Alan321
Registered: Nov 07, 2005
Total Posts: 8686
Country: Australia
|
MFD? Do you mean MTF (modulation transfer function) ?
This is technical stuff and not easily explained in laymans terms, or at least not such that the laymen would be any wiser. However, I'll try.
In general they are calculated rather than actual measured data, but the results still indicate how good or bad a lens design works compared with others of similar focal length.
A set of line patterns passed through the lens would produce a slightly modified set of patterns on the other side. The originals are black and white stripes of equal width. Some are radial (pointing towards the centre of the image) and some are tangential (at right angles to radial). These will produce two sets of lines on the chart, the solid and dashed respectively.
Two pattern line widths are used, corresponding to 30 lines per mm and 10 lines per mm. These will produce two sets of lines on the chart, the blue and black respectively.
Patterns are placed at various distances from the centre of the image to the very corner. This is the horizontal scale of the MTF graph.
Two sets of lines are used - one for maximum aperture and one for f/8 (usually). These are the heavy and thin lines respectively.
After going through the lens the black/white edges of the patterns are somewhat greyed - i.e. they have been softened or blurred. The transition from dark to light is progressive rather than sudden. Because of their proximity to each other the blurred edges may merge together so that there is no longer any pure black or pure white but just various shades of grey from dark to light. The ratio of dark to light in the output is measured as a fraction of the ratio of the black to white in the input. This is the vertical scale of the MTF graph.
We now have 2 x 2 x 2 = 8 lines on the chart.
Values near the top of the chart indicate an excellent reproduction of the patterns whereas lower values indicate greater levels of softening. If the 10 lpmm lines on the chart are high then overall contrast is good. If the 30lpmm lines on the chart are high then fine detail (implying resolution) is good. There are many combinations where lenses do better near the centre than near the edges, or better at smaller apertures than at maximum aperture, or better with radial patterns than tangential patterns.
If the dashed and solid lines of each type (blue/black and heavy/thin) are close to each other then blurring is likely to be smoother (similar in both directions) and more pleasing to the eye. Other than that, higher is always better.
Do a search for the Canon Lens Work III book and look at chapter 10. There is section on MTF.
- Alan
|
