One of the key links in the chain of successful image rendering is the precision with which the lens focuses on the main subject. Generally speaking, a photographic lens only provides optimum rendering at maximum image quality of a two-dimensional plane. This plane runs exactly in parallel to the film or sensor in the camera. Depending on the magnification and the selected aperture, a certain range both in front of and behind the point of optimum focus is also considered to be "adequately sharp". In this context, the magnification refers to the ratio between the image produced by the lens and the object being photographed. Thus, the focal length of the lens, the shooting distance and the size of the film or sensor are responsible for the so-called depth of field. The focusing region designated as the depth of field is the extent of the range in the object space of an imaging optical system. This region is rendered in acceptably sharp focus on the focal plane.You can read about the effect that slight defocusing has on image quality in the article "Measuring lenses objectively" which appears in Camera Lens News Nr. 30 starting on page 24. This article shows how important precise focusing is whenever a photographer intends to produce big enlargements or requires optimum quality for any other reason, and not only when using wide apertures.
Autofocus systems have undergone major developments since they were first introduced in cameras in the 1980s. Manufacturers have steadily improved their systems' performance and efficiency in daily use, and the combination of an increasing number of AF points in the camera bodies and ultrasonic motors in the lenses has facilitated fast and smooth focusing for the phase AF systems typically found in today's SLR cameras. For some applications, it is precisely this speed that marks the key benefit over other AF methods such as the contrast-based AF typically found in digital compact cameras or manual focusing.Good AF systems can generally achieve a very high number of "hits" in terms of the sharp images they produce with long telephoto lenses in situations such as taking shots of cheetahs in the wild, snapping celebrities from within a pack of paparazzi or photographing footballers on the playing field.
For any subject that is either not actively moving away from the photographer's position or that is set to move into the “trap focus” after careful pre-focusing, better results can be achieved with careful manual focusing. Carefully framed landscape shots, images of buildings or architectural details and meticulously arranged tabletop displays in a photographer's studio are unlikely to require the use of autofocus.Equally, both spontaneous portraits and reportage shots can achieve the same focusing precision by manual means as by using an AF system. Macro shots of butterflies on a flower that require a very shallow depth of field – where a tripod is generally a sensible choice – should always be precisely focused using manual methods. In these cases, it is rare for any of the camera's AF points to coincide exactly with the photographer's choice of focal point and, all too frequently, the focusing point covers too much of the subject with a large extension of depth, which means that it is no longer possible to use the automatic mechanism to achieve precise focus.You can read about the effect that slight defocusing has on image quality in the article "Measuring lenses objectively" which appears in Camera Lens News Nr. 30 starting on page 24. This article shows how important precise focusing is whenever a photographer intends to produce big enlargements or requires optimum quality for any other reason, and not only when using wide apertures.
Anyone who has attempted to manually focus modern AF SLR cameras and their corresponding AF lenses will have quickly come to the sobering conclusion that, practically speaking, this is far from easy. Steep-pitch helical mounts, play and backlash in the focus rings of the lenses, dim viewfinders and far-from-suitable screens in the cameras make it very difficult to achieve high-precision focusing. In contrast, it is quite a revelation to go back to one of the good SLR cameras with a suitable lens from the era before the widespread introduction of AF systems and witness the large, bright viewfinder, the eminently practical adjustment aids on the screen and the way in which a subject seems to "spring to life" when it is correctly in focus.Few of today's cameras achieve the innate viewfinder quality of these analog models that have now entered into the annals of history. Nevertheless, some good digital camera models from the medium to top-class brackets, especially full-frame cameras, do offer some advantages for high-precision manual focusing, at least thanks to the glass prism in the viewfinder and corresponding viewfinder magnification.
In most of the Canon EOS, Nikon AF and Pentax AF cameras, whether digital or analog, electronic focus confirmation continues to be available even when using our ZE, ZF or ZK lenses. The camera confirms that the manual focus is correct by illuminating the focus confirmation light or the active AF point. In many of the Nikon models (e.g. the D700 and D3 series), additional support is provided by two arrows in the viewfinder which indicate the direction of rotation for the focus ring when performing fast manual focusing.However, these electronic focusing aids only actually provide a relatively imprecise means of achieving high-precision manual focusing. The region shown as "in focus" when rotating the focus ring is generally quite large and is also dependent on the direction from which the subject is being brought into focus (i.e. whether you are coming from infinity or from the closest focus distance). We therefore recommend comprehensively testing the camera in combination with a manual lens in order to get a photographer's feel for the situations in which you can rely on the AF indicator. Especially when using fast lenses, it is advisable to take a bracketing series with a wide-open aperture and short shooting distances in order to achieve optimum results.It is sometimes possible to fall wide of the mark when taking shots with the aid of the focus confirmation function, but fortunately some recent camera models (e.g. the Nikon D300, D700, D3 series and the Canon EOS 5D MkII) have incorporated an AF fine-tuning option in the menu which obviously also takes effect on the focus indicator during manual focusing. If the AF indicator consistently gives an incorrect response when a certain type of lens is used or even with all lenses, then the focus error can generally be rectified by carrying out meticulous checks and adjustments.
And there is also a “fly in the ointment” when it comes to the use of the Live View function, namely the fact that many current camera models (e.g. the Nikon D700 and the Canon EOS 5D MkII) automatically control the aperture in the Live View mode based on the set ISO values and the ambient light in order to ensure that the image in the display is consistently shown with the correct brightness and contrast values. This means that it is no longer possible to close the aperture to the desired, fixed setting using the preview button. Correct evaluation of the depth of field and focus position is therefore equally out of reach using this method, since the aperture is only closed to the desired setting when the shot is actually taken.