Accurate Focus, etc. (continued)
Lenses for large format cameras are typically corrected for an aperture of f/22, which offers more depth of field than the apertures that smaller digital or film camera lenses are corrected for (typically f/5.6-f/8), with the benefit of increased three-dimensionality in photographs. Since large format images don't need to be enlarged as much as smaller format images, the lenses for large format don't need to be as sharp.
Camera and subject movement can be overcome by using fast shutter speeds and a steady tripod. This may seem like a no-brainer, but there is little sense in buying the sharpest lens money can buy and then hand-holding the camera for photographs you want to be sharp. A light tripod can be made more stabile by attaching an apron (e.g. from Bogen or homemade) to the tripod and then putting weight such as your knapsack on the apron. A monopod is better than nothing, but still no substitute for a good tripod. You should invest more money in a good tripod than in a good lens.
Flare control An effective hood can increase the local contrast of a picture amazingly. The best hood is the bellows type with a baffle on the end to create a darkened box in front of the lens. Hasselblad, Lee and other manufacturers make such hoods for medium format cameras (and they can be used on smaller cameras as well). Sinar makes a highly adjustable bellows hood for view cameras. Or you can make a decent bellows hood and baffle yourself out of flat-black cardboard. A polarizing filter in front of the lens can also reduce reflections that degrade contrast when the light source is at a certain angle to the subject.
Lens aberrations Most modern lenses are corrected for common optical defects like coma and astigmatism, though cheap zoom lenses may suffer from some barrel or pincushion distortion as well as some degree of vignetting. (Beware of camera "kits" for sale with a cheap zoom lens).
Chromatic aberration causes a loss of sharpness and/or color fringing. There are two types of chromatic aberration: one increases with the effective focal length of the lens, and the other increases with the size of the aperture. To avoid chromatic aberration use shorter lenses (less than 180mm) and smaller apertures (f4, f5.6, f8, etc.). Some expensive lenses are apochromatically corrected.
Spherical aberration causes softness in the edges of an image, especially with wide-angle lenses. A lens with an aspherical element may improve resolution at the edges but may increase chromatic aberration. (An aspherical element may also be added to a lens design for marketing purposes even though it doesn't significantly improve image quality.) Wide-angle lenses cause distortion of perspective (enlarged facial features and/or elongated limbs) in close-up pictures, so longer lenses are preferred (app. 100mm for 35mm cameras, 150mm for medium format cameras, 360mm for 4x5" cameras).
Itís widely believed that lenses with more glass elements offer higher quality than lenses with fewer elements, but that isnít necessarily true. More elements are usually needed to correct aberrations that threaten lenses of very wide aperture or especially zoom lenses. Hence, although a lens with a maximum aperture of f/1.4 may have seven or eight elements, that seventh or eighth element merely compensates for the decrease in quality that would otherwise result from the wide aperture (compared to the same lens with a maximum aperture of f/2 or f/1.7 and one less element). In addition, the more glass elements a lens has, the more difficult it it to achieve high contrast. Older lenses with few elements that can be purchased used at low prices are often just as high in contrast as expensive, new lenses.
Lenses designed for digital cameras are often less corrected than older lenses designed for film cameras, since the software in some digital cameras is designed to compensate for some lens aberrations. Hence, you can use older lenses designed for film cameras on new digital cameras (an adapter is sometimes needed) and retain high image quality, but if you use a digital lens on an older film camera then image quality might suffer.
Once you are using a good tripod, bellows hood, polarizer, and focus bracketing, you can start worrying about whether or not there are significant differences in the resolving power of film (slide or negative? 50 or 100 ISO?) or digital sensors (10MP or 39MP?). Other issues in image quality, such as lens contrast, will be analyzed in the comparisons between lenses and between film and digital photographs.