Optics and lens design

[StephenM]

A long time ago, in a thread far, far away (apologies to George Lucas), specifically here

Large format books.

So I am seriously considering going Large Format, most likely a Intrepid 4x5 to start with. Looking for book recommendations for large format shooting. More informative books rather then someone's collection of images. So technique, principles and the like. Cheers.

www.talkphotography.co.uk

I suggested - and then backtracked - on adding a section on lens design and large format lenses to my least read worst seller. For reasons I may explain later - a direct result of a visit to a Steyning Art Trail location recently - I decided to revisit the idea.

It's not written yet, but I have roughed out a preliminary set of headings/topics to cover, and a reasonable introduction to books on optics from a photographer's point of view. I'm copying it here in case anyone wishes to read it, see where I'm going, and comment on what I should include that I don't appear to have covered. It's laid out in the sequence I envisage the final draft to be. Comments may include "don't bother", so feel free to express that view if you wish

*************

Books

In 1900, in the introduction to his translation of Otto Lummer's Contributions to Photographic Optics, Silvanus P. Thomson wrote (his italics):

"The simple reason of the badness of almost all recent British text-books of optics is that, with the exception of one or two works on photographic optics, they are written from a totally false standpoint. They are written, not to teach the reader real optics, but to enable him to pass examinations set by non- optical examiners. The examination curse lies over them all."

When I was a student, the optics text at my university was Richard Longhurst's Geometrical and Physical Optics. In the preface to the first (1957) edition he says that it was written "primarily for students reading for an honours degree in physics" and may therefore be assumed to align with the topics then covered in a physics degree. The contemporary Light by Robert Ditchburn fits the title well, being more concerned with light per se than optics.

The modern text books on optics can fairly be said to cover the courses taught in British (and presumably) American universities, and this has moved on. Sixty or seventy years ago, when the two titles I just referred to were first being written, topics like quantum optics and lasers were not in the standard texts, and were the province of separate books confined to the topic. As an example, a book I've just pulled from the shelf in front of me, T. P. Melia's An Introduction to MASERS and LASERS (capitalisation as in the book title) of 1967 has the following statement in the preface: "Though in the past few years several excellent books and numerous articles have been written on the subject of masers and lasers, there is still a need for an elementary introduction to the subject which is intelligible to the average science undergraduate".

To allow the introduction of modern topics of interest to researchers (and prepare students for topics being actively researched) material previously covered in the standard texts has had to be reduced in scope or cut out completely. Longhurst in the introduction to the third edition (1973) lists the new inclusions, and where discussion has been cut on older topics. It makes perfect sense; if information can be readily found in earlier standard works that should be available in the (university) library, why include it at the expense of topics that are being actively studied in research labs? And so, we see a steady shift in emphasis.

The main conclusion I want to make from this overview is that, for a photographer, the older books are often more relevant. What may be regarded as THE major modern standard work, Born and Wolfe's Principles of Optics starts from the first page with Maxwell's equations; and if geometrical terms like sines, cosines and tangents are off putting, divs, grads and curls will be a total turn off. Photographic optics - optics for the photographer - can omit this level entirely. And the practical application is this; I referred above to "earlier standard works" and if you note the date from which this shift began to happen, it will be clear that many highly relevant books will be out of copyright, and can be found on the Internet Archive, with either a free download or the ability to borrow.

I normally prefer to give not just conclusions, but the reasoning and evidence that support those conclusions. In this chapter, I'm setting that aside, and have no intention of giving anything more than an overview, but I do intend to give pointers as to where further information and detailed proofs can be found. Fortunately, as indicated above, this can be done using older texts that should not require any expense to download and read as they can be found on the Internet Archive. I shall give links to each title where possible.

On lens design specifically, the advent of computers has shifted what it is practically possible to do; simplifications that reduce very significantly the amount of calculations to be done are no longer needed, and programs reduce the lens designers work. However, for the purposes of knowing the principles, this can be ignored - which is just as well given the cost of some modern books on lens design. But even here, the Internet Archive has books available to borrow.

