Lens Speed & Depth Of Field
Although fast lenses are larger, heavier and more expensive than slower ones, they're a better choice for serious photography. As you know, an ƒ/2.8 lens will let you shoot in dimmer light, or at a higher shutter speed in a given light level, than a slower zoom will. And the faster lens will provide better results in selective-focus work because it can produce more limited depth of field when you want that. The faster lenses also provide a brighter image in a DSLR's viewfinder for easier composing and manual focusing/autofocus checking.
Low-light performance is only part of the story. You may be thinking that you don't need a fast lens because you'll be doing most of your landscape shooting at smaller apertures for maximum depth of field, but that's not necessarily the case. The interplay between lens speed, depth of field and the sweet spot come into the equation. In fact, because ƒ/22 is such a small aperture with a wide-angle lens, your maximum depth of field is compromised by diffraction-induced softness. With a fast wide-angle lens, you can get a lot of depth of field at ƒ/8 or even ƒ/5.6, and you'll be shooting at the sweet spot where the lens is sharpest. With a slower lens, you may need to stop down to ƒ/11 or ƒ/16 to be in the sweet spot, but your photograph will be affected by diffraction. For more information, see the sidebars "Sweet Spot" and "Focal Ratio" in this article.
How To Interpret MTF Curves
The gold-standard tool for evaluating a lens is the MTF graph, but for most photographers, MTF data is difficult to interpret, to say the least. An MTF (Modulation Transfer Function) curve is basically a graph that shows how well a lens or an entire optical system transfers the contrast of an original target to the image of the target. The target is a series of bright and dark lines, which gradually become finer and finer, i.e., starting from just a few bright/dark line pairs per millimeter and progressing to 100 line pairs or more per millimeter. An ideal lens/system would reproduce the test chart exactly as it is in real life.
A typical MTF curve plots contrast along the vertical axis and distance from the center of the image frame across the horizontal axis. The vertical axis scale runs from 0 at the bottom (no contrast at all) to 1.0 at the top (indicating the lens reproduces 100% of the target's contrast). The horizontal scale runs from 0 (center of the image frame) to 20 (20mm out from the center) or so, depending on the image format. A chart for a "perfect" lens would show a straight line across the top, meaning the lens reproduces 100% of the test target's contrast all across the frame. Of course, that doesn't happen in real life, so the line on the curve starts out high at the left (representing the center of the image) and curves down as it moves to the right (farther from the image center). As a general rule, MTF lines of 0.8 (80% of the target's contrast) indicate a very good lens; lines around 0.6 (60% contrast) indicate a satisfactory lens (depending, of course, on your idea of satisfactory).
A single line just shows lens performance for a single spatial frequency (number of line pairs per millimeter) at a specific aperture (generally, wide open). So manufacturers include lines for two or three spatial frequencies, generally 10 lp/mm and 30 lp/mm, or 10, 20 and 40 lp/mm. The 10 lp/mm lines provide an indication of the lens' contrast, while the 30 and 40 lp/mm lines provide an indication of the lens' sharpness. Generally, thick lines are used on the MTF graph for the 10 lp/mm curve and thin lines for the 30 lp/mm curve.
Lines are also shown for targets that are aligned parallel to the format's diagonal (corner to corner) and for targets perpendicular to those. The former are called radial, or sagittal, lines; the latter, tangential, or meridional, lines. These are shown as broken lines on the MTF graph; again, thick ones represent 10 lp/mm readings and thin ones, 30 lp/mm readings. The key here is that bokeh is more pleasant when the radial and tangential curves are similar.
Some manufacturers include lines for wide open and a middle aperture (often ƒ/8). These show the effects on performance of stopping the lens down. Lens performance generally increases as you stop them down from maximum aperture, since doing so reduces the effects of various aberrations; then performance decreases as you continue to stop down, and diffraction adversely affects the image.
Of course, for zoom lenses, you'll want to see MTF curves for wide, middle and long focal lengths. Bear in mind that, even for fixed-focal-length (prime) lenses, telephotos generally have better MTF charts than wide-angles because wide-angles present more design challenges.
If you really want to get into this, an online search for "Zeiss MTF Nasse" will lead you to a thorough two-part illustrated discussion of MTF curves and their ramifications (H.H. Nasse is the author).