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Macro Photography for Coral Reef Aquariums

Sanjay Joshi

Macro, or close up pictures of coral provide incredible images with intricate patterns, color and texture that cannot be seen easily with the naked eye. Who among us has not been awed by the sheer beauty that can be seen when the picture magnifies small sections of the corals, fish and other invertebrates. Close up photography has become a lot simpler thanks to modern technology, but it still requires a good understanding of the equipment and techniques, coupled with the tradeoffs created by the challenges that are amplified when shooting at close range and high magnification.


For further exploration into this topic, check out another feature included in this issue, "Marine Life in Macro." In his article, Tim Wijgerde applies many of these very techniques in macro photography, capturing the most exquisite, minute details hidden in a range of fish, zoanthids, clams, and numerous species of corals.

A Short Photography Primer

For the purposes of this article, it is assumed that the user has a basic knowledge of the technical concepts of photography such as Aperture, Shutter Speed and ISO and their relationships. Further it is assumed that the user is using a digital SLR. In my case I use the Nikon DSLR and hence most of my examples will be using the Nikon, however the concepts are equally valid for other cameras.


For the uninitiated a short review is included. The Aperture, Shutter Speed and ISO form the basic trinity of photography and serve as the means to manage the light falling on the sensor (exposure) and the creative elements that are artifacts of these, such as depth of field, motion blur, noise and graininess. For the same exposure it is possible to use a wide range of aperture, shutter speed and ISO combinations each with its own limitations. For example decreasing the size of the aperture (larger f-stop number) while keeping everything else the same will result in a lower amount of light reaching the sensor, and is accompanied by an increase in the depth of field. The reduced light can be compensated for by reducing the shutter speed. However neither of them can be changed ad infinitum without reaching the physical limits or introduction of unwanted artifacts. For example most lenses will limit the aperture to f32 or f22, at its smallest opening. At small apertures diffraction of light causes loss of sharpness and chromatic aberration and impacts the quality of the photograph. At large aperture openings the depth of field (DOF) can become very small causing only part of the object to be in focus. The high end of the shutter speed is usually limited by the mechanics of moving the shutter from its open to close position. High shutter speeds can be used freeze motion. At slow shutter speeds the shutter is open for a longer time and motion and vibrations can cause blurring of the photo. The motion blur is not necessarily bad as it can be used to enhance the creative element of the photo. ISO deals with the sensitivity of the sensor to light. Low ISO numbers indicate low sensitivity. Increasing the ISO numbers will increase the sensitivity of the sensor to the light, however it can result in graininess or noise in the photograph. The same exposure can be achieved by various combinations of these three elements resulting in differing effects.


What is Macro Photography?

First let us start by defining what is considered close up photography or macro photography. It does not really have much to do with how close you can get to an object to take its picture (yes, generally the distances you will be shooting at will be smaller), but rather the relationship between the size of the actual object and its size on the camera sensor (or film). This relationship is called the magnification denoted by m.


It is typically expressed as the ratio with the image size set to “1”. For example, if the image size on the sensor is .5”, and the object size is 2”, the magnification would be .5/2=0.25 or as a ratio 1:4. You may also see it referred to by the symbol “X”, which stands for “times magnification”. In this case .25X.

A typical camera lens generally produces a magnification of 1:10 – 1:4. The term macro or close-up photography ideally refers to magnification ranging from 1X and greater. For practical purposes we could divide the range into:

Close up range – 0.5X to 1X
True Macro – 1X
Extreme Macro – greater than 1X

Note that we are not referring to the ratio between the size of the image as printed or viewed but rather the size on the sensor. The typical size of a sensor on a FX (35mm equivalent) camera is approximately 24 x 36mm and on a DX camera it is 16 x 24 mm. As an example, for a 1:1 macro – a 2mm grain of rice would have the size of 2mm on both the FX and DX sensor.

