One of the major complaints we receive relate to lack of sharpness in photos. Most users attribute the lack of sharpness to their lenses and in some cases, the camera body itself. (Very few accept that their technique or their lack of understanding of the technology has to do anything with it!).
Whilst it is not impossible to have a lens (or a camera body) that is responsible for your mental agony, more often than not, the problem lies with the users not fully understanding the technology behind the scene.
Most entry-level and mid-level digital SLRs use autofocus sensors that are known as vertical line sensors ( Notice that the central sensor is cross-type, which is explained later on). This means that they detect subject contrast along a vertical axis. This further means that if your subject has horizontal lines, the vertical line sensors will easily latch on to them. But if the point where you are trying to autofocus has vertical lines, the vertical line sensors will likely fumble.
The square and the rectangles indicate the position and size of the autofocus sensors. In reality though, the actual sensor is larger than indicated as shown here |
Tip: You cannot physically move the vertical elements in your subject/scene, but you can certainly re-orient your camera so that the vertical sensors now act like horizontal sensors. Instead of trying to autofocus using the camera in horizontal (landscape) position, just turn the camera sideways (use the vertical or portrait orientation). See sketches below.
High-end D-SLRs use, besides the vertical line AF sensors, what is known as cross-type autofocus sensors. They detect contrast in two dimensions (and therefore more accurately) – vertical, as well as horizontal. With these types of AF sensors, it really does not matter whether your subject has vertical lines or horizontal lines on which you want to autofocus.
Notice that in the sketch showing the AF sensors in a particular high-end D-SLR, the central sensor is marked in red whilst the remaining cross-type sensors are marked in blue. This is further explained below.
The sensitivity of the AF sensors also depend on the maximum aperture of the lens being used. We know that autofocus does not work (at least not the way it should work) with lenses having maximum aperture narrower than f/5.6. (Some very high-end D-SLRs may be able to autofocus even with an f/8 lens, but generally, the limit is f/5.6). In our example, if you were to attach a lens having a maximum aperture of f/8, only the central AF sensor will work and it will work only as a vertical line sensor. This means low-to-poor AF capability. If you were to use a lens having a maximum aperture of f/5.6, then all the AF sensors would work, but again, only as vertical line sensors. This means that the AF capability will improve, but is still limited. Change the lens to an f/4 lens, the central sensor will act as a cross-type sensor but all the remaining AF sensors will still work only as vertical line sensors. This would mean a further improvement. Finally, if you were to use an f/2.8 or faster lens, all the cross-type sensors would be activated, resulting in faster and more accurate (most accurate) autofocus.
 Camera in Landscape position |
Camera in Portrait position |
Light Levels and Contrast
In my opinion, light levels and contrast at the point of focus are the two most important considerations to achieve accurate autofocus. Generally speaking, we need good light (intensity) and good contrast. But it is also possible to autofocus successfully in low light if the subject contrast is high. For example, a person standing in low light has clothes with strong contrast. If you point the AF sensor to an area that has sufficient contrast, AF may still be achieved. Alternately, if the subject is low in contrast but the light is strong, the AF sensor in your camera may be able to achieve focus.
 Verticle line plus cross-type AF sensors |
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Subject Movement
Subject movement is another consideration during autofocusing. If the subject is moving, autofocusing becomes more difficult. This, combined with poor lighting and poor subject contrast can cause nightmares for any type of AF sensors.
 Position of the AF sensors when the camera is held vertically |
 When the sensor is parellel to the subject, the focal plane and the subject plane are the same |
More about AF Sensors
The autofocus areas (the rectangles or squares) on your focusing screen are generally not accurately marked. Many a times, the actual sensor is larger than the area shown. This can cause autofocusing problems. Let’s consider the following example:
 The camera is tilted to make use of the central AF point |
 When the camera is repositioned, the point of focus falls behind the subject |
The user has aligned a cross-type sensor on the dog’s eye (which is the right thing to do), and the person is standing in reasonably good light. But, as mentioned earlier, the actual sensor is larger than what is shown on the camera’s focusing screen. Now let’s say that there is a flower in the near background. The flower has good contrast and is in strong light. Chances are that the system will lock focus onto the flower rather than the subject’s eye.
 The active AF sensor ‘sees’ two different elements |
Let’s take another example. This example shows the mistake done by most users. The user is trying to autofocus on the subject’s eye but is using the central AF sensor (after all, the central sensor is considered to be the most efficient). The user intends to autofocus using the central sensor, lock focus by half-pressing the shutter release button, re-compose and shoot. Right?
Wrong! It is better to use the off-centered focus sensor. Why? Let’s consider the geometry we learned a long time back in school, but may we have forgotten! See the sketch. When we focus with the central sensor and then re-align the camera, the focused distance falls behind the subject. So how do you expect the subject to be sharp? Don’t tell me that the depth of field will cover it – we use wider apertures when shooting portraits to throw the background out of focus. And wider the aperture, narrower is the DOF.
Remember, in spite of all the advances in autofocus technology, the AF system is not foolproof. Strange as it may seem, under some conditions, the camera, in spite of not achieving focus, may still display the in-focus indicator and also beep to say that focus has been achieved. This may happen under the following conditions:
a) When there is little or no contrast between the subject and the background.
b) If the active focusing sensor ‘sees’ two different elements at different distances, it can confuse the AF system – for example, in the image below, the sensor sees the bar of a cage and also the animal behind. This is indicated by the AF bracket on the bar, as well as the lion’s nose. In this case it does not know whether the cage is your subject or the animal.
c) Regular geometric patterns, like the row of windows in the skyscraper below.
d) When the subject contains a lot of minute fine details, such as this field filled with flowers, unless you select an AF point, the camera will not know the point of focus you require.
e) If the area you are trying to autofocus on has sharply contrasting brightness.
f) With modern cameras, transfer of information between the body and lens is through the electrical contacts on the body, as well as the lens. If the contacts get dirty (body/lens), communication of information can get disrupted or become erratic. This also could be the reason why the AF doesn’t function smoothly.
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Understand what you are doing
Don’t expect your AF system to perform well if you combine a slow lens with a teleconverter. A f/5.6 lens (most entry-level and mid-level zooms fall in this category) with a 2x converter becomes an f/11 lens and can not successfully autofocus.
Using a Polarizing filter on a slow lens makes it difficult for the AF system to do its work efficiently.
If the subject you are trying to autofocus on is very small (for example, a bird in flight), you won’t be able to achieve pin-sharp autofocus unless the bird is quite large in the frame. This is due to the fact that it is very difficult to align the sensor on a moving target and the fact that most sensors are larger than what is indicated on the focusing screen.
This becomes an ideal situation when the AF system starts to ‘hunt’ for focus.
When buying a lens, consider its usage. If fast and accurate AF is important to you, get a ‘fast’ lens. Fast lenses activate the extra sensitive cross-type sensors in your camera, making quick and accurate autofocus possible.
Here’s an important lesson I learned from Thom Hogan, one of my ‘gurus’, whom I immensely respect. Don’t expect your AF system to latch on to autofocus in Continuous Firing Mode (Burst mode) at a high frame rate unless you have first brought the subject in more or less good focus. When the reflex mirror is up, the AF system does not get the required information about focus and in the Burst Mode, it gets the information only intermittently. Hence pre-focus (as accurately as you can) and then set the Burst mode.
I hope this tutorial helps you in achieving sharper images. And yes, please don’t give up on manual focusing!
Rohinton Mehta