Light
Waves
Particles

Physical optics
Diffraction
Polarisation

Geometrical optics

In geometrical optics, light is treated as if it were particles travelling along rays in a straight line. It is simpler to use this picture and simple geometry to explain how lenses work and how they are designed, although some properties of light are best explained in terms of waves.

Action at a surface

Reflect
Angle
Mirrors
Parabolic reflectors
Specular reflections

Absorb
Total
Partial

Refraction
Refractive index
Snell's law
Total internal
Dispersion

Scattering

Glass
Types and properties.
Flint and crown

Lens design
Preliminary
Effect of heat
Tolerances of materials
Physical constraints: size, weight, size of standard blanks

Gaussian optics
Assumptions
Defects of spherical lenses
Description of aberrations
1st, 3rd, 5th order aberrations (Lummer, Siedel and notes)
Why no even numbers? Approximations and Taylor series

Approximations
Paraxial rays
Effect of computers

Monochromatic aberrations

Overview of how they are corrected (descriptive only, no calculations), tradeoffs

Petzval sum
Degrees of freedom
Auto corrections of symmetrical lenses
Field flatteners

Coatings and flare

Notes on older designs (properties, considerations in using)
Dagor (gold dot etc.)
Artar (Red dot etc)
Tessar
Repro and G Claron

Why you can get focus shift on stopping down
Convertible lenses

Assessing lenses
MTF
Resolution

 

[Terrywoodenpic]

The science of lenses used in photography is perhaps too vast a subject for just about any practicaal.photographer.
Most of us are only interested in Understanding our choices and options. And the pro's and con's of those alternatives.

From my experience, modern users are far from well informed about the image coverage of lenses, and how camera movements should be used to adjust the plane of focus in large format work.

For day to day digital work, recent modern lenses surpass so comprehensively the previous generations of lenses, that one wonders why anyone still uses heritage lenses, except to demonstrate their imperfections.

Today Lens design is so tied up with materials science and electromagnetic wave fronts and quantum effects,, statistics and computer science, that it is far beyond the comprehension of most of us that it might as well be witchcraft.

However the science behind the lenses actually available to us is shrouded in secrecy and trade law. We are never able to make real purchase judgements based on the science involved. The criteria that we can actually use is the same that w have always used. Prices, Reports, Recommendations and basic published technical details.
We buy the best that we can reasonably afford that will serve our purpose.

 

[StephenM]

The problem - and what I had primarily in mind - was the needs of large format photographers, where new lenses are more likely to be new, old stock. I hope I am wrong here, but as far as I know the only new lens you can buy for large format is the Cooke triple convertible, to order only and, last time I looked, about £4,500.

Hence my section on older lenses; what can you expect, which have focus shifts when you stop down, which will increase coverage when you stop down, what happens when you split a Symmar to give the longer focal length, etc. etc. I saw that as being useful.

I could take issue with some of your points on how difficult it is to understand- at least in principle, which is all I was intending to cover - modern lens design.

Thanks for the feedback. I think I'll not waste any more time on this.

 

[StephenM]

One possible error in my post above; if it comes to market, Intrepid have/had a new lens and shutter in development. But thesame was/is true of their 10x8 enlarger, and 5x7 and 10x8 film holders.

 

[Terrywoodenpic]

The problem - and what I had primarily in mind - was the needs of large format photographers, where new lenses are more likely to be new, old stock. I hope I am wrong here, but as far as I know the only new lens you can buy for large format is the Cooke triple convertible, to order only and, last time I looked, about £4,500.

Hence my section on older lenses; what can you expect, which have focus shifts when you stop down, which will increase coverage when you stop down, what happens when you split a Symmar to give the longer focal length, etc. etc. I saw that as being useful.

