The following is technical information you may need to know to expand your knowledge of closeup and macro photography. While this information isn’t critical to the success of your closeup nature photography images, it helps to know as much as possible about the techniques and mathematics that go into the why and the where for of how this all works. We look at flash and synch speeds, depth of field and the Inverse Square Law for Light, tilt/shift lenses and the Scheimpflug Principle, and determining reproduction ratios and magnification.
Flashes and Sync Speeds
Every camera that can accommodate a flash has a maximum shutter speed, or a maximum sync speed, that will work with the flash. Why can’t you go faster? Your shutter is actually made of two separate curtains, both traveling in the same direction. The first curtain opens to allow light to reach the film. After a delay controlled by the shutter speed, the second curtain moves to block the light and end the exposure. At some point, the shutter curtains cannot physically move fast enough, and the second curtain begins closing before the first has completely opened.
If you could look at the movement of the shutter curtains in slow motion at high shutter speeds, you would see a narrow slit that moves across the film to expose a complete frame. Different parts of the frame are exposed at slightly different times.
The fastest shutter speed that your camera is capable of using with the first curtain completely open before the second curtain begins to close is called the maximum shutter sync speed. On most older cameras, this speed will be marked in red on the shutter speed dial. On older cameras, the synch speed may be 1/60 or 1/80 of a second. Better and newer cameras have sync speeds of 1/200 to 1/250 of a second. Newer cameras will refuse to use a faster shutter speed, regardless of the settings. Your camera must time the flash output very precisely so that the flash fires while the entire frame is exposed. At any faster shutter speed, only a portion of the frame would receive the light from the flash, because of the slit effect just mentioned. The newest and best cameras use a sophisticated system that fires a rapid series of flashes that “paint” a portion of the image with each strobe. The speed is so high it appears to us to be a single flash, and this method allows the flash sync speeds of up to 1/4000 of a second.
Inverse Square Law for Light
The farther light travels, the more it spreads out. The more it spreads, the less its intensity. The inverse square law is a law of physics which describes the way light levels change as the distance traveled changes. For photographers, an understanding of how this works is important.
The inverse square law says that the light intensity is inversely proportional to the square of the distance traveled. As distance increases, the intensity decreases. That’s the “inverse” part. In equation form:
|Light intensity at point A = (K)||*||Light intensity at source|
|(Distance from point A to source)2|
We aren’t interested in the actual light intensity, however, because our cameras have meters that figure that out for us. What we want to know is how the light levels between two identical flashes will differ if we place them at various distances from our subject. So let’s modify the equation for a ratio of two distances and see what happens.
If we have two flashes, A and B, the ratio of intensity is:
|Light intensity at point A||=||(K) * Intensity at source
(Distance from point A to source)2
|Light intensity at point B||(K) * Intensity at source
(Distance from point B to source)2
(where (K) represents a scientific constant that we don’t need to worry about. It will disappear in a moment.)
Which reduces to a much simpler relation:
|Light intensity at point A||=||(Distance from point B to source)2|
|Light intensity at point B||(Distance from point A to source)2|
Let’s look at an example. If flash A is placed 1 foot from the subject (remember, we are photographing small subjects) and flash B is placed 2 feet away, the relative intensity is:
|Light intensity at point A||=||22||=||4||=||4|
|Light intensity at point B||12||1|
The light on our subject from flash A will be 4 times as bright as from flash B. That’s a two stop difference. If you want a one stop difference, flash A must be twice as bright as flash B. Working the equation, flash B must be 1.4 times as far from the subject as flash A. We could build a table to make things easier:
|Stop difference||Relative distance||Intensity ratio|
To use the table is simple. If you want a 1 stop difference between the two flashes, one flash must be 1.4 times as far away from the subject as the other. If one flash is 10 inches, the other must be 14 inches. In the same way, if you want a 3 stop difference, one flash must be 2.8 times as far away as the other. The table is easy to memorize, as it’s the same series of numbers as the f-stop series – probably imprinted right on your lens.
