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Aspherical elements e.t.c. Why necesssary?
When I look a little deeper into lens construction, I hear about lots of elements and complicated constructions. Why are these necessary? Why not just have one element, which can move on zoom lenses?Daniel..

Comments (17)

One convex piece of glass, by itself, cannot bring all the light rays hitting it into a common focus (a single point). What is required of a good quality lens is a number of different elements made from special glasses and shaped in special ways to correct the faults or ABBERATIONS inherent in each element..

To answer your question about aspherical elements in particular, these are required to correct a common abberation called, naturally enough, spherical abberation. This is caused because a light ray that hits a piece of convex glass with a surface that is a partial sphere at the edge will come to a different focus to a ray from the same object which strikes the glass in the centre. The aspherical glass element does not have a surface shaped like a partial sphere but is specially shaped to ensure the light rays from the edge of the glass come to the same focus as the light rays from the centre of the glass..

The problem with aspherical elements is that unlike spherical elements, which can be produced inexpensively by polishing machine, require special casting and polishing techniques (read expensive) to produce...

Comment #1

Thanks for the explanation. But why would you want all the light to hit a single point? Sensors are bigger than one point.Daniel..

Comment #2

Profborg wrote:.

Thanks for the explanation. But why would you want all the light tohit a single point? Sensors are bigger than one point..

Simply, each point of the subject should focus at a single point on the sensor. Of course there are many points on the sensor, and many points on the subject too.Regards,Peter..

Comment #3

Just a point, as has been pointed out, different colours focus at different points in front of or behind the focal plane and so - several centuries ago - it was realised that using different types of glass would cure the problem (more or less). In those days the glass types used were flint glass and crown glass..

Since then other glasses have been developed and do the job better. In addition, lens designers often need a glass with a specific quality to fit into the space available; for example between the rear of the lenses and the CCD or film, which is why some exotic glasses are made using various rare elements in them..

Think of glass as a vague word like "brass" which covers a wide range of alloys and the matter might be easier. And, of course, there are the so called optical plastics used in lenses. All of them enabling the designer to tailor various parts of the lens to do a specific job..

Regards, David..

Comment #4

I see your question is already answered to your satisfaction. Here's an short interesting article you may find interesting. Note, the Tessar is prime lens, not a zoom. It's much less complex than a zoom but it demonstrates what others have talked about here. The Tessar type lens is/was very popular and offers good performance, especially for it's simple design..

Http://en.wikipedia.org/wiki/Tessar..

Comment #5

The human "prime lens" manages just fine without any corrective elements, and I dont see any chromatic aberration!Daniel..

Comment #6

Profborg wrote:.

The human "prime lens" manages just fine without any correctiveelements, and I dont see any chromatic aberration!.

Exactly, the fact was noted by Hall; Klingenstierna, Euler and Snell had a bit to do with it too but John Dollond applied for and got the patent. In a nutshell they realised that Newton's theory was wrong as the human eye didn't suffer from CA and - obviously - Snell's law and Newton's theory were not agreeing and so one had to be wrong..

Hall was an experimenter, rather than theorist and used crown and flint glass. Or rather had two lenses made of it as he was a lawyer with an interest in optics. That was the early to mid 1700's: about the time Dollond was born... Some 20 or so years later the man who'd made the lenses (sub-contracted to two opticians) told Dollond about it and, knowing Klingenstierna, Dollond started his experiments..

And, of course, in France Ciairault was also experimenting along those lines..

Dollond's still exist as opticians but older photographers may well have Dollond's lenses for their SLR's (mine is made in Japan but you can't have everything)..

Regards, David.

Interesting little story and probably on the internet somewhere. I happen to have a vague interest in astronomy and so was heavily into telescope construction some time ago. Try a search on "anchromatic" and "doublet" for more info but don't believe all you find..

Regards, David..

Comment #7

Profborg wrote:.

The human "prime lens" manages just fine without any correctiveelements, and I dont see any chromatic aberration!Daniel.

Actually the human "prime lens" is pretty poor. The structure of the retina is far from optimal as well but we have a truly amazing image processing system behind it all which hides most of the defects...

Comment #8

Andrew dB wrote:.

Profborg wrote:.

The human "prime lens" manages just fine without any correctiveelements, and I dont see any chromatic aberration!Daniel.

Actually the human "prime lens" is pretty poor. The structure of theretina is far from optimal as well but we have a truly amazing imageprocessing system behind it all which hides most of the defects..

My guess is that it appears poor to us, and the reason why it appears poor is because we dont understand enough about it..

David, I'm sorry but what theories are you talking about here? And what is the relevance of your story?.

Everyone, admit you looked for chromatic abberation when you read this post!.

Daniel..

Comment #9

Because proper PP-in "DSP" , located near eyeapples and in Vision centers engine in Human Brain..

But historically, for DC vendors - easier - put some money on aspeherical glass(and some other ppieces - up to lens case , than uncover mysterious and[possibly]danger world of writing really smart firmware for their DC's .

And-or - easiest way to make moneys(on lens sales .

Nothing changes until DC's - becomes INTEROPERABLE - btw - you can use optics form vendors A,B, C, D, E on digicam, made by vendor Z...

