Photographing the Unseen World E-Book

Adrian Davies

• Herausgeber: Crowood
• Kategorie: Geisteswissenschaft
• Sprache: Englisch

Seitenzahl: 304

Veröffentlichungsjahr: 2020

Beschreibung

For the first time of its invention, photography has been used to visualize events that are either too fast or too slow for the eye to perceive, or subjects that are outside the spectral range of the human eye. This book shows how you can photograph a range of subjects and see the world as never before. Written with clear and accessible text, it explores and suggests techniques that expose new images in new ways, and pushes the boundaries of the photographer's creative potential. Techniques include: ultraviolet (UV) and infrared (IR) photography; high speed and time-lapse photography; close-up, macro and photography with the aid of a microscope and finally, photography using polarized light. Most of the techniques are accessible ot all photographers using readily available equipment (UV and IR will require some specialist items), and have been relatively unexplored so give the adventurous photographer great potential to experiment and produce unique images.

Leseprobe

Photographing the Unseen World

Photographingthe Unseen World

Art and Techniques

Adrian Davies

First published in 2020 byThe Crowood Press LtdRamsbury, MarlboroughWiltshire SN8 2HR

[email protected]

This e-book first published in 2020

© Adrian Davies 2020

All rights reserved. This e-book is copyright material and must not be copied, reproduced, transferred, distributed, leased, licensed or publicly performed or used in any way except as specifically permitted in writing by the publishers, as allowed under the terms and conditions under which it was purchased or as strictly permitted by applicable copyright law. Any unauthorised distribution or use of this text may be a direct infringement of the author’s and publisher’s rights, and those responsible may be liable in law accordingly.

British Library Cataloguing-in-Publication DataA catalogue record for this book is available from the British Library.

ISBN 978 1 78500 704 0

AcknowledgementsA book like this, covering a wide range of subject matter would not have been possible without the help of numerous friends and colleagues, for help, advice and occasional modelling! Andrea Blum, Bjorn Rorslett, and various members of the ultravioletphotography.com forum; Stephen Bleksley; Paul Buff Inc.; Matt Cass; Lucie Cash, Royal Mail; Paul Colley; Dr Jonathan Crowther; Hannah Davies (and ‘Jack’); Janice Davies; Adam Davies, Maidenhead Aquatics; Robin Davies; Bryony Davies; Miles Herbert, Captive Light Photography; Kat Jack; Yealand Kalfayan; Nicola Musgrave BSc (Hons) MSc., Eurofins Forensic Services; Phred Petersen, RMIT University; Paul Reynolds, Sigma Imaging (UK); Martin Sanders; Hugh Turvey Hon FRPS, FRSA; Paul Wilson; Michael Robbs

CONTENTS

1  Introduction

2  The Electromagnetic Spectrum, Camera Conversions and Filters

3  Ultraviolet Reflectance Photography

4  Ultraviolet Fluorescence Photography

5  Infrared Photography

6  High Speed/Short Duration Photography

7  Time Lapse and Other Time Related Imaging

8  Visualization Through Magnification

9  Polarized Light and Other Visualization Techniques

Resources

Bibliography

Interesting Websites

Index

Chapter 1

Introduction

From its earliest beginnings, photography has been used to help visualize the unseen – events that are either too slow or too fast for the human eye to perceive, to record subjects that are visible only in areas of the spectrum outside the human range of sensitivity, or record detail that is too small for the human eye to see. There are, and have been, many imaging techniques for visualizing the invisible, including X-ray and gamma-ray imaging, various medical scanning techniques such as MRI and CAT scans, Kirlian photography, schlieren photography, electron and light microscopy, thermal imaging and various forms of satellite imaging, though most of these techniques are unavailable to most readers, being either too dangerous, too expensive or requiring unattainable pieces of equipment.

This book is concerned with those techniques that anyone with a good knowledge of photography can achieve with minimal outlay. It will look at ultraviolet (UV) and infrared (IR) photography, high speed and time-lapse photography, close-up, macro and photography with the aid of a microscope, and photography using polarized light and other related techniques. Some of the applications will utilize two or more of the techniques in combination, and do not fit into neat categories.