I could take issue with some of your points on how difficult it is to understand- at least in principle, which is all I was intending to cover - modern lens design.

Thanks for the feedback. I think I'll not waste any more time on this.

Only the early Symmars could be split to extend the focal length. However splitting them increased the aberrations especially colour fringing, they are essentially symmetrical lenses. Like advanced rapid rectilinears .

The Schneider lenses tend to suffer now from the flaking of the black antireflection paint flaking off from the internal metal and going everywhere. This is very common on super angulons. But rarer on angulons.
\
There are plenty of beautiful professional large format lenses available second hand, but still very expensive. I do not know why they are so rich as few people are. Actually using them.

 

[Barney]

On Lenses,

Is it possible to design a lens that would admit all light, without loss, but which would amplify individual colour eg RBY.

I don't mean a filter or blocker.

 

[Garry Edwards]

I applaud your dedication, but frankly don't see the point.

I think of myself as a bit of a backyard practical physicist, and back in the day when I was using LF cameras on a daily basis I learned everything I was capable of learning from a mixture of research and experimentation. Basically, what I learned was:

1. Image circle (coverage) is everything, Scheimflugg depends on it absolutely.
2. LF lenses are crap, compared to modern lenses for small cameras, in terms of resolution and aberrations (or at least mine were) and many or most of them were specifically designed for monochrome, not colour.
3. The more I learned about the technicalities of the optics, the worse my results were, because I was spending too much time and effort on the things that were beyond my own control. The best solution, for me, was almost always to think about the shot and forget about the aberrations that would inevitably result from using whichever lenses I had available, because only I would see those aberrations, and they were unavoidable anyway.

 

[Barney]

I applaud your dedication, but frankly don't see the point.

I think of myself as a bit of a backyard practical physicist, and back in the day when I was using LF cameras on a daily basis I learned everything I was capable of learning from a mixture of research and experimentation. Basically, what I learned was:

1. Image circle (coverage) is everything, Scheimflugg depends on it absolutely.
2. LF lenses are crap, compared to modern lenses for small cameras, in terms of resolution and aberrations (or at least mine were) and many or most of them were specifically designed for monochrome, not colour.
3. The more I learned about the technicalities of the optics, the worse my results were, because I was spending too much time and effort on the things that were beyond my own control. The best solution, for me, was almost always to think about the shot and forget about the aberrations that would inevitably result from using whichever lenses I had available, because only I would see those aberrations, and they were unavoidable anyway.

Thats an interesting perspective Garry,

Do you think that the reason they were craap was in part down to the quality of the actual glass that was available at that time?

There does not seem to be much wrong with the actual optical designs. Apart from simplicity.

There are many wonderful, detailed images from the time when nothing else was available.

I look at some old photos and think that I cannot reproduce the image quality even with the technology at disposal. and look at many other images with a similar problem.

 

[AndrewFlannigan]

I applaud your dedication, but frankly don't see the point.

I hold the same opinion.

On the other hand, as might be said about so many things: if it doesn't frighten the horses...

 

[Garry Edwards]

Thats an interesting perspective Garry,

Do you think that the reason they were craap was in part down to the quality of the actual glass that was available at that time?

There does not seem to be much wrong with the actual optical designs. Apart from simplicity.

There are many wonderful, detailed images from the time when nothing else was available.

I don't know, but my uninformed guess is that most of the aberrations resulted from a lack of perceived need for anything better. All lens faults are exaggerated or become more obvious with enlargement, which is basically why lenses designed for small cameras need to be good. Back in the bad old days, LF shots were typically not enlarged at all, and contact prints were more than good enough, and even when they were enlarged, they were enlarged far less than shots from small format cameras.

I believe that the old, simple designs such as Cooke and variants all pre-dated colour photography, at least in practical terms, therefore things like colour fringing simply didn't matter.

Also, with modern equipment and especially with Photoshop, both standards and expectations are far higher.

