|Lens Focal Length, Working Distance and Closest Focusing Distance|
Perhaps one of the most important factors to consider when choosing a lens for macro photography is its working distance. Working distance of a macro lens is the distance from the front of your lens to the subject (when the lens is set to its highest magnification).
For example the working distance of Canon 100mm f/2.8 IS USM Macro is 5.2" (133mm) at 1:1.
Closest Focusing Distance
Closest focusing distance is measured from the sensor to the subject (with the lens set to 1:1).
For example the closest focusing distance of Canon 100mm f/2.8 IS USM Macro is 1 foot (0.3 m) at 1:1.
Focal length is not a measurement of the actual length of the lens. It is a calculation of an optical distance from the point where light rays converge to form a sharp image of an object to the digital sensor at the focal plane in the camera. The focal length is expressed in millimeters and is determined when the lens is focused at infinity.
The focal length tells us the angle of view of the lens, i.e. how much of the scene will be captured and also the magnification; how large individual elements will be. The longer the focal length, the narrower the angle of view and the higher the magnification. The shorter the focal length, the wider the angle of view and the lower the magnification.
Tips for Choosing a Lens for Macro Photography
When considering lenses for macro photography the two aspects one should consider are the focal length and working distance. These two go hand in hand; generally at any given magnification, the working distance increases with an increase in focal length.
For example, Canon's 100 mm f/2.8 macro lens has a working distance of ~150 mm (6") at 1:1 magnification, whereas Canon's 180 mm f/3.5L macro lens has a working distance of ~300 mm (12") at 1:1 magnification.
The importance of Working Distance in Macro Photography
Working distance of a lens dictates how close you need to get to the subject to capture it at the lens's maximum magnification. This is important for many reasons.
If you are photographing insects or other live subjects the closer you try to get to them, the more the chances of you scaring them away. So longer the focal length / working distance the greater the chances of you getting the shot.
Sometimes when the subject is placed in not very accessible places it is impossible to get close (like when an insect is resting on grass or other foliage) in such cases having a longer working distance is critical.
Another important factor is lighting; with very short working distance you will actually be blocking the light from reaching the subject and it gets very hard to get the light to hit the subject without being blocked by the lens. Even specialist macro lighting equipment like twin flashes and ring flashes find it had to light subjects that are just millimeters away from the lens.
so should one always look for the longest focal length / working distance when choosing a macro lens.
Well the answer is both yes and no. Yes because it makes things easier and No because longer focal length macro lenses tend to be much more expensive than macro lenses of short focal lengths.
If you are shooting subjects that are stationary like flowers then you could easily get the work done with any macro lens irrespective of its working distance, the subject is not going to be scared away and you have many options to set your scene and light it the way you like.
Another important advantage of a macro lens with short focal length is that they provide a more three dimensional feel to the photograph. The viewer feels immersed and not as a spectator. The reason for this is because when a longer focal length lens compresses and flattens perspective a shorter focal length lens widens it and provides a greater sense of depth.
So weigh the pros and cons, think about the intended use of the lens and choose a lens that suits both your nature of work and budget.