I came across a great discussion of the difficulties in getting consistently sharp pictures using medium format digital cameras. Although it was written more than 3 years ago and updated about two years ago, I believe the basic situation still obtains, and therefore what Joseph Holmes has to say about it is important to anyone who wants to get the most out of their expensive medium format equipment, as well as anyone who is contemplating the leap to MF digital capture.
Check it out at: http://www.josephholmes.com/news-medformatprecision.html
A quick synopsis: if the moon and the stars are perfectly aligned, medium format digital image capture equipment is capable of delivering strikingly crisp and detailed images. However, especially with wide-angle lenses, the tolerances are extremely small. Therefore, many things, such as poor quality control in lens manufacture and testing, imprecise positioning and alignment of focusing screens and CCD’s, and focusing difficulties can prevent achieving the quality that the format promises, and often do so.
Holmes’ paper has many examples, and shows a remarkable depth of understanding of both the theoretical world of optics and the practice of photography. Warning: the results of Holmes’ lens tests make depressing reading.
As I’ve said before, if your subject matter will hold still, stitching together images made with normal focal lengths can be an effective way to get clean, crisp, big images.