Photoscience Consultant Addresses The Way Lenses Turn Light Into Images

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TAKE a sheet of plain paper Now hold a lens in front and move it backwards and forwards. At a particular point a sharp image of the scene before it will appear. But what has happened7 The light from the objects in a scene

- photographers call it the subject

- scatters in all directions. To form an image, this light must be gathered and focused. To do this, light rays entering the lens must be bent in to form a sharp, miniature, two-dimensional version of the three-dimensional subject. This bending is called 'refraction', and the universal refractive medium is glass.

Basic lenses

A flat piece of glass such as a window - plane parallel - transmits the light unaltered. The thickness of a piece of glass must be varied in order to refract it and concentrate it into an image. In fact, the glass must vary uniformly across its surface, with the thickest in the middle and thinnest at the edge. In practical terms the surface has to be curved; when looked at in three-dimensions, it should form part of a sphere. Nowadays, however, some photographic lenses use aspheric surfaces, which do not forming part of a sphere (Figure 3).

There are two main types of lens element: positive and negative (Figure 2). The positive lens gathers light and focuses it; a negative lens gathers it and bends it outwards. On the face of it, that's not much use for imaging, but a combination of positive and negative refraction over a number of elements can give a better image than a positive alone. It is this combination that decides how good a lens is.

Focal length Lens types

Figure 1 Figure 2

Figure 1 Figure 2

The focal length of a lens can be measured as the distance between its optical centre These are the main types of spherical lens elements, and the focused image of an object at infinity. The size of the image is proportional Combining these forms, made from appropriate to the focal length. Doubling the focal length also doubles the image size optical glass, will create a well-corrected lens

The focal length of a lens can be measured as the distance between its optical centre These are the main types of spherical lens elements, and the focused image of an object at infinity. The size of the image is proportional Combining these forms, made from appropriate to the focal length. Doubling the focal length also doubles the image size optical glass, will create a well-corrected lens

How lenses work Geoffrey Crawley explains..,

But back to the simple lens: the meniscus (Figure 3). Though it works well, the variation in thickness - the spherical shape - means that the subject is not imaged sharply on a flat surface. The sharp image will instead be found on a curve corresponding to the spherical shape of the lens element. This error is known as spherical aberration (Figure 3). Simple lenses had been used by artists to project an image of a subject onto a screen to help them draw its outlines centuries before photography existed. It was eventually discovered that the increasing fuzziness away from the image centre could be reduced by placing a 'stop' in front of the lens: a circular hole narrower than the lens diameter. We still use the term today. It works by cutting down the input from the outer parts of the lens, creating less spherical aberration.

This produces a sharper image in the corners, but makes more apparent the distortion on imaged buildings: their straight lines are bowed in (pincushion distortion) or out (barrel distortion). Eventually, this was overcome by the British scientist WH Wollaston in 1820, some 20 years before Daguerre launched his photographic process. He used an element with one side curved inwards (concave), and the other curved out

It was found that the increasing fuzziness away from the image centre could be reduced by placing a stop in front of the lens

(convex), and fronted by a stop. When photography then came along, it found in this discovery a ready-made camera lens. However, it only worked at f/15, which was fine for landscapes but not for portraits. No one would want to sit for a 20-minute exposure. So now the speed, or maximum aperture, of a lens became important. The Austrian mathematician Petzval came up with the answer: an f/3.2 design and the first wide-aperture lens.

The eye

The combination of human eye and brain is a remarkable one, but its very qualities introduce problems for the colour photographer. Chief among them is its power of accommodation. It took about 125 years for designers to come up with a camera featuring

an iris diaphragm that responded automatically to the light level via an exposure meter. Humans and other animals have had eyes with auto-irises for many millions of years. Yet the eye has all manner of aberrations and would make a pretty poor camera lens. It is the brain that compensates for these, and allows us to see a spatially and geometrically accurate image. If you were to wear inverting glasses continuously for a time, you would wake up one morning seeing everything the right way up. Spherical aberration and curvilinear distortion are also corrected. The image is sharp everywhere - if you have good eyesight - as the image is built up using its autofocusing scanning ability. Photoshop lens correction tools are a bit feeble in comparison.

