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Photomacrography and Close-Up Photography1 - Close-Up Photography, Lenses for Close-up Photography, Supplementary Lenses

focal image camera distance

MICHAEL R. PERES
Rochester Institute of Technology

JOHN G. DELLY

Close-up and photomacrography, sometimes incorrectly called macro photography, are techniques that allow closer than usual camera-to-subject distances to be used for picture taking. The term close-up normally implies no true image magnification has occurred and will produce images between 1/20 of the normal size down to about life size on the film or charge-coupled device (CCD). This imaging outcome an also be expressed as the image’s reproduction ratio, (image size:object size), which in this case is defined between 1:20 and 1:1 life size. Photomacrography typically covers reproduction ratios from 1:1 to about 50:1, or life size to 50 times magnification, which is often written as ×50. Equipment and
techniques beyond 1:1 magnification become increasingly specialized, so close-up photography and photomacrography will be considered separately in this section, although there is considerable overlap between them. Photography above ×50 magnification is normally regarded as photomicrography and is discussed in that essay.

Close-Up Photography

A close-up picture may play an important role in the documentation of all kinds of subjects, including technical, documentary, artistic, nature, or for personal applications. In medicine for example, almost all dermatological, dental, and ophthalmic pictures are in the close-up realm. Many applications are also found in botanical, biological, and geological photography, close-ups of flower, butterflies, rocks, as well as a fingerprints, coins, or a postage stamp. There are several ways to make close-up images using simple equipment or accessories. In applications where the images are required for scientific or documentation purposes, it is a good practice to include a calibration scale near to the object field.

Lenses for Close-up Photography

Many relatively inexpensive digital cameras are fitted with zoom lenses that are adequately corrected for moderate close-up distances, despite being designed primarily for use at infinity focus. This development is possible because of the digital viewfinder, which bypasses the reflex viewing arrangement necessary for film cameras at close distances allowing shorter lens-to-subject distances to be achieved. However, if the camera is an SLR type, there are a wide variety of close-up lenses available, both in fixed and variable focal lengths. A zoom lens with close-up abilities is often described as macro zoom. True macro lenses may be designed to produce magnifications of 1:1 or greater. They will also operate in situations where the lens-to-detector (or film) distance is always equal to, or greater than, the lens-to-subject distance.

The more expensive macro lenses have elaborate internal mechanisms that allow groups of lens elements to move independently as the lens focusing is extended to close distances and producing excellent life-size (1:1) or slightly larger images. These lenses often have reproduction ratios inscribed on the barrel, have fairly modest maximum apertures, and use longer than normal focal lengths to simplify subject access at close range.

When images are made at a 1:1 reproduction ratio, both the working distance and the image distance will be equal to twice the focal length of the lens. Thus longer focal lenses will provide increased working distance. This feature allows access for more effective lighting and becomes more important when it is not prudent to have close proximity to things such as heat or infectious diseases.

Supplementary Lenses

The design of most fixed focal-length camera lenses is optimized for long camera-to-subject distances so they are usually limited to a minimum focus distance of about 50cm. This distance can be reduced (and the image size increased) by using supplementary or close-up lenses, which are simple magnifying lenses. These lenses will either be single lenses or achromatic doublets that attach like filters. These lenses were sometimes called plus lenses or diopters, and are the easiest way to achieve close-up imagery with a simple camera.

The optical power of supplementary lenses increases as the diopter number, increases, e.g., 1 +, 2 +, up to 20 +. The diopter number of a lens is the reciprocal of its focal length in meters. Hence the formula:

where f is 1000 mm. Thus the focal length can be calculated from:

A 1+ close-up lens will have a focal length of 1m (1000mm); a 4+close-up lens will have a focal length of 0.25m (250mm) and so on.

The dioptric power of the camera lens used can also be computed. For example, an ordinary 50-mm focal length camera lens has a dioptric power of 1000 mm (1 m) divided by 50mm (0.05 m), or 20. To calculate the focal length of a combination of camera lens and supplementary lens, the dioptric power of the supplementary lens is converted to focal length, and then the following equation is used:

where f c is the combined focal length of the camera lens and supplementary lens, f 1 is the focal length of the supplementary lens, and f 2 is the focal length of the camera lens.

Alternatively, one can convert all values to diopters and then convert their sum to focal length. For example, a 50-mm lens has a power of 20 diopters. Add a 2+ supplementary lens and the sum is 22 diopters. The combined focal length is 1000mm divided by 22, or, about 45.4mm.

