Small depth of field (DOF)
Rotating the focusing ring of a lens or using the autofocus function we set the sharpness of the photographed object from a certain distance from the camera. It can easily be noticed that an image is acceptably sharp at a certain distance in front of the object as well as behind it. This range of distance is called the depth of field.
The sharpness is not identical in the whole range but it changes from the maximum in the point of focus to the certain acceptable limit defined by the criterion which is called the circle of confusion. As long as in the conventional photography the DOF can be really large (ultra wide angle lenses produce the sharp image in range even from several dozen centimetres to infinity), in macro photography the DOF is measured in centimetres and even more often in millimetres. The DOF in macro photography is extraordinarily small.
The depth of field depends on:
a. the aperture (opening of the diaphragm),
b. the lens focal length,
c. the distance between the point of focus and a lens,
d. the construction of a lens,
e. the type of a camera.
Ad. a) The bigger the aperture (greater light), the smaller the DOF. It is used among other things in portrait photography in order to isolate the subject from the surrounding background. It macro photography it is quite the opposite. It is usually about achieving the greatest DOF as possible. The easiest way to do it is by closing the aperture (increasing the f-number on a camera lens). But now the hard part begins. Increasing the f-number of a camera lens decreases the amount of light which makes it necessary to increase the exposure time. The longer exposure time means the necessity of using a tripod, but in case of movable object it turns out to be useless. The only solution left is using a flash that freezes the movement. As it turns out the standard flash is not sufficient. You have to use special flashes for macro purposes, however, it can be avoided in some ways. Apart from that, the lenses have such characteristics that initially with decreasing the aperture diameter the resolution increases, then after reaching the maximum, mainly at f/6- f/8 it starts decreasing. You can also increase the ISO sensitivity but it results in degradation of image quality (more image noise).
Ad. b) The DOF depends also on the lens focal length. It is its actual size that matters and not its conversion size depending on the size of a sensor. The longer the focal length of the lens we used to take a photograph of the same frame, the smaller the DOF. It seems that the lens with shorter focal length would be better here but they require approaching the object closer. It works in case the object is motionless. It gets more difficult when the object is an insect or a splashing drop of water. The aforesaid things concern the situation in which the lens is attached to the camera in a standard way. You will get to know later that the lens can be attached using different accessories allowing greater magnification. In which case, the DOF decreases.
Ad. c) Having a certain camera with attached lens we can take photographs of a given object from various distances. Soon we will notice that the closer the object on which we focus is, the smaller its DOF. In this way we come to a point beyond which the sharpness is not possible to achieve. The distance between this point and a sensor is called the minimum focusing distance.
Ad.d) The DOF depends also on the structure of the lens. There are the so called tilt-shift lenses allowing the perspective and DOF control thanks to their front part which can change the position of its axis in respect to its back part e.g. Nikkor 45mm micro. However, I’m not sure if the results are worth the prize of this particular lens. There are also different inventions such as Lensbaby lenses which thanks to their elastic construction (a metal ball joint) allow similar results, but their simple construction does not go hand in hand with the good quality.
Ad. e) The DOF changes with the size of the sensor of the camera. The bigger the sensor, the smaller DOF for the same frame. This relation will be described in the chapter dealing with the choice of the camera.
Usually the more advanced cameras are equipped with “DOF preview” button. It enables us to view the DOF of the final image. Using this function we can shot a particular object from such a perspective that e.g. shows the whole of it.
In the photo of the caterpillar only the head is sharp. Taking a photo from its side we can see its whole image.