Tilt/Shift Lenses and the Scheimpflug Principle
When trying to compose a photograph, it sometimes seems impossible to get everything that is important to be in focus. Even with the aperture set at f 32, it is possible that something will be too close or too far away to be acceptably sharp. This problem gets worse with the high magnification of macro photography.
The Schleimpflug principle was discovered at the beginning of the 20th century by Jules Carpentier and Theodor Scheimpflug. It is useful if your subject lies essentially in a single plane that is not parallel with the film plane. A perfect example is a field of flowers that extends to the horizon. The ground is flat and the field of flowers is horizontal, while your film plane is vertical. The flowers near the camera and at the horizon will probably not be acceptably sharp. See the diagram below:
If we could tilt the plane of focus to lie along the tops of the flowers, then it would be possible to photograph the field of flowers with every blossom sharply focused, even with a wide open aperture. This is what the tilt/shift lenses do, and is the practical result of the Scheimpflug principle, which states that the subject plane, the lens plane, and the film plane all intersect at a common point. The front of the lens tilts (as much as plus or minus eight degrees with the Canon lenses), and this brings almost any subject plane in focus. See diagram:
For years, Canon has offered this kind of lens in three versions: 24mm, 45mm, and 90mm. Nikon, in the past, made a special “short-mount’ 100mm lens that worked with a bellows to achieve the same effect, but it was discontinued long ago. Now, Nikon offers two lenses similar to the Canon, a 28mm and an 85mm. The 85mm or 90mm are by far the most useful for macro photography, because their greater focal lengths give the greatest working distances. They allow the photographer to choose the camera position for convenience, and then tilt the lens to align the subject plane with the plane of focus of the lens. The Canon 90mm tilt/shift lens is very sharp, works well with teleconverters and extension tubes, and makes a very flexible tool for macro photography. We haven’t used the Nikon, although we expect it to be equally sharp. All of these are manual focus lenses, and are similar in features with one exception. The Nikon aperture is not automatic, and must be manually opened for focusing and stopped down during exposure. Quality is not cheap, and list prices are about $1900 US for either make. You can expect to pay around $1100 by mail order to purchase the Canons, or about $1200 to $1300 for the Nikons.
The basic rule for calculating reproduction ratio is that with the lens focused at infinity:
Reproduction Ratio = mm of Extension / Focal length
To show how this works, assume that you have a 50mm lens. In order to achieve a reproduction ratio of 1/2X life size, you need 25mm of extension, because 25mm of extension divided by 50mm of focal length equals 1/2X reproduction ratio.
If you want to achieve life-size magnification, or 1X, with the same lens, you need 50mm of extension. 50mm divided by 50mm equals 1X. This, again, is with the lens focused at infinity.
Lets take a more complicated example. The Nikon macro lens that we used for years was a 55mm lens that could focus to 1/2X magnification at its closest focusing distance. This means that the lens had built-in extension (notice that we’ve rearranged the equation algebraically) of:
|mm of Extension||=(Rep. Ratio)(Focal length)|
If we want to use this lens to achieve life-size magnification, how much extension do we have to add? We know that we need 55mm of extension to reach 1 to 1 (life-size, or 1X) with a 55mm lens. But the lens already has 27.5mm built-in. So we need to add another
(55mm – 27.5mm)=27.5mm of extension
How do you add extension? You use a device called an extension tube, which is little more than a hollow tube that mounts between the lens and the camera body. It pushes the lens away from the body, and allows the lens to focus closer. Nikon manufacturers an extension tube that is 27.5mm long, and it is called the PK-13.
With the PK-13 installed between the 55mm macro lens and the camera, and the focusing ring of the lens set at infinity, the reproduction ratio will be
|Rep. Ratio||=(0mm from lens + 27.5mm from tube)||/ 55mm focal length|
If we then focus the lens to its closest focusing distance, we get
|Rep. Ratio||=(27.5mm from lens + 27.5mm from tube)||/ 55mm focal length|
Any Nikon tube can be used on any Nikon lens to make it focus closer. Likewise, any Canon tube, or any Canon-compatible tube, can be used on any Canon lens to make it focus closer, and the same is true for any other brand of equipment. The entire secret is that when the lens is moved farther from the camera body, the lens will focus closer, and the reproduction ratio will increase.