Lens Contrast (Part 6 : MTF Chart)
Taken with a Nikon D50 and AF Zoom-Nikkor 70-300mm f/4-5.6G lens
You'll note that most MTF charts have two graph lines per frequency, one solid and one dotted. This just measures object lines ("object" in opticspeak corresponds to what we'd call the "subject", what the lens is pointed at and focused on) that are either parallel to the radius of the image circle (called "sagittal") and those that are perpendicular to the radii ("tangential"). Most lenses are unable to do equally well with both simultaneously.
Technically speaking, MTF measures both contrast and resolution more or less simultaneously. In a photographer's reading of an MTF chart, however, generally the position of the topmost lines (typically 10 lp/mm, sometimes 5) will have the highest correlation to visible lens contrast. The lowest set of lines (30 or 40 lp/mm) will correspond best to actual resolving power. Personally, I pretty much ignore the lowest set or sets of lines when reading an MTF chart.
You should note here that different manufacturers provide different MTF frequency measurements. One company may provide 5 lp/mm graph lines, which makes their lenses look good. These lines are often very close to the top boundary of the chart. Other manufacturers may provide lines for 10 lp/mm as the coarsest structures they measure. The two shouldn't be compared directly. In fact, MTF charts from two different sources shouldn't be compared directly, either. There are enough experimental and procedural variations to make direct comparisons meaningless.
Labels: AF Zoom-Nikkor 70-300mm f/4-5.6G, Nikon D50, Photography