The very act of viewing a subject through a magnifying glass can reveal previously unseen details within a subject, and the higher the power of the magnifying glass, or microscope, the more detail will be revealed. In particular, 3D electron microscopy not only reveals otherwise invisible detail, but also produces, at times, stunningly beautiful images. Electron microscopes are very expensive, and well outside the scope of this book, but a brief discussion of light microscopy will be given in Chapter 8.

The whole of life lies in the verb seeing.

Pierre Teilhard de Chardin

X-ray images of everyday subjects often reveal beautiful and startling details, such as the carnivorous pitcher plants shown here, alongside UV images of the same plant.

Schlieren photography is another technique used by scientists, and restricted to laboratory use, but capable of revealing heat and pressure patterns around suitable subjects such as the heat emitted from the soldering iron shown here. The cover image for John Martyn’s Solid Air album, from 1973, was a schlieren image of the heat emitted from a hand.

Fig. 1.1Kirlian image of stinging nettle (Urtica dioica) leaf. The leaf was placed on a sheet of 5 × 4” transparency film in a dark room, which itself was placed on a metal sheet. A high voltage electrical current was passed through the metal sheet, resulting in a Kirlian image.

Home made Kirlian imaging device. 5" × 4" Fujichrome film. Exposure approx. 30 seconds.

Kirlian photography (see Fig. 1.1) is another visualization technique, used to capture the phenomenon of electrical coronal discharges (often referred to as the aura around a subject). It is named after Semyon Kirlian, who, in 1939, accidentally discovered that if an object is placed on a photographic plate that is connected to a high-voltage source, it produces an image. It became popular in the 1980s, and various claims for the technique were made with regard to it being able to predict illness or other issues, or reveal the natural aura around a subject. There is little evidence for these claims, and the technique has largely fallen out of use.

Fig. 1.2Image of lilac (Syringa sp.) pollen at a magnification of ×3,000, using a scanning electron microscope.

Authors: Yapryntsev, A.D; Baranchikov, A.E; Ivanov, V.K. This file is licensed under the Creative Commons Attribution 4.0 International license.

Fig. 1.3Several different imaging techniques were used to illustrate different aspects of a carnivorous plant (Nepenthes spp.), resulting in very different images and differing information about the subject in Figs 1.3 to 1.6.Conventional image of Nepenthes spathulata × (copelandii × truncata) shot with visible light.

Fig. 1.4X-ray image of carnivorous pitcher plant (Nepenthes ventricosa × talangensis) yielding a truly beautiful image, showing remarkable clarity and detail. Unfortunately, the equipment required is not available to most people.

Image copyright Hugh Turvey, Hon FRPS, FRSA.

UV photography, in particular, is nowadays helping us get an idea of how other animals see the world. As early as 1864, Hermann Vogel, a German chemist, photographed a woman using a photographic plate that at that time was only sensitive to UV and blue light, using a simple, uncoated lens that transmitted good quantities of UV. The processed image showed several black spots on the woman’s face, invisible in normal daylight. A few days later the woman was diagnosed with smallpox. The significance of this UV reflectance image was not realized for another forty years or so.

Today, UV photography is used by forensic scientists for detecting trace evidence, by art historians examining works of art, and by botanists and entomologists investigating the hidden patterns on flowers, invisible to us, but visible to insect pollinators.

Fig. 1.5UV fluorescence image shot with UV light, showing fluorescent rim (peristome) possibly acting as an attractant to prey.

Fig. 1.6UV reflectance image of pitcher showing how visible light patterns disappear, and how nectar absorbs UV to become more apparent to potential prey.

IR photography, also, has a long photographic history, for penetrating haze in landscapes, camouflage detection, visualizing subcutaneous veins, revealing alterations to paintings, and helping diagnose plant diseases for example. It has enjoyed a great resurgence in recent years, with the ease of getting cameras converted to record IR.

