Now to what's called MTF measurements. MTF stands for "Modulation Transfer Function." What it refers to is the contrast between black and white lines that are spaced very close together. As the spacing decreases, the lines become less and less distinct. Both the the white lines and the black lines start to become gray, so the contrast is less.

For the photographic MTF tests, sharpness for a particular camera/lens combination is defined as the number of line widths per picture height (LW/PH) when the contrast between lines is 50%.

To perform the tests, a set of two black and two white rectangles (seen in the photo below) is photographed at a standard distance, in this case 1 meter. In Imatest, the MTF sequence is opened, and a small vertical section of the edge between one of the black and one of the white rectangles is selected, and Imatest does the calculations.

The results are shown below, for the Sigma SD14, using the 15mm-30mm lens, set to 15mm and f/8.

In the top graph is shown the Edge Profile, the black line indicates the sharpness of the camera/lens combination, which states 1606 LW/PH. The red line is a standardized line, calculated to be what the sharpness would be if the sharpening were optimal, namely 1992 LW/PH. The steeper the curve, the better. In other words, the faster the rise, the better. In this case, the 10-90% rise is 1.91 pixels.

In the second graph, MTF is shown. The camera/lens combination showed 1596 LW/PH, and is considered to be 18.8% undersharpened, compared to the standard sharpening curve shown in red.

In this third graph, the chromatic aberration of the camera/lens combination was measured to be 0.107 pixels.

Here are the date for this same lens, set to 30mm and f/8.

The rise is 2.33 pixels, and sharpness at 1321 LW/PH. The MTF results showed 1367 LW/PH and a 29.7% undersharpening. Chromatic aberration was 0.537 pixels.

 

With the Sigma 50mm lens, below are the results.

Rise was 3.7 pixels, sharpness was 830.2 LW/PH. MTF showed 870.2 LW/PH and 27.3% undersharpening. CA was 0.34 pixels.

Using the 28mm-70mm lens, set to 28mm, rise was 2.36 pixels, 1299 pixels per picture height. MTF was 1363 LW/PH, undersharpening 29.9%, and CA was 0.241 pixels.

At 70mm, the results follow below.

Go to Part VII.