It is necessary to know the combined focal length of the camera lens and supplementary if it is required to calculate the lens-to-subject distance using the well-known lens equation:

where u is object distance and v is image distance.

Supplementary lenses can be combined to obtain even greater magnification. However, when combining close-up lenses, the strongest should be closest to the camera lens. Image degradation will increase dramatically when more than two supplementary lenses are combined. Supplementary lenses can also be used in combination with extension rings and bellows in order to minimize the total amount of lens extension. Shorter lens extension produces shorter exposures. Shorter exposures may, for many subjects, offset the theoretical advantage of using a camera lens with extension tubes alone. Using a small lens aperture may minimize the aberrations introduced when using simple, positive close-up lenses. Simple supplementary lenses are unsuitable for telephoto lenses. Two-element achromats are available for longer focal-length lenses for improved performance.

Teleconverters are precision optical accessories that can be mounted in between compatible lenses and the camera body. Teleconverters increase the focal length of the primary lens and create a shorter working (object) distance. A teleconverter used in combination with a 200-mm close-focusing lens, for example, will yield a life-size (1:1) image and does so while maintaining a 71-cm (28 in.) minimum object distance. This amount of working distance is advantageous for many biomedical, industrial, and natural history applications. An increased working distance allows for considerable freedom in arranging lighting.

Lens Extension for Close-Up Photography

Cameras with interchangeable lenses will allow for extension tubes or a bellows to be used, which extend the lens-image distance. Increasing the image distance rather than changing the focal length will often produce a better optical result than through the use of supplementary lenses. Extension tubes are cylindrical tubes of various lengths, used singly or in combination to change the reproduction ratio in fixed steps. Tube lengths vary from 5 to 100 mm, and extensions of 250 mm or more are often used. They are fitted together with threads or bayonet-type mechanisms and some allow the camera’s automatic diaphragm features to be retained.

Extension-tube sets are relatively inexpensive but cannot achieve continuously variable lengths. This shortfall is overcome by the use of bellows. A bellows is more costly but is essential for professional work because it will allow precise control over the lens-image distance, which results in more
control of image magnification. Of course, large format view cameras are already so equipped.

The chief problem in the use of extension tubes or bellows for close-up work lies in the use of ordinary lenses close to the subject. Most ordinary camera lenses are corrected for long object-to-lens distances and short lens-to-detector distances. When they are used with short working distances, the resultant images may be degraded as a result of lens aberrations. Optical performance can be markedly improved by using the normal camera lens reverse mounted, so that the rear element of the lens is closest to the subject. Manufacturers supply reversing rings for this purpose.

A key asset of a good close-up lens is its ability to deliver edge-to-edge image sharpness with no curvilinear distortion or curvature of field. Many enlarger lens are designed with similar optical characteristics and can be adapted for many close-up applications. This type of lens is called PLAN or flat field.

Focusing, Depth of Field, and Diffraction

To achieve a specific image size in close-up photography using an SLR camera, traditional technique will need to be abandoned. To achieve critical focus in close-up work, the required reproduction ratio should be determined and (if vailable) set on the lens barrel. The camera should now be focused by changing the working distance, which usually involves moving the subject or camera closer, or further from one another. This may be most conveniently done with a focusing rail. A good rail may have calibration marks, smooth adjustment slides, and locks for precise control over its movements.

The range of sharp focus in a picture is referred to as its depth of field (DOF). Assessment of this is subjective and viewer dependent, but in close-up photography, depth of field will be noticeably small, and becomes smaller still with increasing magnification. In everyday photography the image DOF can be increased by using smaller apertures, but as the magnification of 1:1 is approached a very small aperture will produce diffraction that ultimately softens the image’s crispness. Diffraction effects are dependent on a sample’s characteristics. In general photography there tends to be more DOF behind the subject than in front. In close-up photographs though, the distribution is more equal in the front of and behind the subject.

Determination of Exposure

As image magnification increases, the amount of light reaching the detector from a given area of the subject is correspondingly reduced. However, exposure evaluation with through-the-lens (TTL) metering equipment is straightforward if the system permits and the subject is “average” in its reflection. With a hand-held exposure meter, allowances must be made for light loss due to the lens-to-detector distance. As an example, at 1:1 magnification the rear element of a 50-mm lens may be up to 100mm from the detector. At this reproduction ratio, there will be two full stops of light loss. The amount of light loss for any reproduction ratio can be calculated. The good equation for use with close-up photography provides the exposure factor in terms of reproduction ratio:

Exposure factor = ( R + 1) 2 .