Comment #10

Profborg wrote:.

Andrew dB wrote:.

Profborg wrote:.

The human "prime lens" manages just fine without any correctiveelements, and I dont see any chromatic aberration!Daniel.

Actually the human "prime lens" is pretty poor. The structure of theretina is far from optimal as well but we have a truly amazing imageprocessing system behind it all which hides most of the defects..

My guess is that it appears poor to us, and the reason why it appearspoor is because we dont understand enough about it..

It's very well understood and the optics really aren't anything special which is at least partly to do with their size. The eye is limited to no better than around a minute of arc resolving power (a good lens might be 50-100 times better depending on atmospheric conditions) and a dynamic range of apparently not much more than 100:1..

When you get to the retina, you find there is no colour sensitivity at any significant distance from the fovea (perception of colour at the periphery of vision is put their by your brain), visual acuity also plummets off axis and the light sensitive cells are overlain by a web of nerves and blood vessels which does a good job of obscuring them and requires some rather fancy image processing to correct. Apparently half of the visual information from the retina comes from the central 1% so you don't notice the massive loss in resolving power in the rest of the eye...

Comment #11

Great explanation! But could you put that measurement of DR in normal format for me, so I can relate it to a camera's DR?Daniel..

Comment #12

Profborg wrote:.

Great explanation! But could you put that measurement of DR in normalformat for me, so I can relate it to a camera's DR?Daniel.

It's a bit more than 6.5 stops. I think that's better than slide film but worse than negative film and almost all digital cameras...

Comment #13

Profborg wrote:.

David, I'm sorry but what theories are you talking about here? Andwhat is the relevance of your story?.

Hi,.

Well, a while ago (about three days) a post appeared from someone with a name very much like yours (it could even be you) and in it they asked "I hear about lots of elements and complicated constructions. Why are these necessary? Why not just have one element, which can move on zoom lenses?" and then later on added "The human "prime lens" manages just fine without any corrective elements, and I dont see any chromatic aberration!"..

Newton said roughly that CA was inevitable and Hall and Dollond both made lenses from two varieties of glass to overcome CA. Both wondered why CA wasn't seen in the human eye and so thought about it. Actually, you do get CA with the human eye but - as I see it - the eye is backed by some very good software..

Can you see the connection between the human eye not seeing CA, compound lenses and the opening post?.

Newton went on to invent the Newtonian telescope (still one of the classics in use today) and Snell's Law of Refraction is just too well known to waste time on. Klingenstierna was the mathematician who had suggested Newton was wrong and supposed ways around it. As an aside I assume the obvious link between Snell's Law and the Olympus range of cameras..

I hope you've been taking notes as there's likely to be a question later on about it. </;-).

Regards, David..

Comment #14

David Hughes wrote:.

As an aside Iassume the obvious link between Snell's Law and the Olympus range of cameras..

As far as I'm aware, the naming is intended to signify "micro" in the colloquial meaning of "very small", hence that range of cameras are fairly compact. To expect the purchasers of the camera to relate to Snell's law would be a stretch too far by the even the most ambitious marketer.Regards,Peter..

Comment #15

HI,.

I assumed that as well but didn't say anything as it would lead to a little more typing. My guess was is was ambiguous as a joke for the band or printers might be. And please don't ask me to explain that but it's a very "in" joke delivered in public for a small minority to spot....

Regards, David.

Sherwoodpete wrote:.

David Hughes wrote:.

As an aside Iassume the obvious link between Snell's Law and the Olympus range of cameras..

As far as I'm aware, the naming is intended to signify "micro" inthe colloquial meaning of "very small", hence that range of camerasare fairly compact. To expect the purchasers of the camera to relateto Snell's law would be a stretch too far by the even the mostambitious marketer.Regards,Peter..

Comment #16

David Hughes wrote:.

Profborg wrote:.

David, I'm sorry but what theories are you talking about here? Andwhat is the relevance of your story?.

Hi,.

Well, a while ago (about three days) a post appeared from someonewith a name very much like yours (it could even be you) and in itthey asked "I hear about lots of elements and complicatedconstructions. Why are these necessary? Why not just have oneelement, which can move on zoom lenses?" and then later on added "Thehuman "prime lens" manages just fine without any corrective elements,and I dont see any chromatic aberration!"..

Newton said roughly that CA was inevitable and Hall and Dollond bothmade lenses from two varieties of glass to overcome CA. Both wonderedwhy CA wasn't seen in the human eye and so thought about it.Actually, you do get CA with the human eye but - as I see it - theeye is backed by some very good software..

Can you see the connection between the human eye not seeing CA,compound lenses and the opening post?.

No!.

Newton went on to invent the Newtonian telescope (still one of theclassics in use today) and Snell's Law of Refraction is just too wellknown to waste time on. Klingenstierna was the mathematician who hadsuggested Newton was wrong and supposed ways around it. As an aside Iassume the obvious link between Snell's Law and the Olympus range of cameras..

I hope you've been taking notes as there's likely to be a questionlater on about it. </;-).

You are right! Thanks for your patience though....

Regards, David.

Daniel..

Comment #17

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This question was taken from a support group/message board and re-posted here so others can learn from it.

 

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