At first glance it would seem that close up pictures could be trivially obtained by taking pictures of small objects with a normal lens and subsequently cropping out the extraneous portions and “blowing up the image”. This has more to do with the final magnification of the image size. In this case the final magnification is indicated by:





While this can result in pictures with large final magnification, there is a limit to how far this can be done. Cropping a picture reduces the number of pixels in the picture and enlarging the fewer pixels into a larger image size results in pictures that are not sharp, blurry and with significant loss of resolution. Using a macro picture and further magnifying the final print size will result in a much better quality picture.

Some Fundamentals of Lens Optics


Before looking at the various ways in which macro photography can be achieved, let us get a better fundamental understanding of how the images are created and what factors impact the image size. For a given camera and sensor size, the image size will be controlled by the lens focal length, and the optical distances between the object and the image planes from the lens. The lenses are typically characterized by their focal length and maximum aperture. Lenses with fixed focal lengths are called prime lenses and lenses with variable focal length are called zoom lenses. The function of the lens is to project the image of the object at a given distance from the optical center (called the object distance) onto the sensor which is at a certain distance from the optical sensor (called image distance). These are related to each other by the basic simplified formula:


While this formula is applicable to unit lenses, it may not be exactly applicable to the compound lenses used in the camera, but it serves to highlight the relationships that still impact the image size.

From this it can be seen that the when the object distance is at infinity, the focal length will be the same as the image distance, and this is the number specified in the lens designation. A lens with a focal length of 50mm will have an image distance of 50mm when the object is at infinity. Focusing on objects that are less than infinity distance away will result in a smaller object distance and hence a larger image distance. When the object distance is 2X focal length and is equal to the image distance, m =1. If we need a magnification greater than 1, one way to get this is to move the object distance closer so it is less than 2x focal distance and correspondingly the image distance will have to be changed to satisfy the equation. There is a physical limit to how close the object can be moved, since at object distance equal to f, the image distance will have to be infinity. The physical limit to how much this object distance can be reduced to get higher magnification will be limited by how much the image distance can be physically changed.


The object/image distance pairs are different for each focal length. For a fixed object distance the longer focal length will provide larger images than a short focal length. This can be easily seen when you switch from a shorter focal length lens (wide angle) to a longer focal length lens (telephoto). Conversely for a given image distance a shorter focal length will produce more magnification than a longer focal length lens.

There are essentially three elements that can be brought into play when attempting to increase the magnification. The focal length of the lens, the image distance (or extension), and the closeness of the object. Through the manipulation of these, various macro photographing techniques have been developed:

1. Macro Lenses
2. Close up or Extension Tubes
3. Bellows
4. Diopters
5. Reversing and Stacking Lenses
6. Combinations of 1-6


Tradeoffs, Tips and Issues with Macro Photography

Magnifying a subject comes with its own set of issues and problems. When trying to achieve higher magnification the biggest issue is the loss of depth of field (DOF). Depth of field refers to the distance between the nearest and the furthest objects in the scene that are sharp in the image. The higher the magnification the shallower the depth of field will become, resulting in only a small part of the image being sharp and in focus, as shown in the equation below:


Where:
c = circle of confusion (0.03mm for Nikon D800)
f = aperture and;
m = magnification

As can be seen from the above equation, for a magnification of 1X, at aperture f 5.6 the DOF is .67mm. Changing to f11, changes this to 1.32 and going all the way to f32 would make the DOF 3.84mm. While it may seem that a higher f stop is the better choice to get deeper depth of field, increasing the f stop beyond f11-f16 will often result is loss of sharpness due the optical phenomenon of diffraction. In macro photography you may be willing to trade off some sharpness for a higher depth of field. It is best to experiment and see which one is suitable for the subject at hand.