In this equation 1 represents one focal length of any lens used. R is the reproduction ratio expressed as a decimal, but since the reproduction ratio always corresponds to the increase in extension, the R can be thought of as an increase. In the case of a reproduction ratio of 1:5 (i.e., 1/5 or 0.2), the exposure factor is (0.2 + 1) 2 = (1.2) 2 = ~1.4.

Another equation for the exposure factor where there are bigger changes of image distance or focal length is:

Exposure factor The ( R + 1) 2 version is easier to use if scales or rules are substituted in the specimen plane to determine reproduction ratio. The (image distance/focal length) 2 version is easier to use if graduated bellows are used to determine image distance (remember to add the focal length to the bellows extension reading to get the entire image distance if the bellows scale reads 0 at the infinity focus position). The exposure time indicated for a selected f-number, with hand-held exposure meters, is multiplied by the exposure factor.

There have been many pioneers in this application of photography including H. L. Gibson, Alfred Blaker, and Lester Lefkowitz, all who authored important books at their time. Copies of many of these books still exist and can be found in used books stores or on eBay: Gibson, H. L., Close-up Photography and Photomacrography , Rochester, NY: Kodak, 1970; Lefkowitz, L., Manual of Close-up Photography , Garden City, NY: Amphoto, 1979; and Blaker, A., Handbook for Scientific Photography , San Francisco, CA: W. H. Freeman, 1977, ISBN 0716702851.

Photomacrography

Photomacrography usually is defined as producing image magnifications in the range of 1:1 (life size) up to about 50:1 (fifty times life size), but is more aptly defined by the equipment that was used to create that magnification. Photomacrography equipment is generally more elaborate and professionally-oriented than the equipment used for close-up work.

There are several ways to produce photomacrographs, including the use of a simple microscope, which is defined as using a single stage of magnification such as a magnifying lens. An enlarging lens is a de facto macro lens, and is as good in quality as a fixed focallength camera lens when used reverse mounted. However, the best macro lenses are those designed for this purpose and have special features that will be shared below. When using a simple microscope, the object is placed at an distance equal to or closer than two focal lengths from the lens but not nearer than one focal length. Additionally, the image must travel a distance to the detector that is greater than two times the focal length of the lens.

In this magnification range, it is essential to have a rigid support for the components, ideally equipped with some means of moving each independent element (object, lens, and detector) precisely along the optical axis and securing it firmly. This is why photographs in this magnification range are often made with the ubiquitous low-power stereomicroscope. Stereo microscopes will often have a zoom objective lens and possess a relatively low numerical aperture. It is excellent to use for long working distances and usually equipped with rack and pinion adjustment of object distance influencing the fine focus. However, photographers should be aware that the tilted optical axis necessary for stereo viewing can result in photographs that have similarly tilted DOF effects. A binocular microscope has two eyepieces, but does not produce a stereo images.

Bellows and Laboratory Setups

A bellows is, in effect, a variable length extension tube with a lens board at one end and a location for the camera to be attached. The camera and lens boards will be mounted on a dovetail, or similar focusing rail. The bellows extension can easily be adjusted over a very wide range to achieve the correct and precise magnification for the sample under evaluation. Although a bellows is a seemingly simple piece of equipment, it often causes problems for photographers, mainly because the spacing of the key elements (detector, lens, and subject) is
not fixed, which means that working distance, magnification, and focus are independent and interdependent. In a “normal” camera, two of these three variables are usually fixed. Although some bellows can accept a wide variety of cameras and lenses, many are designed for specific manufacturers’ lens mounts.

While a bellows may be more portable than a stereo-microscope, the distinctly non-portable, laboratory version of a photomacrography setup is built around a vertical bellows, which is fixed on a sturdy rail mounted on a large, solid stand that can also double as a copy stand. The focal plane and lens are often on a dovetail slide, which is in turn is on a separate dovetail to raise and lower the camera as one unit. The camera end of the bellows may have eye-level reflex viewing and fittings for large format photography. The base often has a transillumination system with several condensers to give a large, evenly lit field of view for transparent specimens, and sometimes fitted lamps for reflected-light photography. This allows excellent control over combined top and transmitted light for translucent subjects such as petri dishes. Most cameras of this type are no longer manufactured but are easily modified to accommodate digital cameras.

