What is an MTF chart?
Have you ever wondered what those MTF charts that are displayed below each lens are? MTF stands for Modulation Transfer Function. An MTF chart is a measurement of sharpness, with sharpness being defined by contrast and resolution. Typically, contrast and resolution are measured based on the reading of closely stacked black and white lines. The contrast would be how well one can differentiate a black line from a white line. If the white line is not white enough and the black line is not dark enough, it translates into a mixture of greyish lines, which makes it very difficult to see the separation between lines. The resolution would be how many lines can be visually separated in a spacial frequency. The number of lines per millimeter or L/mm defines a given spatial frequency. Both contrast and resolution work hand in hand to create sharpness.
How do you read a Canon MTF chart?
Reading an MTF chart is the equivalent of looking at a measurement of a modulation of contrast. Ideally, an optical device or lens in our case, would let 100% of the light go through to the sensor. Losses of light are variances or modulation of contrast. This modulation of contrast is measured at two different spatial frequencies: at 10L/mm and at 30L/mm. The 10L/mm spatial frequency is used to measure the level of contrast, while the 30L/mm spatial frequency is used to measure the level of resolution. In the MTF chart, the vertical axis represents a % of contrast from 0 to 1, where 1 is 100% contrast. The horizontal axis represents the distance from the center of the image. Quickly said, the closer to 1 the lines are, the better it is.
Now, one will see eight different types of line in an MTF chart:
- The black lines are measurements with the aperture wide open, while the blue lines are measurements taken at f/8
- The thick line is dedicated to the 10L/mm spatial frequency and hence measures the level of contrast, while the thin line is dedicated to the 30L/mm spatial frequency and measures the level of resolution
- The solid lines represent meridonial lines, while the dotted lines represent the sagittal lines
- Another important point, the out of focus or bokeh will be more pleasing if the solid lines and dotted lines are very close to each other
- The Black lines are noticeably lower than the blue lines, meaning that the lens is measurably less sharp when the aperture is wide open at f4. Shots at f8 are quite sharp though
- The thick lines are a bit further up than the thin lines, meaning that contrast is a bit better rendered than resolution
- As solid lines and dotted lines are not too far apart, the bokeh will be descently pleasing