Lens Contrast (Part 5 : MTF Chart)
Taken with a Nikon D50 and AF Zoom-Nikkor 70-300mm f/4-5.6G lens
Basically, how lenses are evaluated is by looking at how well they transmit evenly spaced lines of black and white; ten, 20, or 30 "line pairs per millimeter" (lp/mm) means exactly what it says. As these lines get more and more closely spaced, the "noise" between them blurs the edges and makes the black lines look dark gray and the white lines look light gray to the lens (and also to your eye, especially as you get farther away). As the lines get closer and closer together, pretty soon the lens canþt distinguish them tonally, and the lens "sees" one undifferentiated gray. This ability on the part of the lens, charted graphically, is what MTF, or Modulation Transfer Function, is all about.
MTF graphs typically chart a lens's performance at various "image heights". This just means the distance from the optical axis, which would correspond to the center of the negative. The exact center would have a height of zero, and so forth out to the corners. Thus, the left-hand side of most MTF graphs corresponds to the center of the image, and as the graph line moves to the right, it corresponds to areas of the negative further from the center. So the MTF chart describes a radius of the image circle cast by the lens. Any other radius from the optical center is presumed to mirror the one that's charted (which assumes the lens elements are perfectly centered, but manufacturing defects and quality control is another article).
Labels: AF Zoom-Nikkor 70-300mm f/4-5.6G, Nikon D50, Photography