Our eyes have a set of sphincter muscles that contract the pupil, as well as dilator muscles that open it. These muscles are constantly working to adjust the size of the aperture through which we see

Yet most important for imaging in colour is the ability of the eye and brain to accommodate the quality of light illuminating the subject. A sheet of white paper viewed in domestic lighting looks just as white as it does in bright sunlight. But photographs taken on film or with a digital camera, and balanced for sunlight, will show a white that is sickly yellow under artificial light. Using film or a digital camera balanced for artificial light produce a white that looks blue under sunlight. A photographer can accommodate for this by using a blue filter with a daylight balance film or a warm filter with an artificially balanced film respectively. With digital imaging, simply select an appropriate white balance setting. Neither method, however, is likely to do as good a job as the eye and brain.

Spherical aberration

a saucer shape. It is caused by those rays entering the outer margins of the lens being refracted more than the central ones, which brings them to focus closer to the lens. Stopping down will improve the image by increasing depth of focus in the image plane

^k Spherical surface

The use of aspheric-surfaced ^

elements has brought major Jfl enhancements in lens design.

Unlike the classic spherical J^M Aspherical surface elements, their surface curves do not form part of a circle or sphere.

They were first investigated by

French philosopher Descartes

(1596-1650)

Curvilinear distortion

Curvilinear distortion bends straight lines and the distance from their correct position at the centre, as a percentage of the length of the line, gives the measure of the distortion. In the diagram the distortion errors C-D and E-F amount to about -14% and +14% respectively

Geoffrey Crawley explains... How lenses work

The science

Chromatic aberration control

Since the materials photographers used in the early days were only sensitive to blue light, chromatic aberrations were not that important. Chromatic aberration causes unsharp images when light of different colours and wavelengths focuses at different distances from the lens. As photographers became more critical, it was noticed that an image could be sharp on the focusing screen but somewhat less sharp when the plate was developed. It occurred because the human eye is most sensitive in the yellow/green wavelength and uses it to focus. The early plates would 'see' in blue and this focus difference caused the blur. From then on, the elimination of chromatic aberration has been a prime task of lens designers, as it is a major cause of poor-quality colour images. Digital cameras are particularly affected as they are susceptible to colour fringing. This is why AP tests highlight how well colour corrected a lens is.

All the lenses we use - except for those made for very inexpensive cameras - are at least 'achromatic'. That means they are corrected for two major colour bands in the spectrum: blue to yellow/green, tailing off towards the deeper red. A few are 'apochromats', identified by the 'apo' suffix or prefix. This term should really mean that full colour correction extends to a third colour, into the deep red. However, It is now sometimes used a little loosely, meaning that the correction into the red is more than an achromat. Performance is improved, though, notably when used on digital cameras where the image projected by the lens is further processed by the microlenses that face the light-sensitive photosites. However, these have their own aberrations.

Modern optics

The very high image quality obtained with modern lenses was finally made possible by the development of new optical glasses in the 1960s and 70s. Familiar suffixes to modern lens names such as ED, LD, SLD or ULD refer to the use of glass with a high refractive index, capable of bending light to a high degree. But unlike normal glasses they bring the colours in the spectrum much closer to a common focus: they have low colour dispersion, and make it possible to achieve a higher level of chromatic correction (Figure 5a and 5b). At about the same time the possibilities of aspheric surfaces also began to be expiated. These enable fewer elements to be used and spherical aberration to be more easily corrected.

Keeping down the number of elements in zoom lenses - 13-16 is common - is important, as some light loss is unavoidable and reflections between the surfaces increases. Those reflections dilute colours and contrast - more so in a digital camera since the highly polished surface of its sensor acts as a mirror. That is why so much attention is paid to the anti-reflection coating of lens surfaces. These thin films reflect the light out of step - or phase - with the reflection from the lens glass beneath it. This interference cancels out its energy, so removing the reflection and preventing unwanted light straying around inside the lens.