Fig. 1.7Schlieren image of otherwise invisible heat waves rising from a soldering iron. Schlieren photography involves focusing a collimated beam of light to a point. If a knife edge is positioned precisely at that point, then the slightest disturbance in the medium will divert the rays of light to produce changes in brightness. It is used infrequently nowadays but was used to show changes in fluid density and refractive index of a substance in glass for example.

Author: Ian Smith. This file is licensed under the Creative Commons Attribution 3.0 Unported license.

Fig. 1.8Infrared (IR) image (850nm) of female model wearing plastic tinted sunglasses. The plastic becomes transparent in IR. Note too the veins in the neck and forehead revealed by the IR.

In 1872, the English photographer Eadweard Muybridge undertook photographic studies of animals in motion. These were carried out primarily for a commission from the ex-governor of California, Leland Stanford, who had a wager with a friend that horses had all four legs off the ground at the same time when running and trotting. By using from twelve to twenty-four cameras triggered with tripwires, Muybridge produced some of the first studies of animals in motion, and also helped prove that horses did indeed have all four legs off the ground at the same time, though not in the rocking horse attitude so often depicted in contemporary paintings! Muybridge rigged up a primitive projection system, and showed the images in quick succession, an early form of animation, giving the impression of movement. He produced a book entitled Animal Locomotion in 1887 that to this day remains one of the most comprehensive studies of animal motion of its type.

Taking this type of photography one stage further is time-lapse imaging, where a subject is photographed over a long period of hours, months or even years, and the result condensed into a short time to show precisely how plants grow or glaciers move down a hill for example.

One of the great pioneers of high speed photography was Harold E. Edgerton, who was a graduate student at the Massachusetts Institute of Technology in the 1930s. He was working on the reaction of electric motors to various loads. He was experimenting with arc rectifiers that, when close enough to the rotating motor, created a stroboscopic effect. Edgerton went on to develop the stroboscopic light and short duration electronic flashgun. He produced some, now iconic, high speed and stroboscopic images of milk splashes, sporting events such as golf swings, and light bulbs being smashed by bullets. In the 1950s he was hired by the US government to photograph atomic explosions, a feat that required a shutter speed of one-hundred-millionth of a second and requiring him to solve numerous technical difficulties.

Fig. 1.9Sequence of horse galloping (Daisy). Eadweard Muybridge, c. 1880. Twelve cameras were triggered by trip wires to show the movement of the horse. This, and images like it, proved for the first time that horses had all four legs off the ground at the same time.

Source: Creative Commons.

Fig. 1.10High-speed image of a milk drop striking a surface. This was shot in 2019, using readily available cameras and flashguns. Harold Edgerton’s famous version was shot on a camera, taking single sheets of film and potentially lethal short-duration flash equipment.

The high-speed image of a milk drop splash shown in Fig. 1.10 was shot as an homage to Edgerton’s famous image of a milk drop splash, shot after a huge amount of research in 1936, with relatively primitive equipment, and using single sheets of film, with a flash exposure 1/10,000th second.

Natural history photography has always made great use of high-speed flash, in particular, to photograph birds in flight. The British photographer Eric Hosking, working in the 1940s and 1950s produced some extraordinary images of owls, nightjars and other species in the field, using primitive and potentially highly dangerous electronic flash equipment.

One of the greatest challenges for nature photographers was the photography of smaller subjects such as insects in flight, which was taken up by Stephen Dalton in the 1980s. He developed a powerful short-duration electronic flash system together with a highly sensitive light-beam triggering system and high-speed camera shutter to photograph insects in free flight for the first time. He went on to take high-speed photographs of birds, amphibians, bats and other animals in extraordinary detail.

Today, it is possible to buy such pieces of equipment off the shelf, and its use is limited only by one’s imagination.

Many of the techniques described in this book can be carried out with conventional cameras, lenses and flashguns, generally only requiring the ability to alter shutter speed and aperture manually. You will often be using the camera in manual mode, setting shutter speed and aperture rather than using the camera’s automatic settings. However, most of the techniques are not necessarily straightforward, and much of the work will require ingenuity, improvisation, re-purposing of equipment, and a good working knowledge of photographic techniques and equipment to solve specific problems. In many cases, logistics becomes the most important part of the image-making process, getting the right subject in front of the camera at the right time.