 

[myotis]

A long time ago, in a thread far, far away (apologies to George Lucas), specifically here

Large format books.

So I am seriously considering going Large Format, most likely a Intrepid 4x5 to start with. Looking for book recommendations for large format shooting. More informative books rather then someone's collection of images. So technique, principles and the like. Cheers.

www.talkphotography.co.uk

I suggested - and then backtracked - on adding a section on lens design and large format lenses to my least read worst seller. For reasons I may explain later - a direct result of a visit to a Steyning Art Trail location recently - I decided to revisit the idea.

It's not written yet, but I have roughed out a preliminary set of headings/topics to cover, and a reasonable introduction to books on optics from a photographer's point of view. I'm copying it here in case anyone wishes to read it, see where I'm going, and comment on what I should include that I don't appear to have covered. It's laid out in the sequence I envisage the final draft to be. Comments may include "don't bother", so feel free to express that view if you wish

*************

Books

In 1900, in the introduction to his translation of Otto Lummer's Contributions to Photographic Optics, Silvanus P. Thomson wrote (his italics):

"The simple reason of the badness of almost all recent British text-books of optics is that, with the exception of one or two works on photographic optics, they are written from a totally false standpoint. They are written, not to teach the reader real optics, but to enable him to pass examinations set by non- optical examiners. The examination curse lies over them all."

When I was a student, the optics text at my university was Richard Longhurst's Geometrical and Physical Optics. In the preface to the first (1957) edition he says that it was written "primarily for students reading for an honours degree in physics" and may therefore be assumed to align with the topics then covered in a physics degree. The contemporary Light by Robert Ditchburn fits the title well, being more concerned with light per se than optics.

The modern text books on optics can fairly be said to cover the courses taught in British (and presumably) American universities, and this has moved on. Sixty or seventy years ago, when the two titles I just referred to were first being written, topics like quantum optics and lasers were not in the standard texts, and were the province of separate books confined to the topic. As an example, a book I've just pulled from the shelf in front of me, T. P. Melia's An Introduction to MASERS and LASERS (capitalisation as in the book title) of 1967 has the following statement in the preface: "Though in the past few years several excellent books and numerous articles have been written on the subject of masers and lasers, there is still a need for an elementary introduction to the subject which is intelligible to the average science undergraduate".

To allow the introduction of modern topics of interest to researchers (and prepare students for topics being actively researched) material previously covered in the standard texts has had to be reduced in scope or cut out completely. Longhurst in the introduction to the third edition (1973) lists the new inclusions, and where discussion has been cut on older topics. It makes perfect sense; if information can be readily found in earlier standard works that should be available in the (university) library, why include it at the expense of topics that are being actively studied in research labs? And so, we see a steady shift in emphasis.

The main conclusion I want to make from this overview is that, for a photographer, the older books are often more relevant. What may be regarded as THE major modern standard work, Born and Wolfe's Principles of Optics starts from the first page with Maxwell's equations; and if geometrical terms like sines, cosines and tangents are off putting, divs, grads and curls will be a total turn off. Photographic optics - optics for the photographer - can omit this level entirely. And the practical application is this; I referred above to "earlier standard works" and if you note the date from which this shift began to happen, it will be clear that many highly relevant books will be out of copyright, and can be found on the Internet Archive, with either a free download or the ability to borrow.

I normally prefer to give not just conclusions, but the reasoning and evidence that support those conclusions. In this chapter, I'm setting that aside, and have no intention of giving anything more than an overview, but I do intend to give pointers as to where further information and detailed proofs can be found. Fortunately, as indicated above, this can be done using older texts that should not require any expense to download and read as they can be found on the Internet Archive. I shall give links to each title where possible.

On lens design specifically, the advent of computers has shifted what it is practically possible to do; simplifications that reduce very significantly the amount of calculations to be done are no longer needed, and programs reduce the lens designers work. However, for the purposes of knowing the principles, this can be ignored - which is just as well given the cost of some modern books on lens design. But even here, the Internet Archive has books available to borrow.