Image-forming light

We see objects by the light they reflect, which presupposes that a source of light is present - no light, no see. There are two 'constant' natural light sources: the sun and the moon, though the moon just reflects the sun's light. Then there are artificial light sources, including incandescent lamps (such as domestic bulbs), fluorescent lamps, halogen lamps and electronic flash - even candles. All light sources, whether natural or artificial, radiate electromagnetic energy. The unit is the photon, which is considered as a particle with a waveform or shape. The higher the

Hie high image quality obtained with modern lenses was made possible by the development of new optical glasses in the 1960s and 70s

Figure 5a Figure 5b

Chromatic aberration

Figure 5a Figure 5b

Axial chromatic aberration disperses the colour in a line vertical to the image plane according to the spectrum

Special low colour dispersion optical glasses reduce axial colour errors. Firms have their own brand names for these glasses, shortened to ED, LD, SLD, ULD, and so on

How lenses work Geoffrey Crawley explains...

Lateral chromatic aberration causes a point in the subject to be spread according to its colour content across the image. It is especially damaging in digital imaging since it is made worse by the aberrations in the microlenses fronting the photosites. The result is colour fringing energy, the more waves are packed into the particle, so the distance between the peaks or troughs of the waves - the 'wavelength' - shortens as energy increases.

The light energy emitted or reflected by an object scatters in all directions. As it does so, it attenuates, spreading out increasingly with distance. It does so according to the inverse square law: double the distance from the object and the volume of light falling on a given area is reduced to a quarter In order to assemble an image of an object, some of this scattering light has to be captured. To do this a device has to be used that can gather a sample and reassemble the contents into a facsimile of the object. We call such a device a lens, after the lentil (Greek) beam whose shape a simple version resembles.

Light from the various areas within a lens view arrives at different angles to its surface. It is then refracted to fall into place in a small, two-dimensional representation of the original. As it is two-dimensional and the subject is, typically, three-dimensional, it is necessary to move the lens forward or back to produce a sharp image of an object at a particular distance from the lens. This is known as focusing. Perhaps in the future it will be possible to produce a recording medium having 3D depth. A lens will then be able to record a miniature in-depth facsimile of a scene in one exposure.

When the refractive power of certain shapes of transparent materials was first realised is unknown, but it is difficult to believe that advanced cultures such as the Chinese, Egyptians and Greeks failed to notice at least the magnifying ability of some pieces of glass. However, we had to wait for relatively modern times before 'lenses' were put to use. One philosopher has suggested that all the inventions that will ever be made are already out there waiting to be realised. They will be discovered when the need for them arises. Hero of Alexandria built a toy steam engine 1,800 years before the Industrial Revolution demanded that it be developed In any event, this year has seen the 400th anniversary of a Dutchman using two lenses to produce an enlarged image of distant objects. He had discovered the telescope. Shortly after, Galileo put the device to astronomical observation. What he saw challenged some fundamental dogma of the Church and got him into deep trouble. The reality of what a lens records was not yet recognised.

Conclusion

Finally, this overview would not be complete without a tribute to the unsung engineers who manage to cram complex gears and cams into compact lens mounts. These enable internal focusing and zooming, as well as moving of groups of elements to and fro to maintain aberration corrections over the zoom and focusing ranges. Thank you. AP

Though they look similar, fringing isn't necessarily chromatic aberration. Fringing often occurs around highlights or dark areas when one or two colour channels burn out before another. The microlenses over sensor photosites can also cause purple fringing through chromatic aberration

Chromatic aberration is often most noticeable along strong contrast edges, such as in this shot of a dark statue against a bright overcast sky. Both red and cyan aberrations can be seen in this example. It would be relatively easy to adjust an image like this by desaturating the selected colours, but this technique isn't suitable for all images. Fortunately, many image-editing software packages, such as Photoshop CS3, now offer a chromatic aberration correction tool. These effectively shift the colour bands until they disappear, but there are usually blank areas around the image that must be cropped out

Roger Hicks

ROGER HICKS is a much-published author on photography. He has written more than three dozen books on the subject, many in partnership with his wife, Frances Schultz. Roger started photography as a teenager in the 1960s and worked professionally in a London advertising studio in the mid-1970s. He has been a freelance photographer/writer since 1981, contributing to many photography magazines, including 'Shutterbug' in America. Visit his website at www. rogerandfrances.com.