It is very important to consider the purpose of the images. Are you a scientist wanting to gather data from the images, in that case you will probably need to shoot comparative control images, or include a scale for example, or are you primarily a photographer looking for new, challenging subjects with a pictorial emphasis? Some of the techniques described in the book, such as UV reflectance photography, are little used today, and one of the aims of the book is to re-kindle an interest in these areas. There is great scope for citizen scientists to discover new subjects and techniques during their photographic journey.

Photography is one of the few subjects where science and art come together to produce images, and some of the most beautiful and striking photographic images are those shot with some of the techniques in this book.

Note: Throughout the book, the terms IR and UV light will be used, even though it is strictly more technically correct to use the terms UV and IR radiation.

Chapter 2

The Electromagnetic Spectrum, Camera Conversions and Filters

In order to undertake UV and/or IR photography, you will need a digital camera that has been converted to make it record UV and/or IR wavelengths. It is sometimes possible to shoot UV and IR with unmodified cameras and appropriate filters, which we will discuss later. In order to fully understand the principles of UV and IR invisible light photography, and be able to make informed decisions as to what type of camera conversion you will need for your specific needs, it is useful to have a good working knowledge of the electromagnetic spectrum, and how the light that we see (visible light) is just one relatively small region of a very large spectrum of electromagnetic wavelengths that can be recorded by different types of imaging system. Different specialist filters and lenses will also be required, and these are discussed at the end of the chapter.

Fig. 2.1UV reflected light portrait showing the application of sunscreen to one half of the face. Note how some areas have been missed and may become vulnerable to sun damage.

Nikon D300 full spectrum converted camera with 105mm El-NIKKOR lens and Baader U filter. Two full-spectrum converted Metz 45 CT1 flashguns.

THE ELECTROMAGNETIC SPECTRUM

When a beam of sunlight passes through a prism it is split into its component parts. We see the classic rainbow of colours, from red through yellow and green to blue and violet. Many of us will have learned a mnemonic in school to help remember the order of colours e.g. Richard Of York Gave Battle In Vain (red, orange, yellow, green, blue, indigo, violet). This spectrum is continuous, with the colours gradually merging into each other rather than fitting into neat discreet boxes. It is often quoted that the colour (or hue) green, for example has a wavelength of 550 nanometres (nm), but this is not a precise figure, and green wavelengths will in fact cover a range of wavelengths.

Outside this visible (to the human eye) spectrum, however, are a whole host of other forms of electromagnetic radiation. Beyond the red wavelengths are IR and heat (an old-style tungsten light bulb emits around 10 per cent visible light, and 90 per cent IR and heat, obviously highly inefficient as a light source), followed by increasingly long wavelength radio waves. At the other end of the spectrum, beyond the violet wavelengths, are UV, divided into three distinct regions (UVA from 400–320nm, UVB from 320–290nm, and UVC 290–100nm), followed by X-rays and gamma rays, both used in medical diagnostic imaging for example. In general, shorter wavelengths have higher energy than longer wavelengths, making them potentially dangerous in certain circumstances.

UVC is a very damaging type of UV but is filtered out by the ozone layer in the atmosphere. It is used as a biocide, for killing off microbes in various environments such as swimming pools and food processing plants.

UVB is highly dangerous but cannot penetrate superficial skin layers. It is responsible for delayed tanning and burning, enhances skin ageing and promotes the development of skin cancers such as melanomas.

Fig. 2.2The electromagnetic spectrum, showing visible light, and positions of UV and IR wavelengths, arranged according to frequency, energy and wavelength.

UVA is responsible for the immediate tanning of the skin, contributing also to skin ageing and wrinkles.