Another critical element when working with macro photography is the “Working Distance”. All lenses have a minimum focusing distance. Recall that to get higher magnification you will need to be shooting with the object distance being as small as possible. The closest focusing distance for any lens is measured from the plane of the sensor located in the camera body. The lens and the camera body occupy some of this distance, leaving a smaller distance to work with between the end of the lens and the object. This is called the working distance. A smaller working distance will require you to be closer to the object to get maximum magnification. This can create problems especially in an aquarium environment where it may not be physically possible to get close enough without hitting the aquarium glass. Lenses with longer focal lengths have larger working distances and are more suitable for working in an aquarium environment.

Another problem with macro photography is that working with higher f stops reduces the amount of light reaching the sensor. For proper exposure it may require lowering the shutter speed. When shooting corals in an aquarium, light is often not enough to begin with, and shooting at higher f stops will require reducing the shutter speeds to values where hand holding the camera will lead to motion blur being caused by shaky hands. Increasing the ISO values may allow for an increase in shutter speeds, but depending on the camera the image could get grainy or noisy at high ISO values. Most cameras will start showing some noise effects at ISO values around 800, while others may be useable at ISO 1600-3200 after some post processing to remove noise in software.


A tripod becomes absolutely necessary especially at lower shutter speeds. My advice is to get the most rigid tripod you can find, as at higher magnification even the slightest movements get magnified. Small movements such as vibrations caused by the mirror slap, pressing the shutter release button, can manifest themselves as motion blur and result in loss of sharpness. Use the camera’s mirror lock (or mirror up) function to force the mirror to withdraw before taking the picture, using timed delays to trigger the shutter release and using remote control to trigger the shutter release are additional ways to minimize the impact of small motions, and become absolutely necessary at higher resolutions.

Often light becomes a limiting factor and the use of a flash may be necessary to provide additional light. Camera mounted flash may not be the best option due to the lens itself shading the flash at the close distances between the object being photographed and the lens. Mounting the flash off camera may provide better lighting angles.


Another consideration when shooting macro photographs of aquariums is the fact that you are shooting though air, glass and water. The effect of changes in the refractive index of these mediums can be exaggerated if the plane of the camera is not parallel to the plane of the aquarium glass. This will result in pictures being out of focus, blurry and not as sharp. Carefully align the camera so the lens axis is perpendicular to the glass pane of the aquarium.

Manual control of the camera functions is preferred, both for focusing as well as setting the aperture, shutter speed and ISO. With some of the techniques the in camera metering system may be disabled. Autofocusing may result in the camera “hunting” for the focus point.

Additionally, turn off all water flow in the aquarium as even slight movement of the polyps will be exaggerated resulting in blurry pictures if the shutter speed is too slow. Make sure the glass is clean both inside and outside.


The pictures above show some of the pitfalls of macro photography. The pictures were taken with a Nikon D700 using the 200mm Macro lens. The settings were as as follows: aperture f22, shutter speed 1/4s and ISO 400. As can been seen, the full coral is not in focus, either the front or the back is in focus and sharp. This is due to the lack of DOF. The shutter speed is ¼ which is too slow for hand held photography. Reducing the aperture to f8 would allow for a higher shutter speed but result in further loss of DOF. Increasing the ISO to 800 or more would increase the noise in the pictures. This is the challenge with macro photography in the aquarium.





All macro photos by Sanjay Joshi.

Approaches to Macro Photography

1. Macro Lenses


These are lenses specifically designed for macro photography and have the capability to shoot at magnification of 1:1 or larger, and are the easiest to use. For example Nikon makes several macro lenses (called micro by Nikon) with different focal lengths 60mm, 105mm, and 200 mm. Canon has its own set of macro lenses. One specific Canon lens the Canon MP-E 65 mm f/2.8 can achieve magnification of 5:1 ! Various other third party lens manufactures also have their own set of macro lenses eg. Sigma, Tamron, etc. The lens construction allows for internally changing the image distance thus requiring a corresponding reduction in the object distance too. Physically this means moving closer to the object to get the higher magnifications, thus reducing the working distance. In an aquarium setting this may make it difficult to shoot macro pictures of corals that are further away from the glass panes of the tank. A way around this is to choose a longer focal length lens. For example for the Nikon 60mm micro lens, the minimum working distance to get 1:1 reproduction is about 3.5”, the working distance for the same magnification or the 105mm will be larger. The table below shows the differences in working distances for the various macro lenses that I own:


While these working distances are for these lenses on a FX (full frame) camera, the focal length of these lenses will by multiplied by the crop factor when using them on DX cameras. For the Nikon DX camera the crop factor is approximately 1.5. Hence a 60mm FX macro lens will effectively behave like a 90mm macro lens on a DX camera and result is larger working distance.

