Resolution - MTF Chart results
As demonstrated by the MTF chart below, the 135 STF is an outstanding performer.
The 135 STF's MTF Chart
The values are indicated for wide open (green) and f8.0 (orange) for a spatial frequency of 10 line pairs/mm (the thick lines) and 30 line pairs/mm (the thin lines) in each case. Solid lines represent Radial measurements made from the optical centre out to the periphery. Dotted lines represent Tangential mreasurements, made at 90 degree angle to radial measurements.
The excellence of the 135 STF are illustrated in this chart. The constantly high contrast and sharpness achieved this lens, from the centre of frame to the edge. The small distance between the two curves indicates besides the outstanding bokeh.
Reading MTF Charts
In case you're unfamiliar with MTF charts (Modulation Transfer Function Charts), here is a brief description:
MTF charts (short for Modulation Transfer Function) provide a graph analyzing a lens' ability to resolve sharp details in very fine sets of parallel lines, and a lens' contrast or ability to provide a sharp transfer between light and dark areas in sets of thicker parallel lines. Fine repeating line sets are created parallel to a diagonal line running from corner to corner of the 35mm frame, directly through the exact center of the image area. These are called Radial lines (or sometimes sagittal lines) on MTF charts. At a 90 degree angle to these, additional sets of repeating lines are drawn, called Tangential lines (sometime referred to as Meridional line sets). Repeating extremely fine short parallel lines spaced at 10 and 30 lines per millimeter measure the lens' ability to record fine details, or its resolution.
In theory, a perfect lens would produce nothing but straight horizontal lines across the very top of an MTF chart, indicating 100% accurate reproduction from the center of the picture (toward the left of the chart) to its outermost corners (at the right side of the chart). Of course, no such thing as a perfect lens exists from any SLR manufacturer, so MTF charts typically show lines that tend to curve downward as they move left to right (tracking the lens' performance from center to corner of the frame). The lower the line, the poorer the lens performance, and the greater the curve, the more performance drops off toward the edges of the images produced.
The vertical axis shows numbers from 0 to 100, representing percentage contrast - so 50 means 50% contrast. The horizontal axis is in millimeters and shows the distance from the center of the image toward the edges. In other words, 0 on the horizontal axis is the center of the lens and 20 is 20mm out from the center.
Here are some rules of thumb for reading a chart:
- the higher up the chart the 10 LP/mm line is (the thick lines), the higher the contrast reproduction capability of the lens will be.
- the higher up the chart the 30 LP/mm line is (the thin lines), the higher the resolving power and thus subjective sharpness of the lens will be.
- a lens whose thick lines (10 LP/mm) are above 80 on the chart should be regarded as having excellent image quality. Above 60 is regarded as 'satisfactory'.
- the closer the solid and dotted line pairs are to each other, the smoother bokeh is likely to appear.
- the smaller the gap between the green and red curves, the more consistent the performance across the aperture range.