UV of course causes sun tanning and sunburn, and it is becoming increasingly apparent that we should protect our skin with the correct application of sunscreen. There is also some evidence that IR radiation can cause skin damage, and several sunscreens are now available to protect against that as well. Photography and video are being used increasingly by manufacturers of sunscreens to show the effect of applying sunscreen incorrectly (seeFig. 2.1).

It is worth noting at this stage that many insects, birds and other animals can see UV wavelengths, and a few animals can see IR, and even detect heat patterns, usually from their prey.

The various areas of the electromagnetic spectrum have specific wavelengths, the length of which is described in metres or fractions of a metre. The visible light that the human eye can see ranges from around 400 nanometres (nm) for violet, to around 700 nm (red). Green is usually described as being in the region of 550nm. One nanometre is equivalent to one billionth of a metre (10-9 m). Radio waves can stretch to 1500 metres in length and beyond, while X-rays extend down to 0.01 to 10 nm for example, so the range of scale of the electromagnetic spectrum is vast.

THERMAL IMAGING

Heat is a major part of the electromagnetic spectrum beyond the IR, and specialist thermal cameras are available for recording the heat emitted by a subject. Converted digital cameras are not capable of recording heat. Thermal imaging cameras have a range of industrial uses, by the electrical utilities industry for example, when investigating early fire detection, as well as by fire fighters looking for any hot spots remaining after a fire has been extinguished. Various thermal imaging devices are now available, including some that attach to smartphones, such as the FLIR ONE®.

HUMAN VISION

Most humans can see the colours of the spectrum, from violet through to red. The cone shaped colour sensitive cells on the retina, (cones) are sensitive to red, green and blue light, and the human brain simulates all other colours from these three receptors. This is known as trichromatic colour vision. Some people are colour blind meaning that they have less sensitivity to some colours, often blue. Other people, very rarely, are known to have extra receptors in their eyes (giving them tetrachromatic vision) enabling them to see a wider range of colours than others. There is good evidence too that some people who have had cataract operations are able to see subjects illuminated only by UV light of around 365nm.

The human eye cannot see wavelengths longer than around 750nm, due to the light sensitive chemical called rhodopsin, which is the basis for most animal vision. The chemical reaction needed to produce a signal on the retina has an energy requirement that is higher than the energy emitted by radiation longer than 750nm.

It is also the case that we do not all see the same object as the same colour. A few years ago an image of a blue and black dress went viral on social media when many people claimed it to be white and gold rather than blue and black!

Other animal species have different colour vision to us. As might be imagined, the subject is highly complex, and only generalizations are possible here, but honeybees (Apis mellifera), for example, have trichromatic colour vision, with sensitivity to UV, blue and green, while butterflies generally have tetrachromatic vision, seeing further into the red end of the spectrum (some species of the swallowtail butterfly can see wavelengths from around 300–700nm), while many birds are also sensitive to UV. Dogs and horses are known to be dichromatic seeing mainly blue and yellow (though some may see UV), while bats are not thought to perceive colour, though some nectar feeding species may have some UV sensitivity.

It is possible, by using a suitable combination of filters, to simulate animal vision, and this will be covered further in Chapter 3.

Some snakes, such as pit vipers, can detect IR and also heat, which enables them to detect warm-blooded prey.

THE PENETRATING ABILITY OF UV AND IR

One of the most interesting facets of UV and IR photography is their ability to penetrate certain substances, to reveal hidden detail beneath the surface. IR, for example, penetrates 1–3mm into the surface of skin revealing invisible blood vessels, and also can show underpainting when photographing artworks. Some plastics are effectively transparent to IR, such as the sunglasses shown in Fig. 1.8. UV can penetrate water, depending on the turbidity and amount of organic substances in the water. These various characteristics are used by forensic and museum photographers, for example.

BASIC COLOUR THEORY

As we have seen, the human trichromatic system uses red, green and blue sensitive cells on the retina to produce all other colours. When dealing with light, when red, green and blue wavelengths are combined in equal proportions, white light is created. Varying the relative amounts of red, green and blue produces different colour sensations such as orange or purple. In terms of light, red, green and blue are known as the primary colours (artists use red, blue and yellow as their primary colours when mixing paint).

