a parabolic shape generates the sharpest focus of light rays. So what shape is an 'aspherical' lens surface following?

Click to expand...

Here you are correct about the shape of the lens. Spherical aberrations are caused by the spherical shape of the glass and (assuming one colour to simplify matters) are caused by the fact that a spherical lens surface will not bring the light rays striking the centre and edge to the same point on the sensor. A parabola is indeed the correct shape to bring the light from all part of the lens to the same point of focus.

Historically, this has been well known for a long time but the cost of individually "figuring" or grinding and polishing the correct shape has been prohibitive. I believe that aspherical lens elements are able to be cast in an aspherical shape and this has reduced the cost of the elements and the lenses that use them. From memory, the very first cameras to use a cast aspherical element in their construction were the old Kodak Disc cameras (remember them?)

james laubscher said:

Thanks.
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jamesza

Click to expand...

Thank you to all above for the contributions. That was really informative, even if not all agree as to the benefit of parabolic surfaces! Whatever, I now have a better idea of what goes on in compound lenses.

The complexities involved make the size of P&S long zoom lenses really an amazing engineering feat. We take a lot for granted these days but when we hold a modern camera, we are holding a miracle of micro-engineering in our hands. That is why I'm still not used to the disposable consumerist philosophy that pervades the modern camera scene. When I was starting out, quite a time back, a camera was a big investment that was intended to last at least ten years. Possibly some of today's cameras could be used for ten years without failure but who would want to do that with an improvement coming out every 6 to 12 months?

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jamesza

james laubscher wrote:

Possibly some of today's cameras could be used for ten years without failure but who would want to do that with an improvement coming out every 6 to 12 months?

I am still using a Minolta D7ug from 2001, which is nearly 10 years old now. It has been relegated to Infra Red shooting, but could be used for normal stuff just by taking the dark light (IR passing) filter off it.

Also, I'm using my Konica Minolta A2 from 2004, because I like it a lot. Indeed, I have two of them, so I can have one permanently set up with a wide angle adaptor (22mm equivelant).... and be protected from the inevitable ultimate camera failure just that bit longer.

Note re A2: THAT model has not been updated/replaced.... not least because Konica Minolta got out of the camera business... (sigh)
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Regards,
Baz

Well, I'll see your Cher, and your Streisand... and I'll raise you an Alice Babs!

Barrie Davis said:

james laubscher wrote:

Possibly some of today's cameras could be used for ten years without failure but who would want to do that with an improvement coming out every 6 to 12 months?

Click to expand...

I am still using a Minolta D7ug from 2001, which is nearly 10 years old now. It has been relegated to Infra Red shooting, but could be used for normal stuff just by taking the dark light (IR passing) filter off it.

Also, I'm using my Konica Minolta A2 from 2004, because I like it a lot. Indeed, I have two of them, so I can have one permanently set up with a wide angle adaptor (22mm equivelant).... and be protected from the inevitable ultimate camera failure just that bit longer.

Note re A2: THAT model has not been updated/replaced.... not least because Konica Minolta got out of the camera business ... (sigh)

--
Regards,
Baz

Well, I'll see your Cher, and your Streisand... and I'll raise you an Alice Babs!