Autofocus: Razor-sharp images and high resolution sensors

by Dietmar Temps
Autofocus systems are an inherent part of modern digital cameras. Additional autofocus assistants like face- or eye-recognition support the photographer to accurately capture common subjects. However, in the field of creative photography there are a few possible autofocus pitfalls in combination with high resolution image sensors. This applies also to sport-, action and macro photography. It is always an advantage to have a profound knowledge about the autofocus technology to avoid unpleasant surprises later in the post-processing workflow. These issues can often only be detected in 100% view of an image. Only an absolutely correct position of the focal plane guarantees razor-sharp images when using high resolution full-frame sensors.


Advanced amateurs and professional photographers often try to separate the main subject from the background by choosing a longer focal length combined with a large focal aperture. Such shooting situations are still a challenge even for sophisticated autofocus systems, particularly when using high resolution image sensors. When shooting portraits it can happen that the nose is sharp, but the eye facing the camera is slightly out of focus. Even if it is perhaps only a minor error of the position of the focal plane, we generally perceive such images as “slightly out of focus”.



AF fine adjustment
In case of SLR cameras the AF sensor is not located on the actual image sensor and is addressed via the mirror. The different optical path could be the cause that the focal plane of the AF sensor and the image sensor don’t match accurately. This effect is known as the “front focus” or “back focus” issue: the focal plane of the image is slightly ahead or behind the targeted autofocus point. Fortunately, this error can be corrected very easily with the assistance of the AF fine adjustment of the camera. Creative photographers who like to work with fixed focal lengths and large focal apertures should make sure when purchasing a camera body that the SLR camera provides an AF fine adjustment feature. There are also supporting tools (for example the SpyderLENSCAL focus tool from Datacolor), which makes AF fine adjustment easy and reliable.  Mirrorless system cameras use the actual image sensor for the autofocus and have an advantage because no AF fine adjustment is necessary.


Correct AF point selection
Modern AF sensors of SLR cameras can evaluate a lot of information from the subject of the image; mirrorless system cameras can even use the information of the entire image sensor. One might expect that this information almost excludes focus errors. However, only scenes such as faces and eyes are reliably detected. Other image information like for instance the color is predominantly used for AF tracking. If no subject can be detected, the AF sensor’s algorithm usually prefers the point closest to the camera as the optimum initial focal plane, which is not always the best result. For this reason, experienced photographers often choose the initial autofocus point by themself, leaving only the automatic tracking of the subject to the AF system.



Face recognition and eye recognition
The face- and eye-recognition of modern AF sensors are great tools, but as often the devil is in the detail. If the camera only provides face-recognition the autofocus might be too inaccurate. This can lead to issues like the ear or the nose of a portrait image is sharp, but the eyes are slightly out of focus. Although the eye-recognition is more accurate and extremely useful for portraits in most situations, the limits are shown in close-up shots. In case the face is the image-filling subject the usable depth of field is very narrow. These close-up scenarios in combination with high-resolution full-frame image sensors are a huge challenge for the accuracy of AF modules. It is a difference whether the AF point is positioned on the eyelashes, the iris or the skin below the eye. For example, portraits of children’s faces may appear slightly out of focus when the typical long eyelashes of children are blurred. On the contrary, a portrait of an old person full of character may be classified as out of focus if the wrinkles of the face are slightly blurred although the eyelashes might be razor-sharp.


Diffraction and focus stacking
When using high-resolution full-frame sensors the issue with diffraction is quite noticeable in combination with small focal apertures.  The undesirable diffraction of the light beams at small focal apertures can lead to soft picture results. At first glance the soft images often look like the result of an autofocus error. Unfortunately, a small aperture cannot always be avoided, especially in landscape photography. The problem can be solved in static scenes with “focus-stacking”: The landscape scenery (see the example below) was first exposed with the aperture 22 in order to optimally depict the sun stars of the setting sun on the horizon. In the second shot, the focus point was set on the foreground with the aperture 11. In the subsequent post-processing workflow, both exposures have been combined and a razor-sharp image is obtained over the entire image area.