Fig. 2.3Approximate penetration of different wavelengths into human skin.

Fig. 2.4, 2.5, 2.6Portrait in reflected UV (365nm), visible and IR (850nm) light, showing different renditions of the skin in different wavelengths.

Fig. 2.7When mixed in equal proportions, the three primary colours of light red, green and blue make white light.

If the three primary colours of light are projected in equal proportions so that they overlap, white light is created in the centre. Where only two of the primary colours overlap, this is white light minus a primary.

(cyan and red are complementary)

(yellow and blue are complementary)

(green and magenta are complementary)

DESCRIBING COLOUR

Describing colour is surprisingly difficult – there are many versions of white for example. The generally accepted method is to use the HSL model: Hue, Saturation and Lightness. The hue is the name of the colour (e.g. blue); saturation is the intensity of the hue – a highly saturated or pastel shade, i.e. how much white is mixed with the pure colour; and lightness is the brightness of the colour.

In Adobe® Photoshop and other image-processing programs there is a Color Picker dialog box. Here, it is easy to move the cursor around the screen and see the values for hue, saturation and lightness (Photoshop® uses B for brightness).

Fig. 2.8The ColorPicker in Photoshop®, showing the various ways the yellow colour can be described.

CAMERA CONVERSIONS

Images are recorded by digital cameras (including those on smartphones) through the use of a light sensitive imaging sensor – usually a CCD (charge-coupled device) or CMOS (complementary metal-oxide semiconductor) type – situated in the focal plane of the camera. These devices contain a grid (matrix or array) of light sensitive photo diodes called picture elements, or pixels. The light falling onto an individual pixel creates an electrical current, in proportion to the amount of light. The values from the pixels are converted into a stream of digital data by an ADC (analogue to digital converter) in the camera. To record colour, the pixels are covered with transparent RGB (red, green or blue) filters, so that incoming light is effectively measured in terms of its red, green and blue content. Human colour vision is not equally sensitive to the various colours (it is more sensitive to green light than red or blue), and this is reflected in the colour pattern of the sensor in the camera. The pattern of the filters overlying the pixels on the sensor is very specific, there being two green filtered pixels for every red and blue one. This mosaic pattern is known as a Bayer array, and is designed to match the sensitivity of the human eye as closely as possible.

The imaging sensor is sensitive not only to visible light but also to UV (to around 320nm) and IR (to around 1100nm). Because the majority of photographers want to photograph in normal visible light without the effects of UV and IR as well, manufacturers filter out the UV and IR by placing one or more filters over the imaging sensor at the time of manufacture. This filter varies from one brand to another but is often called a hot mirror filter and is often incorporated with another called an anti-aliasing filter, designed to help prevent aliasing (or moiré), when photographing regular patterns with a regular pattern of pixels. This filter can be removed, to enable the camera to record UV and IR wavelengths.

When choosing which camera to convert, it will be best to have one that is capable of shutter and aperture priority settings, as well as fully manual settings. One that has a PC flash synchronization socket will also be useful, though this is not essential if it has a hot shoe. A Live view facility would also be useful, as discussed later. Digital SLRs, mirrorless cameras and compact type fixed lens cameras can all be converted, though a camera with interchangeable lenses will obviously be most flexible for a range of uses.

In order to convert a digital camera to record UV and/or IR, the UV and IR absorbing filter needs to be removed. It is possible to do this yourself, and there are many tutorials on YouTube and other internet sites telling you how to do it. However, it is not for the faint-hearted. The sensor is very delicate, and the most expensive element of the camera, and must be handled with extreme care. Not only will you require the right tools, but also a dust and static free environment in which to carry out the operation, together with having a good knowledge of the cameras being converted. Some Nikon models for example, have an internal IR shutter monitor that will contaminate images in a converted camera in which the internal UV/IR blocking filter has been removed.

Fig. 2.12The Nikon D70 camera is capable of producing good UV and IR results without modification. Here it is shown with an 80mm El-NIKKOR lens and extension tubes to enable focusing.