Light
Waves
Particles

Physical optics
Diffraction
Polarisation

Geometrical optics

In geometrical optics, light is treated as if it were particles travelling along rays in a straight line. It is simpler to use this picture and simple geometry to explain how lenses work and how they are designed, although some properties of light are best explained in terms of waves.

Action at a surface

Reflect
Angle
Mirrors
Parabolic reflectors
Specular reflections

Absorb
Total
Partial

Refraction
Refractive index
Snell's law
Total internal
Dispersion

Scattering

Glass
Types and properties.
Flint and crown

Lens design
Preliminary
Effect of heat
Tolerances of materials
Physical constraints: size, weight, size of standard blanks

Gaussian optics
Assumptions
Defects of spherical lenses
Description of aberrations
1st, 3rd, 5th order aberrations (Lummer, Siedel and notes)
Why no even numbers? Approximations and Taylor series

Approximations
Paraxial rays
Effect of computers

Monochromatic aberrations

Overview of how they are corrected (descriptive only, no calculations), tradeoffs

Petzval sum
Degrees of freedom
Auto corrections of symmetrical lenses
Field flatteners

Coatings and flare

Notes on older designs (properties, considerations in using)
Dagor (gold dot etc.)
Artar (Red dot etc)
Tessar
Repro and G Claron

Why you can get focus shift on stopping down
Convertible lenses

Assessing lenses
MTF
Resolution

I think this a great idea, but sounds like a lot of work.

It's intrinsically interesting to have some understanding of how the tools of our trade work, and how this might, or might not, affect our image making.

It would be useful for people interested in the history of photography, especially when old designs still get mentioned with reference to modern lenses.

And it sounds useful for those looking at the second hand market for large format lenses.

 

[Barney]

I think that if the level of technicality was kept to the level of, for instance, "The Royal Institute Christmas lectures", it would suite my level of understanding and would be a valuable addition to any photography book. In order of importance to photography in general I would place it first.

Lens
automatic metering
automatic focusing

after that its mostly flannel, blather and sales patter.

 

[StephenM]

I think this a great idea, but sounds like a lot of work.

It's intrinsically interesting to have some understanding of how the tools of our trade work, and how this might, or might not, affect our image making.

It would be useful for people interested in the history of photography, especially when old designs still get mentioned with reference to modern lenses.

And it sounds useful for those looking at the second hand market for large format lenses.

That was my original opinion; to write something that would inform photographers of some of the history of their craft, and, perhaps even more, allow informed choices to be made when buying second hand lenses for large format cameras. Especially since virtually no large format lenses are being made now, and second hand is the only source. In the time since I started this thread I have become more aware of the lack of interest in photographers in the history of photography, in the lack of interest in the important photographers of the past (and even more so of the inevitable response to that statement - "who decides who's important - it's all subjective anyway").

I applaud your dedication, but frankly don't see the point.

And that's the killer argument to me for not bothering. Although Terry's post makes the futility even more apparent:

Today Lens design is so tied up with materials science and electromagnetic wave fronts and quantum effects,, statistics and computer science, that it is far beyond the comprehension of most of us that it might as well be witchcraft.

However the science behind the lenses actually available to us is shrouded in secrecy and trade law. We are never able to make real purchase judgements based on the science involved. The criteria that we can actually use is the same that w have always used. Prices, Reports, Recommendations and basic published technical details.
We buy the best that we can reasonably afford that will serve our purpose.

Summing up: it's impossible to explain (I honestly thought it was, as mathematics can be kept to an almost non-existent minimum if all that's being explained are the principles (just as you can explain a central heating system without having to go through the calculations on heat output from radiators, room sizes, levels of insulation etc. that were gone though when we had central heating installed). I was honestly unaware of the "secrecy and trade law" issues, since lens design patents are matters of public information. Exactly how the aspherical lenses are manufactured might be - I've never bothered trying to understand the mechanics (although at least two text books are available on this side of lens making, and I have one of them somewhere).