The main advantage of macro lenses is the ease of use and coupling with the camera’s metering system. The disadvantage is the higher cost of the lenses.

2. Extension Tubes


As seen in the theory, one way to increase the magnification is to increase the image to sensor distance. A convenient way to do this is to use extension tubes. These are basically hollow tubes with no glass elements that are inserted between the lens and the camera body and increase the image distance. The main advantage here is that non macro lenses can be converted for macro use. Kenko makes some excellent extension tubes and these are typically sold as a set which includes 12mm, 20mm and 36mm tubes for a total extension of 68mm. The Kenko tubes also provide the necessary electrical connections (customized for the difference camera systems eg. Nikon and Canon) allowing for full through the lens (TTL) metering control by the camera. The simple equation:


gives an estimate of the magnification that can be achieved by the extension tubes. For example a 50mm lens with a 50mm extension will roughly provide a magnification of 1:1. The biggest advantage of using extension tubes is that non macro lenses can be used for macro photography, thus providing a low cost approach.

Adding extension tubes have several disadvantages:

a) Adding extension tubes results in a drop in the light reaching the sensor. This can be compensated for by appropriate adjustments to the aperture, shutter speed and ISO settings and the addition of extra light sources such as flash and evaluating the appropriate trade offs.

b) Further Loss of depth of field

c) Reduced working distance

3. Bellows

Photo by Richard de Stoutz.


Bellows are simply another form of extension tubes that provide the ability to have higher increase in image distances to generate high magnification. They are typically mounted on rails with an accordion type pleated extension tube, with knobs to adjust the amount of extension in a continuous manner. The same limitations as the extensions tube apply. Most often the bellows are not electrically coupled to the lens and require manual settings since the camera loses all TTL metering capability.

4. Diopters or Close Up Lenses


A diopter is a magnifying lens element that is mounted to the front of the camera lens by screwing it into the filter ring of the existing lens. They are inexpensive and easy to use. The strength or power of these lenses is rated by diopters, where:


So, a +2 diopter will have a focal length of 500mm. Several filter manufacturers such as Tiffen, Hoya etc. manufacture diopter lenses that can be purchased singly or in sets (+1,+2,+5 etc.) These diopters can be stacked to increase the strength. Stacking a +2 and +5 diopter would result in a diopter of +7. The diopters work by reducing the focal length of the lens. An approximate magnification can be computed by:


For example, using a 105mm lens with a +2 diopter (focal length = 500mm) will result in a magnification of approximately 0.2X.

Adding diopters results in additional layers of glass added to the lens and the quality of the glass can impact the image quality. However unlike extension tubes and bellows they do not result in light losses and are relatively cheap. Since the diopters are designed to screw into the front of the lens, and different lenses have different front end diameters it could require purchasing different sets of diopters for each lens on which they will be used. One way around this problem is to buy the set for the largest diameter lens, and use step up/down rings to fit it to smaller diameter lenses.