Fig. 2.13Images from the unmodified D70, of a carnivorous pitcher plant (Sarracenia flava) showing how UV makes the veining pattern invisible, but instead makes the UV absorbing liquid nectar droplets very dark.

Nikon D70 with 80mm El-NIKKOR lens and Baader U filter.

Ever since digital cameras were first introduced there have been photographers wanting to use them for UV and or IR photography. Kodak marketed IR versions of their early digital cameras, including the Nikon based DCS420 IR, DCS560 IR and DCS 720× IR aimed specifically at forensic and medical photographers; while Fuji marketed the IS Pro in 2007, a full spectrum camera targeted again at forensic professionals. This model was supplied with a full set of UV and IR filters.

A couple of early DSLR models with CCD sensors, such as the Nikon D40× and D70, did record UV and IR well without any modification to the sensor, presumably because the filter fitted over the sensor was not very effective at blocking UV and IR. These models can often be found relatively cheaply on the used camera market and are a good starting point for those readers wanting to try out invisible radiation photography. Despite being only 6Mp, they can yield excellent results. They do not have a Live view facility.

SIGMA CAMERAS

While the vast majority of digital cameras on the market use CCD or CMOS imaging sensors, one manufacturer, Sigma, use a different system. It uses an imaging sensor called a Foveon ×3 sensor. Rather than the mosaic structure of a Bayer array sensor, the Foveon sensor captures colour vertically, recording hue, saturation and brightness for each pixel, making use of the fact that silicon absorbs red, green and blue light at different depths. Uniquely, Sigma’s interchangeable lens cameras such as the sd Quattro series have a user removable dust protector filter, which incorporates a hot mirror type IR blocking filter. Removing this filter enables the camera to record IR images, as well as UV reflectance images, when used with appropriate lenses and filters over the lens.

The camera was tested and found to have good sensitivity to both UV and IR wavelengths. You will need various adapters such as those shown in the image. For UV reflectance imaging, a Sigma to M42 thread was coupled with an M42 to Nikon adapter to enable a Nikon mounted El-NIKKOR enlarger lens to be used. For IR imaging you can use the normal Sigma lenses, and place appropriate filters over the front of them. The dust protector and filter should be replaced for conventional photography.

Fig. 2.9A Sigma sd Quattro camera with dust protector (hot mirror filter) removed, effectively converting it to full-spectrum sensitivity. Shown here with 80mm El-NIKKOR enlarging lens and Baader U filter for UV reflected photography.

Fig. 2.10Flower (Rudbeckia sp.) in visible and UV reflected light with Sigma sd Quattro camera.

Fig. 2.11Churchyard tomb in visible and IR, with Sigma sd Quattro camera.

SENSOR SIZE AND FOCAL LENGTH

One factor to consider when choosing which camera to convert is that of sensor size. There are various sensors in digital cameras, including full frame 35.9 × 24mm, APS-C 23.6 × 15.8mm and micro four thirds 21.6 × 17.3mm. They can contain different numbers of pixels, from a massive 45 million (a million pixels is 1 megapixel, Mp) to around 12Mp or so. In reality, a 12Mp sensor will give excellent results, good enough for a full-page reproduction in a book such as this one, or an A3 ink jet print. The main effect of different sensor size is the relationship with the focal length of the lens being used.

The focal length engraved on the lens barrel (for example 100mm) is strictly only valid when it is used with a full frame camera with a sensor of 24 × 36mm (the size of 35mm film). When the same lens is used on a camera with a smaller sensor, there will be a magnifying effect, (known as the crop factor) because the sensor is only seeing a smaller area of the image projected by the lens. An APS-C (Advanced Photo System type-C) sized sensor will have an approximate magnifying effect of ×1.5. The 80mm El-Nikkor lens, recommended later for UV reflected photography will effectively have an approximate focal length of 120mm when used with an APS-C sized sensor for example. A longer focal length lens will give a greater working distance from the camera to the subject, which may be useful when lighting small subjects with various lights, or photographing nervous insects, but may be too long for mid-range subjects such as portraits for example.