Macro Lenses and Optical Considerations

The laboratory photomicrography camera system described above will usually accept a wide range of apochromatic macro lenses optimized for specific magnification ranges, but will be versatile enough to be used with a true compound microscope and eyepieces. This link between micro and macro endured for many years, since macro lenses often had Royal Microscopical Society (RMS) treads for mounting.

True macro lenses are special purpose lenses designed for magnifications greater than infinity. These lenses may have other names reflecting their size, such as short mount or thimble lenses. Macro lenses are unlike close-up lenses in several respects. The first and most obvious difference will be that this lens has no focusing collar because the lenses are used with a bellows on a stand. Consequently, most macro lenses will have only a lens diaphragm ring and focus is controlled through changes in working distance as a function of bellows length. The aperture range is usually smaller than for traditional lenses used for other technical purposes. True macro lenses will typically have maximum apertures of f/2.8 and minimum apertures of f/l6 or f/22. At high magnification, small apertures produce undesirable diffraction effects and poor resolution and are rarely selected.

The diaphragm rings of macro optics are seldom marked with conventional f-numbers. The f-numbers on ordinary camera lenses are defined in terms of an object photographed at infinity, which is never the case. These lenses work at a short lens-to-subject distance. The effective f-number therefore increases as bellows extension increases. For this reason, some macro lens diaphragm settings are designated by a simple numerical sequence, for example, 1, 2, 3, 4, 5, 6, where each number is a factor of two (one stop) difference in exposure. Other lenses use Stolze numbers, for example, 1, 2, 4, 8, 15, 30, where each step is proportional to relative exposure time.

Alternative Lenses

Different manufacturers such as Carl Zeiss, Nikon, Leitz, Canon, and Olympus have produced true specialized macro lenses of excellent quality in the recent past. In most of these companies, these lenses are not currently sold. They may be found in the used camera equipment marketplace. Perfectly serviceable lenses by Bausch & Lomb or Wollensak lenses might also be found there. Many of these lenses are readily available on www.ebay.com. Also, as a consequence of the evolution from movie film to video in recent past, wide-angle and normal focal lenses for 16mm cine cameras can easily be found and adapted for use in this magnification range. The 16, 25, and 50mm lenses work well for photomacrography when reverse-mounted. Cine lenses are not as well corrected as true macro lenses, but they can make a satisfactory “poor man’s” substitute.

Exposure Compensation

As the lens is moved further from the imager to increase magnification, significant light loss occurs in the image plane, following the inverse square law of illumination. A camera with an internal metering system will automatically adjust for this loss; however, with many set-ups, especially large-format cameras and macro stands, exposure must be determined by an external meter. Some meters for scientific applications have sufficient sensitivity so that readings can be made at the image plane, which automatically compensates for the loss of light in the system. Most photographers will need to determine the exposure by reading the incident light with a hand-held exposure meter or reflected light with spot meter and apply an This is commonly referred to as the bellows factor and is applied to the indicated reading to obtain correct exposure.

There are several equations that can be used to determine this exposure factor, but the two most widely used are

This equation is equivalent to the previous one because the image distance is the lens focal length plus any extension, making the equation

but extension/focal length is equal to R , and the second term is 1, which bring us back to

On many film cameras the exact position of the focal plane is indicated by a circle with a line through it, which is inscribed on the camera body. The exact position to measure on a lens (the conjugate point) is not inscribed but is generally accepted to measure from the diaphragm location. In digital cameras a “best guess” is suggested as to where the CCD is located.

The best way to calculate the magnification of a single lens system is by using the equation:

v = (m + 1) F

where v is the image distance, m is the magnification of the system, and F is the focal length used. This equation can be used effectively for magnification calculations in close-up photography or photomacrography, independent of the format of the camera system utilized.

In the case of film photography, exposure compensation must be calculated taking into consideration film reciprocity law failure, the non-linear relationship between the total light intercepted by the film and the intensity of that light. A long exposure to very faint light does not have the same photographic effect as the same amount of light arriving in 1/100 of a second. This is only of practical importance for exposures longer than one second and shorter than about 1/1000 of a second. However, exposures of one second or longer are not uncommon with the high magnification found in photomacrography. Manufacturer’s data usually includes information about this correction, which may include a color shift. With digital cameras reciprocity failure with long exposures is not as much of a concern as is the build up of digital noise. A camera with a cooled chip has great value in this application or a camera with noise reduction software and long exposure compensation.