5. Reversing and Stacking Lenses


Reversing a lens and mounting it on the camera is another option for magnifications greater than on 1:1. Standard camera lenses have a small image distance and larger object distances. For magnifications greater than 1:1, the image distance needs to be greater than the object distance. Reversing a standard lens provides this capability. Reversing the lens and mounting it on the camera requires the use of a special adapter and mounting ring. For my Nikon, the Nikon BR-2A is the 52mm macro adaptor ring and will allow for attaching a reversed lens with a 52mm diameter. When using a lens with a larger diameter e.g 62mm the Nikon BR-5 mount adapter ring will screw into the BR-2A and allow a 62mm lens to be mounted in reverse. Additionally, a Nikon BR-3 can be used at the end of the reversed lens to allow adding other filter attachments. When reverse mounting a lens it loses all its capability for TTL metering. Additionally, the aperture has to be set on the lens manually, hence only lenses that have an aperture control ring can be used. The advantage is that for the similar magnification (obtained through extension tubes) the working distance is improved. A 50mm lens when reversed will give a magnification of roughly 1.2X with a working distance of about 3.25”, to get the same magnification with extension tubes the working distance would be approximately 1.5”. The smaller the focal length of the lens, the larger the magnification. A reversed 28 mm lens would give a magnification of 2.3:1 without the use of any extension tubes, and a working distance of about 2”. [ref]


Another way of using reversed lenses is by reversing the lens and stacking it with a regularly mounted lens. This has the same effect of using a diopter. A longer focal length lens is usually mounted on the camera and a shorter focal length lens is reverse mounted to it's filter ring using a lens stacking/reversing ring. The reversing ring diameters must match the camera lens and reversed lens diameter. The resulting magnification is:


For example, using a 200 mm lens, with a 50mm lens reversed and attached to it, will result in a 4X magnification. This technique can be used effectively to get high magnification, however the tradeoff is the significant loss of depth of field along with considerably reduced working distance.

6. Teleconverters


Teleconverters are lens accessories that are added between the lens and the camera to increase the focal length of the lens and are designed for use with telephoto lenses. Teleconverters are rated by their magnification factor and 1.4X, 1.7X, 2X teleconverters are available. The main benefit of using them for macro photography is that they are a multiplier of magnification and increase the magnification already obtained by macro lenses or extension tubes. The other advantage is that the addition of a teleconverter does not reduce the working distance. The disadvantage is the loss of light which must be compensated for by changes in aperture, shutter speed and ISO. The introduction of additional glass can degrade the picture quality.

7. Combinations of 1-6
Magnification is typically obtained by changing focal length and/or changing the amount of extension. The individual techniques described above can also be combined to create higher magnifications. For example, diopters can be combined with the use of extension tubes, teleconverters and extension tubes can be used in conjunction with each other. Feel free to experiment with the different techniques to see which ones give you the best results.


Dealing with Small Depth of Field

One common technique to use to increase the depth of field is through the use of focus stacking techniques. Multiple images at different focus distances are combined in software to create a single stacked image with large depth of field that would not be possible in a single photo. Several software packages such as Photoshop, Helicon Focus, Combine Z, etc. are available that allow for focus stacking to be done in an automated manner.


When photographing corals the DOF is quite small and small increments in focus distances are often needed to create the stacked set of images. To perform this effectively, the use of focusing rails is highly recommended. The camera can be set to focus at the front of the coral, and without changing the camera settings the focus point is incrementally moved forward using the focusing rails. A complete set of pictures can be taken and combined in software to create a composite image with a high depth of field. The picture below shows the results of combining 6 images in Helicon Focus and with some post processing in Photoshop.


Macro photography of corals can yield some beautiful pictures showing the detail that is often not visible to the naked eye, but it is not without its pitfalls. Hopefully this article has addressed the techniques and challenges. While these techniques are applicable to all macrophotography, some of them are not useable given the short working distances and limitations of working with subjects that are confined to a glass box. Which technique to use will often depend on the equipment you already own, budget constraints on what you might be willing to purchase, required magnification, resolution requirements, and working distance. The best way to learn and improve is to get your camera out and start taking macro pictures.

References and Further Reading

The Manual of Close-Up Photography, Lester Lefkowitz, American Photographic Book Publishing Co., Inc., 1979.

http://www.cambridgeincolour.com/tut...aphy-intro.htm

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