Macro Photography - Also known as Photomacrography or Macrography, is a type of photography that involves extreme close-up, usually of small objects. Normally a photograph is classified as a macro photograph if the subject was captured in the sensor at its actual size or greater. However, some lens manufacturers use the Macro label as an indicator that the lens is able to focus at a very close distance from the subject. Although having a very close minimum focusing distance (MFD) is one factor of achieving a true Macro photograph, this still does not qualify as a true Macro if it cannot reproduce a life-size image on the sensor. When buying lenses for Macro photography, always be sure that the lens is a real Macro lens.
Reproduction Ratio - In order to have a real sense on the magnification capability of your Macro setup, it is important to know it's reproduction ratio. Reproduction Ratio refers to the ratio between the size of the image captured on the sensor and the actual size of the object. When using a full frame sensor with a size of 36mm x 24mm, for example, an object that is 12mm wide must occupy at least 1/3 of the width of the sensor (36/12 = 3). For smaller sensors like Canon's APS-C which is 22.2mm x 14.8mm, the same image must occupy at least 45% of the sensor's width. This means that for the same 1:1 reproduction ratio, the image coming from a cropped sensor will exhibit a smaller field of view compared to the image from a full frame sensor.
The easiest way to determine the actual reproduction ratio of your macro setup is to take a horizontal picture of the scales in a ruler and compare the captured measurement to the actual sensor width. For example, if your image shows only 11mm of the scales of the ruler and you are using a Canon APS-C camera, then the reproduction ratio is 22.2:11 or approximately 2:1. This means that your setup is able to capture 2x of the actual size of the object. Often, reproduction ratio greater than 1:1 is classified as photomicrographs, however Canon's MP-E 65 lens is able to capture up to 5:1 magnification but it is still classified as a Macro lens, so the classification is a little vague.
Factors in Macro Photography
When doing macro photography there are several factors that you need to know and understand in order to make your setup more adequate to the type of subjects that you intend to photograph. Since Macro photography can be applied to various kinds of subject such as small insects, plants and plant parts, mechanical parts, small products, small body parts, etc., you have to know what setup works best. The same way you must also know what works best for a certain type of output, such as for artistic effect, for product magazines, for educational books, or merely for web-sized fun photos.
Minimum Focusing Distance (MFD) - This is the closest distance between the sensor plane and the subject that the lens can acquire perfect focus. For Macro lenses this should provide at least a 1:1 reproduction ratio.
Working Distance - This is the distance between the front element of the lens and the subject when the subject is in perfect focus. If the macro lens is set to focus at its MFD, the working distance at this point is the space needed to acquire a 100% magnification (1:1 reproduction ratio). Going farther will make the image smaller, while if you go closer, the lens will not be able to acquire perfect focus. It is important to know the working distance of a macro setup specially if you plan to photograph subjects that react easily to nearby moving objects, such as skittish insects.
Focal Length - Various brands of macro lenses offer them in different focal lengths from 35mm to 180mm. Most common ranges are in the 60mm to 100mm. These lenses are designed with a MFD that allows a 1:1 Reproduction Ratio. The most obvious difference when using different focal lengths is the view angle or perspective. Depending on the design of the lens, the working distance may not be that much different, however most lenses with longer focal lengths tend to have larger working distance because of the inherent magnification of the lens.