The great thing about macro photography is that there is never any dearth of subjects. A macro lens will present you a whole new world of photographic opportunities right within your own backyard. Macro subjects are everywhere to be found, the intricate patterns in a flower petal, numerous bugs ,beetles and other insects, mushrooms, fungi etc. all make great subjects for macro photography.
However the most important feature of a macro photograph is the amount of detail and sharpness it has; and attaining it requires a strong understanding of photographic concepts like focal length, magnification, sensor- size, depth of field, diffraction etc.
When we speak about magnification of a macro lens we are referring to the magnification at the image plane, meaning the size at which an object will appear on the camera’s sensor in relation to its original size. For example if the size of the object on the image sensor is 50% of its actual size then the lens is said to have a magnification factor of 1:2 or 0.50X.
|Photograph at .25x magnification|
So greater the magnification factor of the lens the smaller the size of the subject with which you will be able to fill the frame. The two lens properties that determine the amount of magnification of a lens are focal length and focusing distance. The longer the focal length of the lens, the greater its magnification and the closer a lens could focus the greater its magnification.
Sensor Size and Magnification
Other than the magnification of the lens, one important factor that determines the size of the smallest object with which one can fill the frame is the sensor size of the camera.
For example if a lens with a magnification factor of 1:2 or 0.05X is used to photograph a subject, even though the image formed by the lens on the image plane is the same, a smaller sensor will only capture the center portion of the image cropping out the rest. So the smaller the sensor the greater is the effective magnification. Thus everything else being the same, a smaller sensor is capable of photographing even smaller subjects.
|macro photography magnification and sensor size|
Close up and Macro Photography
Any lens which has a minimum magnification factor of 1:10 or greater is called a close up lens and photography at those magnification is called close up photography. True macro photography is when one captures objects at life size (same size as the actual object) 1:1 magnification or higher. In general people use the term macro broadly to include anything from 1:10 to 1:1. However a lens is only categorized as a macro lens only if it is able to achieve magnification of 1:1 or greater.
Macro Focal Lengths
Macro lenses come in Normal, Short Telephoto and Telephoto focal lengths; one may choose the focal length appropriate for the type of subject and also depending on the working distance needed and the perspective one wish to achieve. Shooting closer to the subject expands perspective, while shooting from farther away compresses it.
Macro lenses with normal focal length (50mm for a full-frame sensor camera) achieves 1:1 magnification and provide a working distance of about 7 – 9 inches. Those with short tele photo focal lengths (100mm for a full-frame sensor camera) provide of about 12 inches while Tele photo macro lenses (200mm for a full-frame sensor camera) give an even greater working distance of about 19 inches.
Increased working distance helps in more than one ways. It allows one to shoot without getting much closer to the subjects that are easily scared away. The longer working distance lets one to easily light the subject.
Working Distance and Minimum Focusing Distance
Working distance means the distance from the front of the lens to the subject. It is an important consideration when choosing between macro lenses of different focal lengths. Generally at any given magnification, the working distance increases with focal length.
Minimum focusing distance is the distance from the focal plane to the subject when the lens is focused at its closest focusing point.
Lens Extension and Effective F Stop
We have seen earlier that when a lens is focused at infinity the lens elements are arranged closest to the image plane and when a lens is focused at nearby objects, such as in macro photography the lens elements are at extended further from the camera, the higher the magnification the farther the lens elements are.
This is called ‘extension’; for relatively lower magnifications the extension is very small so the lens elements remain at the expected distance of roughly one focal length from the sensor.
As magnification increases to o.25x or higher, the lens elements move further away from the sensor and this make the lens behave as if having a higher focal length.
At life size or 1:1 magnification, the lens elements move out twice the focal length from the camera’s sensor.
When you are shooting a subject at 1:1 magnification and at f/2.8 you will actually be getting results as if an aperture of f/5.6 was set on the camera.
This increase in effective focal length changes the effective f stop of the lens. With a change in the lenses focal length, the effective f stop increases , so it increases the depth of field , requires a longer exposure time and the lens also becomes susceptible to diffraction. So the lens now displays all the characteristics of a longer focal length lens and the focal length had indeed changed, it is only addressed as effective focal length because most cameras do not show the change in f stop in its settings. And since the camera's metering system automatically compensates for the drop in light when it calculates the exposure settings most people does not take notice of it.
Since the camera's metering system automatically compensates for the drop in light when it calculates the exposure settings most people does not take notice of it.
Depth of Field and Macro
Depth of field is one of the major factors influencing macro photography, It becomes shallower as the magnification increases, the higher the magnification the shallower the depth of field.
With magnifications of 1:1 or higher, one is working with depth of field of only millimeters. One technique; macro photographers use is to align their subject along with the image plane so as to get most parts of the subject in the plane of sharp focus.
Where sufficient depth of field is critical; photographers need to stop down the lens considerably and in such cases; it is better to know the diffraction limited resolution of your camera.
It’s a common belief that depth of field is more for cameras with smaller sensors. In one way it is true, for the same f stop a smaller sensor camera will have larger depth of field when compared to one with a larger sensor.
But it is to be noted that the larger sensor also could use larger f numbers before diffraction starts to get in and reduce picture quality. Results produced from both sensors at their diffraction limited resolution is comparable; both will have the same depth of field.
The advantage of cameras with smaller sensors is really in the shorter exposure time it requires to achieve the same depth of field.
Some interesting facts about Macro Depth of Field
At lower magnifications the available depth of field is distributed more towards the back of the plane of focus than to the front. But when is increased; this distribution of depth of field becomes evenly balanced towards the front and rear.
Depth of field is independent of focal length. A 50mm, 100mm, or 200mm lens set to the same f stop and at 1x magnification will all have the same depth of field.
Macro Photography and Diffraction Limit
Macro photographers are always looking for ways to increase depth of field in their pictures, and they do this by using higher f stops. But one thing to be aware of is that your lens is not sharpest at its narrowest apertures, in fact after a certain point higher apertures will only mean lower image quality due to diffraction.
Diffraction is more pronounced at higher f stops and is also influenced by the characteristics of your specific lens. To produce great macro photographs one should be aware of this phenomenon and also should be willing to trade diffraction induced softening for greater depth of field or greater sharpness for lesser depth of field depending upon the situation.
However when working at high magnifications always remember that it is the effective f-stop that determines the diffraction limit and not the one set by your camera.
Macro Lenses and Image Quality
The higher subject magnification achieved in macro photography also highlights the various imperfections in your camera lens including chromatic aberrations, image distortion and blurring. The problems will be more visible towards the edges of the frame than at the center.
This is one reason why people struggle to achieve great image quality at higher magnifications with non -macro lenses and when shooting with compact cameras in macro mode.
However macro lenses by design are tuned to achieve optimal image quality near its minimum focusing distance and thus produce great results.
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