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MTF Charts – Understanding Modulation Transfer Function Charts – the Basics, the Vocabulary โญโญโญโญ๐ŸŒ๐Ÿ˜Ž๐Ÿ˜Ž๐Ÿ˜Ž๐Ÿ˜Ž

August 6, 2013

โšซ MTF Charts Made Simple – Understanding Modulation Transfer Function Charts – the Basics and Vocabulary – Resolution, Contrast. MTF sounds very Complicated and Unnecessary “techno-babble.” However, take a few moments and read through this article as it will explain this valuable measuring method to determine a Lens’ Contrast and Resolution characteristics, a/k/a Optical Performance.

โšซ This topic sounds very complicated – It is really not that exotoic. After you’ve read this article, you will better understand how to read these valuable MTF charts. This is not written for optical engineers. This has been written as talking-points to be discussed as a segment of our Principles of Digital Photography classes.

โšซ MTF Charts – Understanding Modulation Transfer Function Charts – the Basics and Vocabulary. In theory, an ideal lens would transmit 100% of the light that passes though it perfectly, with unlimited (high) resolution and with no distortion. However, we know intuitively, optical lenses are not perfect and therefore image quality degradation occurs, "image quality losses."

โšซ Optical Losses are measured in terms of contrast, called the modulation of contrast, by optics engineers. What level of contrast is lost, or level of modulation, simply “the change in contrast.” Modulation of contrast is measured at different spatial frequencies. Spatial frequencies are the number of black-and-white bars on a test-target, ranging from 1 line every millimeter, or 10 lines per millimeter, 20 LPM, 30 LPM, or 100 lines per millimeter for a very high resolution chart.

โšซ The MTF Chart below, shows the MTF chart for a Canon 16-35mm f2.8L USM zoom lens, the Left Graph is with the lens set to maximum focal length of 35mm; the Right Graph is with the lens set to minimum focal length of 16mm. Looking at the Vertical and Horizontal Axes, the vertical axis shows values from 0 at the bottom-left corner, increasing by tenths, 0.1, 0.2, 0.3, proceeding UP to the top-left of the graph to a value of 1.0 which could be translated to q scale of 0% to 100%. Therefore a value of 0.10 could be thought of as 10% contrast, 0.5 equates to 50% contrast, and 1.0 equals 100% contrast. In other words, the higher the Value, on the Vertical Axis, the Better.

โšซ The Horizontal Axis is delineated in Millimeters, and shows the distance from the exact-center of the image, scaling outward toward the edges of the image. For example, the value of 0 on the Horizontal axis is the exact-center of the Lens, the value of 5 represents characteristics, 5mm from the center of the image, the value of 10 represents characteristics, 10mm from the center of the image, and the value of 20 represents characteristics, 20mm from the center of the image. In sum, the higher a plotted-point is on the chart, the higher the contrast transfer capability; the further to the right for a plotted-point is on the chart, the further from the center of the image.

โšซ Since a 35mm (film) frame is exactly 36mm across, on the long-edge of the image frame, a value of 20 represents the outer-edge of the image frame. Recall that a 35mm film frame, or the present day Full Frame Imager has a dimension of: 24mm by 36mm, and about 43.2mm diagonally.

โšซ MTF measurements are displayed by the wavy-lines of plotted-points. Notice on the chart above that there are blue lines, black lines, thick lines and thin lines. Each of these line-types are also shown as solid and dotted, for a total of 8 different line-types, that represents:

โœ” Thick lines are measurements taken at 10 Lines per Millimeter (10 LPM is low spatial frequency, or low resolution).

โœ” Thin lines are at 30 Lines per Millimeter, (30 LPM is a higher spatial frequency, or higher resolution).

โœ” Black lines are measurements taken with the lens aperture set wide open, in this case f2.8.

โœ” Blue lines are with the lens aperture set at f/8.

โœ” Solid lines are Meridonial. (Meridonial lines are dawn at a 90ยฐangle to these diagonal lines, additional sets of repeating lines are drawn, called Meridonial (or โ€œMโ€) line sets.

โœ” Dotted lines represent Sagittal measurements. (Fine repeating dotted-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 sagital lines, sometimes designated โ€œSโ€ on Canonโ€™s MTF charts.)

โœ” Bokeh is an important image-quality issue that a MTF chart helps to evaluate. Bokeh is a Japanese word which describes the manner in which a lens reproduces the “out-of-focus” areas of an image. Some lenses deliver relatively harsh out-of-focus results, while best performing lenses produce a more pleasing out of focus image.