IN THIS DIGITAL AGE, THERE IS AN EVER-INCREASING WALL OF UNREALITY BETWEEN US AND WHAT WE DO

NE of the reasons steam locomotives are so popular, I am convinced, is that they are so comprehensible. Or at least, they seem to be: as the son of a steam engineer, I know that there is rather more to steam than meets the eye. We think we understand steam engines, though, and we do understand the basics. Water turns to steam, it expands, pushes a piston, and the piston turns the wheel. There's no need for a gearbox as a steam engine delivers maximum torque at zero revolutions.

Likewise, a mechanical camera is broadly comprehensible. On my 1936 Leica Ilia, a twist of the wind-on knob advances the film, returns the shutter to its start position, and winds up the springs that drive the shutter on its next journey. As you lift, twist and drop the shutter speed dial, a little peg drops into a hole (you can't see this unless you take the top off) to govern the separation between the shutter blinds: the bigger the separauon, the more exposure the film gets. Then you focus the lens, on a helical mount: you can see it mcwe. You set the aperture: little steel leaves in the diaphragm create a bigger or smaller hole. Finally, you press the shutter release: there's a satisfying muted clop! as it fires, and a slight vibration. You know you've taken a picture.

Admittedly, a lot of our apparent understanding is illusory. Could you build a steam locomotive, let alone a Leica? If I were transported back to the 18th century, and given enough resources, I could almost certainly build a beam engine of the kind that was used to pump out the mines (and indeed transport the miners) in my native Cornwall. If I could do that, I'm pretty sure I could refine it with an automatically operated separate condenser chamber. But these are still atmosphenc engines, not steam: a triple-expansion Burrell traction engine from the mid-to-late 19th century, let alone a locomotive, would be completely beyond me.

Likewise, I could probably remember enough to 'invent' photography in the 18th century, maybe even getting as far as the dry plate, but equally I might only get as far as the Calotype (I don't think I'd bother with daguerreotypes). I certainly couldn't start coating llford HP5 Plus at several metres per second.

But even if a lot of what we think is understanding is, in fact, illusion, there's also plenty that isn't With many mechanical faults, or, for that matter, with a defective negative, I can look at what is wrong, without any tools or instruments, and work out the likely cause. Maybe I couldn't build the machine, or synthesize the chemicals that are needed to make up the developer, but I can look at a made thing, and understand the nature of its making, and work out what to do. lean form a very good idea of what is going on, just by looking at it

Which I can't with a CD. Recently, I had to make up some new business cards. The file for doing this is on a CD, and the only practical way to find the CD is by seeing what is written on it. Admittedly, I could put every CD I own through the CD reader, and do a visual search, but that would take until Christmas - and not necessarily Christmas this year This brings me into 'primaiy and 'secondary1 searches.

With negatives or slides, I put them on the lightbox or hold them up to the light. I search the things themselves: a primary search. With the CD, on the other hand, I must first search through the CD, using a completely unrelated primary search system - writing, not the image itself - before I can look for the image on the CD, which is a secondary search.

How much does this matter7 I'm not sure. I am sure, though, that there is an ever-thicker wall between me, and what I do. It is what Sylvia Plath called the bell jar, a layer of unreality.

Everyone has their own comfort level with this unreality, and I have to say that I would far rather write on a computer than in longhand -1 hate to disappoint those of you who think I use a quill. On the other hand, I do fear that as we move further and further from the physical, we lose more than we realise. I can look at words and pictures on a screen, but they are ephemeral and no substitute for a crisp, freshly printed book or magazine, or a well-made silver halide print. So let's get physical. AP

fiS Even if a lot of what we think is understanding is, in fact, illusion, there's also plenty that isn'tSS

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