Superb image quality in today’s world of photography is fostered by a whole host of different factors such as the high numbers of pixels in digital SLR cameras, large format sensors in full-frame 35mm format or traditional almost-medium format, intelligent image processing techniques and noise reduction algorithms. The results are also dependent on how the camera interacts with the lens, which is why high-grade optics have such a vital role to play in fulfilling photographers’ increasingly sophisticated demands with regard to image quality.
The weakest link
Achieving top-quality shots with high-resolution cameras means keeping all the parameters that have a direct or indirect impact on image quality within strict boundaries. The idea that “a chain is only as strong as its weakest link” has particular relevance for photography and image reproduction.
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 plain. This plain 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.....
Via Thomas Menk