โœ” Bokeh is where the meridonial and sagital lines come into focus, pardon the pun. Meridonial and sagital lines the dotted and dashed lines on the MTF chart.

Simplified Key for Interpreting MTF Charts, Modulation Transfer Function Charts: Recall, the higher Up the chart a line is, the higher the contrast, and that 0 to 20 across the bottom scale represents the distsance in millimeters from the center of the lens to its edge. Ignoring the dotted vs. dashed lines for the moment. Here is some guidance in reading MTF Charts:

โœ” The higher UP the chart the 10 LPM line is (the thick lines), the higher the contrast reproduction capability of the lens will be.

โœ” The higher UP the chart the 30 LPM line is (the thin lines), the higher the resolving power and thus subjective sharpness of the lens will be.

โœ” The solid black lines show the lens wide open, f2.8 the maximum aperture.

โœ” The solid blue lines show the lens stopped down to exactly f/8 for all lenses.

โœ” The closer each paired-sets of solid-lines are to each other within the pair, the better the performance of the lens when used at its maximum aperture of f2.8 for this particular lens.

โœ” The closer the the dotted and dashed line-pairs on the MTF chart, are to each other, the more pleasing the Bokeh of the lens. Meridonial and sagital lines are also used used to evaluate astigmatism distortion and field curvature.

โœ” The very best performing lenses will have the Black and the Blue lines closer together. The further apart the Black and Blue lines are, the lens would be of lesser overall performance

โœ” The lens whose thick lines (10 LPM) are above 0.8 (or 80%) on the chart should be regarded as having excellent image quality. Values above 0.6 (or 60%) is regarded as “satisfactory”. Line-values below 0.6 (or 60%) may be considered below satisfactory.

โœ” MTF charts “say” much about lenses; however, it does not expose everything about a lens’ optical characteristics. Variables such as vignetting, linear distortion, and a lens’ resistance to flare are among the things not shown on MTF charts.

๐Ÿ”ท Lens sharpness is vague term and can mean different things at different times to different people. MTF on the other hand is the terminology that lens designers and optical engineers use; MTF presentations is a topic photographers need to understand when describing the complex interaction between resolution and contrast.

๐Ÿ”ท Resolution and contrast are inseperably bound. Think of a series of alternating lines; black, then white, then blank, then white again. What differentiates them is their contrast. Make the black lines lighter and the white ones darker and eventually you won’t be able to tell them apart. In other words, if you have a white line on white paper you have no contrast and therefore no resolution. The line will be invisible. Keep this in mind, because a discussion of resolution without taking contrast into account is meaningless.

๐Ÿ”ท Resolution is usually measured in lines per millimeter, or line pairs per millimeter. Be careful not to confuse the two. Typically engineering types refer to lines per millimeter, assuming that to have a black line one must also have a white line in between. But, photographers tend to be less rigorous in their thinking and so Line Pairs per millimeter is commonly mis-used. Be aware that numerically speaking L/mm is double LP/mm. The value of 50 L/mm to an engineer, means 50 line-pairs housed with a One Millimeter segment, as every black line has a white line of equal gauge. There is a similar confusion regarding DPI (dots per inch) and PPI (pixels per inch).

๐Ÿ”ท Resolution is not a quantifiable scientific absolute, as resolution subjectively determined by balancing contrast with sharpness. While one observer may be able to see a resolution of 50 L/mm on a particular test, another may see 45 L/mm or 55/ L/mm. A 10% variance with the same observer at the same time is not at all unusual. Add to this the other variables, as focusing errors, and have heard by experts that a variance of as much as 30% in such tests is not uncommon. And, to make things even more indeterminate, it can clearly be shown that the actual image quality produced by a lens does not necessarily correlate with its measured resolving power. For this reason resolution test results alone should not be considered a sole measure of a lens’ quality. These factors are why the optical designers and engineers, and professional photographers rely on the lens’ MTF Charting.

๐Ÿ”ท For optical designers and engineers, contrast and resolution are in conflict. Increase one and the other is effected adversly. Various lens makers have differing philosophies for the balance of these two characteristics.

๐Ÿ”ท Historically, Zeiss has a reputation of designing lenses for maximum resolution, while Leica apparently favored maximum contrast.

๐Ÿ”ท Canon has the reputation of delivering a high-quality balance of Resolution and Contrast optical characteristics, including Image Stabilization for their professional lenses with higher focal-lengths. It is these lens design decisions that account for differing performance characteristics of different lens brands.

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