Copyright Michael Karbo, Denmark, Europe.
Chapter 31. Sharp images
One the photographer’s most important tasks is to produce sharp images. This means that a camera’s lens has to be focused at the right distance when an image is taken.
This is what happens in by far the most situations because cameras have very effective autofocusing systems. And should the autofocus fail at first, then there are other ways of ensuring a sharp image.
Figuree 140. How annoying! The camera has focused on the bottles in the background, which wasn’t the idea.
What is sharpness?
It is not important how well an image is exposed or how fine the colours are - if the image isn’t sharp, then it just isn’t good enough. Unsharp images are created in two ways:
Normally, it is possible to avoid shaken images; you just have to keep an eye on the shutter speed and take it into consideration. It’s worse when the camera’s lens isn’t focusing properly.
The problem is that it is often difficult to see that the focusing is wrong. The LCD screen is often too small and imprecise to reveal unsharp exposures during display. This means that you first discover the unsharpness when you get home and see the images on a computer’s screen. And by then it is too late and very annoying!
Let’s look closer at the mechanisms of focusing. There are several helpful functions.
The lens’ focus point is found in front of the camera. This focus point can be moved so that the camera focuses on something, which is closer or something, which is further away.
Figure 141. The camera focuses at the distance, the lens has been set to. The focus distance can be from a few cm to the ”infinite”, as one says.
Focusing means to “make sharp”, and this is done in the camera’s objective, where the lenses can be moved in relation to each other. By moving the lenses the focus distanceis changed.
Normal lenses can focus at a distance from ca. 50 cm up to ”infinite", which is the biggest distance you can choose. This is typically ten meters and applies to everything, which is further away. If you, for example, are taking a portrait then the focusing point will be about 1.2 meter. If you are photographing a landscape, then the distance is set to infinite.
If you want to photograph at a shorter distance than the ca. 50 cm, which is the normal minimum focusing distance, then the lens has to be set to macro exposure.
Autofocus with contrast measurement
If a camera knows the distance to a subject, it is easy to focus. But the camera does not know this; in practice, it has to guess the distance.
All cameras attempt to find a focus distance with the help of the function autofocus (or AF). This is done quite automatically, typically when the release is pressed halfway down. The camera’s electronics tries to identify the main subject and the distance to it, so that it can focus. There is a little motor in the lens, which can move the optics, so that the focus is altered.
Figure 142. Contrast is bigger, when the image is focused as on the left. Autofocus examines the image to find the area, which has the largest contrast.
The focusing itself is done with the help of contrast measurement of the light, which comes through the lenses. The image sensor measures the contrast in selected areas of the image’s pixels. The area, which contains the biggest contrasts, is identified as the focus area. Then the lenses are just moved back and forth until there is maximum contrast in the area in question.
When the contrast is at a maximum, the camera assumes that it is focused on the main subject. The contrast is examined in the points and the lens is set to focus on the point, which has the biggest contrast. This point is often accentuated in the viewer or on the LCD screen.
There are two problems with autofocus: If there isn’t enough light for taking an exposure, then the system doesn’t work; there has to be light for measuring the contrast. The other problem is that the autofocus has to find the distance to the right subject. This isn’t always easy.
AF sensor points
Autofocus doesn’t work with all the area of an image. It doesn’t, for example, try to focus on a subject in the image’s top left corner. There are instead a handful of points, which the autofocus system works with – either automatically or manually. There can be 5-, 7- or 11-points (as in Figure 143), all dependent, on which camera is being used.
The autofocus points are located within a central area in the middle of the image (focus area). This is why the main subject is to be found within this focus area, if the function is to be reliable.
When the release has been pressed halfway down, the AF system measures the contrast in the points. The point that has the biggest contrast, is assumed to be the subject, and is, therefore, focused on. The active focus point is often lit up with green or marked in another way, when the camera has decided where it should focus. There might also be a little bleep in the camera’s loudspeaker indicating that the focus point has been found.You just have to be sure that it is the right subject the camera is focusing on. If you are at a concert, for example, and want to take a photograph of the musicians, then it is not particularly smart if the camera is focused on a bald head in the row just in front of you.
Figure 143. With autofocus the camera places a number of points in a focus frame on the subject, as shown here.
When the camera can’t focus
What if the autofocus for one reason or another isn’t working? Then there are a couple of tricks you can resort to.
With some cameras you get the option of selecting autofocus points yourself (Autofocus Select). You can activate one of the autofocus points with the help of the camera body’s tilt button. The selected autofocus point is framed or marked with a colour in the viewer. Press the release halfway down, and the activated point will focus. This can be of help when placing the focus at the right spot in the subject.
Figure 144. The autofocus here is Select-mode. The active AF point is moved to the right by pressing on the camera’s tilt key.
The most used trick is using the focus lock, when the main subject is located outside the focus frame. You place the main subject in the middle of the viewer field, so that you can focus on it.
Press the release button halfway down and the focus is locked. Then you can, while the button is still held halfway down, recompose the image so that the main subject, for example, is moved over to one of the sides. Press all the way down and the image is taken with the locked focus.
Some cameras have an illuminator, which can be activated if there isn’t enough light for the autofocus to work with. The illuminator (AF assist beam) is a special lamp, which lights the subject up so that the autofocus can work. The beam can be white, red or green – it varies from model to model. The beam is activated automatically, when there is too little light to focus with and it has a range of about 6 meters.
Figure 145. The illuminator casts a beam of light out of a little lamp opening on the camera body. (Sony F828)
The autofocus normally works as soon as the release has been pressed halfway down. That’s when the camera’s autofunction starts finding the focus point and focusing on the chosen subject. This type of autofocus is usually called Single AF or One-shot AF.
You can also decide to let the autofocus function constantly. This option can probably be found in the menu system under something like Continuous AF, Servo AF or Fulltime AF.
There are three focus options on the dial: Continuous AF (C-AF), which works
continuously. With the option Single AF (S-AF) the release has to be pressed
halfway down to activate the autofocus. Finally at the bottom you can see
the manual focusing, called MF. (
Some cameras have this value as standard, which means it has to be switched off, if you don’t want the constant AF activity. Constant focus has, you see, both advantages and disadvantages. When it is switched on, the camera focuses all the time on something new every time it is moved. This gives continuous noise from the lens, which can be quite irritating. It also uses a lot of power. The advantage is that is faster to take snaps. The camera is in focus all the time “all on its own”, so you don’t have to waste time focusing when the release is pressed down.
Constant autofocus is activated here. (
All the automatics are switched off with manual focus. This can be of great help with:
In all these situations, there is a risk that the autofocus won’t work properly. So there isn’t anything else to do than activate the manual focus.
Figure 148. Photographing through a window. The autofocus focuses on the window’s (dirty) surface, which wasn’t intended. The solution is choosing manual focus.
Manual focus is usually activated by a little press or tilt button. When you change over to manual focus, something usually happens on the LCD screen. Either you get a little section of an image powerfully enlarged or else the whole image is enlarged to twice its size. The idea being that you yourself ought to be able to see when the required subject is focused on.
Figure 149. Shift between autofocus and manual focus. (Sony F828)
The focus can be changed by either pushing on a button or by twisting a ring on the lens. This last way of focusing is really great to work with and is found on the biggest zoom cameras:
Figure 150. Macro photography with manual focus. The top focus is set to 7 cm, but the trinket isn’t sharp. The focus ring is twisted, so that it is set to 15 cm, but this is too far away; the trinket is still not sharp (the image in the middle). 10 cm is the correct focus distance. You can see this on the LCD screen at the bottom, where the trinket is focused with maximum contrast. (Sony F828)