Lighting

Lighting for photomacrography can be very simple to the very complicated, depending on the intention of the photographer. The nature of the specimen will often dictate which illumination methods should be used. Fiber-optic light sources and electronic flash units are popular ways of lighting specimens in photomacrography because they do not create heat, which is an important consideration for many kinds of specimens, including living things.

For transparent specimens, transmitted light (diascopic illumination) is used, often from a built-in condenser system. For opaque specimens various episcopic (surface) illumination options are available, including the use of one or more auxiliary tungsten lights, fiber-optic units, or flash. Flatter illumination is achieved by means of a ring light made of either incandescent, fluorescent, or flash. Direct contrasty light can be achieved using an axial half-mirror attachment together with a wide open aperture on the front of the lens, or through the use of using a very thin piece of flat glass supported at a 45 degree angle above the specimen that will work in a similar way. A Lieberkuhn mirror—a type of mirror with a concave surface—reflects light coming from below and is useful when mixed transmitted-reflected lighting is needed. The mirror surrounds the lens, which protrudes from a hole in its center.

Depth of Field

The DOF is the distance in front of and behind the object plane that is in acceptable focus. It increases as the lens is stopped down, but unfortunately there is a fundamental limit as to how far a lens can be stopped down without degrading the image, even in the plane of sharpest focus. That effect is diffraction, which is caused by the bending of light as it passes through a very small slit or opening.

Resolution of the image is assumed to be diffraction limited, and points on either side of the specimen focal plane will be blurred by the combined effects of optical and diffraction blur. Subsequent enlarger magnifications further affect the final results. In practice, any aperture can be used when magnifications are less than ×5, but as the magnification goes beyond this point, image degradation becomes obvious both in the viewfinder and on the subsequent print.

Scanning Photomacrography 1

Originally patented by Peter Zampol in 1954, deep field scanning photomacrography is a method of obtaining images of small, three-dimensional objects with a high degree of resolution as well as a large DOF. Those two desirable qualities have long been considered mutually exclusive in photomacrography.

In scanning photomacrography, the lens aperture is set to its highest resolution potential (large aperture), which creates a very shallow DOF. The sample is photographed during time
exposure, by moving the subject slowly and precisely along the optical axis. The object passes through a sheet of light that is perpendicular to the optical axis and positioned at the focal point of the lens. The sheets of light may be created using narrow light beams collimated so that they are narrower than the DOF. Thus, only in-focus parts of the specimen are illuminated and photographed. Multiple light sources may be used to create 360 degree coverage.

The specimen is mounted on a motorized platform capable of controlled, vertical movement that introduces no vibration. Beginning with the specimen below the illuminated plane, the stage slowly moves the entire specimen through the light beam. Exposure is principally controlled by varying the speed of the sample stage. The slit width can also be varied in small ways or the lens aperture can be changed to modify the image’s exposure.

An observer looking at the ground glass of the camera would see a brightly lit contour of the specimen focused in one plane, constantly changing shape as it traverses the beams of light. The final image is the summation of all of these changing exposure locations on the capture material. Only on seeing the processed image can the effect of the process be known. The procedure works best with opaque objects that have no concave surfaces. Specimens that absorb light, such as certain fibers and crystals, are not suitable for this technique. One of the most interesting aspects of images produced using this technique is the fact these images have no perspective. Since all locations in the image will be photographed at the same object distance, there is no vanishing point of magnification differences from the front to the back. This phenomenon is described as resulting in an image with zero perspective (isoperspective image), which is useful for measurement since there is no shape factor change.

Scanning Photomacrography 2

Some flatbed scanners make remarkably good photomacrographs of specimens that are reasonably flat such as flowers and leaves, insects, coins, and postage stamps as well as other similarly sized objects. Some models combine very high resolution with a useful DOF, especially those with CCD detectors. These scanners have complex optical arrangements and fluorescent tube illuminators and are quite bulky. Slimline scanners use a different optical system and have a much shallower DOF. DOF can easily be determined by scanning a reflective plastic ruler with one end of it lifted off the scanning surface by a known amount.

Scanner and print resolution can translate directly into useful magnification. If an image is made with a flatbed scanner with true optical resolution of 1000 ppi, and the resulting file is printed at 250 dpi, the image magnification will be ×4.

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