To me, it seems impossible to "buy the best we can reasonably afford" if we have no idea as to what the different characteristics of various lenses are; buying on name or price seems the only option left. A knowledge of the history of lenses shows that photographers once upon a time chose the lens according to the subject being photographed - Petzval designs, very sharp on axis, blurry as you moved to the corners were ideal for portraits, but less so for architecture. The only way to be able (in my opinion) to make an informed choice is to have the information (sounds tautological, doesn't it?). That was what I hoped to make possibly more easily available, knowing how much I had to read through to gather it. Even how much effort it was to locate the sources online.

As a final point, I'd just like to address this comment very briefly:

I don't know, but my uninformed guess is that most of the aberrations resulted from a lack of perceived need for anything better.

Given the vast number of different designs that were patented, sold and then discarded I'd say that in actual fact makers large and small were attempting to improve lenses to improve their bank balances; there was an undoubted demand. Lens designers tended to be individuals, and to some extent rather set in their ways. It's a simple fact of history that lenses predated photography, and that the Petzval lens was the first that was mathematically computed rather than made by trial and error. Lens designers did tend to have a modern photographer's disdain for the theoretical, and the groundbreaking book on aberrations was unknown in the English speaking world before it was translated in the 1920s. One British lens designer (died, from memory, in the 1940s) was refining his designs by regrinding rather than recalculating right up to the end.

Designers were working to remove aberrations as best they could, in the early years sadly lacking in the mathematical tools to do so.

It is of course true that enlargement enlarges defects, and when you were using 20"x24" plates you probably weren't intending to enlarge them much .

One day, if I return to the Ryedale Folk Museum, I may test out the superiority of modern lenses by recreating a photograph I made with a 1950s Symmar lens using a more modern lens, and see how much more (or less) detail it can cram into a full frame digital sensor (to compare old with modern takes on apparatus).

Summing up, based on the responses here, the only conclusions I can draw are:

1. I'd be taking on a project that to most forum members (and, by extension, photographers) is irrelevant. I base this on the replies and more importantly, lack of replies, in this thread;

2. It would involve some time and effort on my part, although I have enough information in my memory to write the bones without needing to consult other works;

3. That it can't be done anyway as it's too hard for "normal" people to understand.

In Lord of the Rings, Gandalf remarks of Saruman's knowledge of ring lore, that learning, however deep, has a source. I know nothing that others cannot discover for themselves, given the time and effort. And for me, the extra effort of writing it down, even though it would help fix it in my mind, doesn't have a commensurate return. So, in the well known catchphrase, I'm out. But many thanks to all who replied and crystallised my thoughts - and thereby saved me time and effort.

 

[Terrywoodenpic]

Thats an interesting perspective Garry,

Do you think that the reason they were craap was in part down to the quality of the actual glass that was available at that time?

There does not seem to be much wrong with the actual optical designs. Apart from simplicity.

There are many wonderful, detailed images from the time when nothing else was available.

I look at some old photos and think that I cannot reproduce the image quality even with the technology at disposal. and look at many other images with a similar problem.

Large format lenses were as good as they needed to be. a 20x 16 print only needed a 4x enlargement from a 5x4 negative..
The best lenses were designed for reprographic use and had both flat fields and fantastic resolution and were usually also apochromatic.
Going back even earlier, the lenses used for daguerreotypes produced fantastic detail but no one cared the least about things like spherical aberration and the like.
From the 50's onwards I mostly used 5x4 and 5x7 professionally,I never felt short of quality. I mostly used Tessars, Symmars angulons and super angulons. Probably the sharpest lens that I ever used was a 500mmm Tele Tessars on a 5x7.