USING UNMODIFIED CAMERAS FOR IR

Although for most purposes best results will be obtained with a converted camera, it is possible to use an unmodified camera for IR, using an appropriate filter and a long exposure, probably in the region of five to ten seconds at f/5.6 at 400 ISO (discussed more fully in Chapter 6).

One test that you can easily perform to see if your camera can record IR images without conversion is to point it at a TV (or similar) remote control in a darkened room. If the camera has a Live view facility, turn this on and look at the screen while pressing the remote button. If you see a bright light on the screen then the camera has some sensitivity to IR. If it doesn’t have a Live view facility, you will need to shoot an image of the remote in a darkened room. You will probably need to have the camera on a tripod to do this. Unfortunately there is not a similar test for UV sensitivity.

CAMERA CONVERSIONS

There are a number of companies specializing in camera conversions, and many are listed in the resources section at the end of the book. They can convert one of your own cameras or may offer a range of already converted cameras. Most types of digital camera can be converted – DSLR, bridge, mirrorless and compact, though an interchangeable lens camera will certainly offer most scope and flexibility.

There are three major options when converting a camera for recording UV and IR: UV only, IR only, and full spectrum (UV, visible and IR).

When the hot mirror filter is removed, it can be replaced with a UV only transmitting filter, an IR only transmitting filter (there is a range of options here, discussed more fully in Chapter 5), or a sheet of plain quartz glass allowing UV, IR and visible light (full spectrum) to reach the surface. It is worth thinking very carefully about the type of conversion you need, as they are expensive, and may limit the type of photography that you can carry out.

UV-ONLY CONVERSION

With this type of conversion, the hot mirror filter is replaced with a UV transmitting filter (usually transmitting around 365nm), which absorbs all visible light and IR. Depending on the conversion company used and your own requirements, specific wavelength UV filters can sometimes be fitted. No filter is required over the front of the lens (that needs to be capable of transmitting good quantities of UV), so the camera viewfinder can be used normally to compose and focus the image. The converted camera cannot be used for conventional visible light photography, or IR. This is generally an expensive conversion, due to the cost of the UV filter.

IR-ONLY CONVERSION

With this type of conversion, the hot mirror filter is replaced with an IR transmitting filter, which absorbs all visible light and UV. Depending on the conversion company used, specific wavelength IR filters can be fitted. No filter is required over the front of the lens (most lenses can be used for IR photography), so the viewfinder can be used normally to compose and focus the image, enabling it to be used for moving subjects. The converted camera cannot be used for conventional visible light photography or UV.

WHITE BALANCING A VISIBLE LIGHT IMAGE

When using a converted camera for normal light images, using a hot mirror filter, you will probably need to white balance this image with a photographic 18 per cent grey card, or similar device, to achieve the correct colour. Shoot two images, one of which includes the grey card, which should receive the same lighting as the main subject. In the raw converter such as Adobe® Camera Raw, select the image with the grey card, then place the white balance tool on the card and click. This should neutralize it to grey. Now synchronize the two images (in the Filmstrip: Select All > Sync Settings) to apply the setting from the grey card image to the one without. It can also be used for multi-spectral imaging, discussed later in the book.

FULL SPECTRUM

This type of conversion replaces the hot mirror filter with a plain quartz glass filter transmitting UV, visible and IR wavelengths. Specific filters will then need to be placed over the front of the lens to transmit and absorb the required wavelengths. The converted camera can also be used to record normal visible light images by placing a hot mirror filter (such as the Kolari Vision Hot Mirror filter, or Schott S8612 filter) over the front of the lens, effectively converting it back to its original state. For many applications you will want to record normal visible light control images, for comparison with the invisible light images, and this conversion allows you to do that without changing the camera.

Focusing and composition through the camera viewfinder will usually not be possible when the UV or IR filter is placed over the lens, though some models with a Live view facility (particularly mirrorless models) will show an image, albeit probably very dim, on the rear LCD screen.