When looking at lens specifications, MTF curves are often encountered. We’ll take a look at what they mean, how to interpret them and, most importantly, how to use them when selecting your next lens.
MTF Characteristics explained
MTF stands for Modular Transfer Function and is a method of evaluating the lens performance based on its contrast and resolution.
Contrast expresses how well an image differentiates between the object’s black and white lines.
Resolution expresses how well object detail is expressed, how sharp it is. A high-resolution image will display crisp edges and shows much detail while in a low-resolution image detail can run together causing blur (also expressed as “softness”).
MTF charts are created by measuring how well lenses capture a series of evenly spaced lines of black and white. By measuring multiple “densities (how close together these black and white lines are), we get meaningful information at how well the lens can distinguish the boundary between black and white.
The Canon MTF curves (which are the ones we’ll be looking at) display results measured at 10 lines per millimeter and at 30 lines per millimeter. Generally speaking, the 10 lines/mm curve is a useful indication of contrast while the 30 lines/mm is a useful indication of resolving power. Many photographers are primarily interested in the 10 line/mm curve.
Measuring these characteristics is of course influenced by the aperture used so the MTF charts typically display two sets. A black set for measurements at the lens’ maximum aperture (note that this can vary by lens) and a blue set for measurements at f/8 (always f/8).
Another characteristic that can influence the lens’ performance, is the lens curvature from center to edge. This is reflected in the chart on the horizontal axis where the numbers “0” to 20″ are millimeters. Zero is the center of the lens.
The dotted lines vs the solid lines are measurements representing the orientation of the black and white lines. Strangely enough, most lenses resolve these differently as you can see in the difference curves below.
Putting this all together in the illustration below, we can see an MTF chart (in this case of the Canon EF 300mm f/4 L IS lens).
As indicated in the illustration, any line above 0.6 is considered good. Above 0.8 it is considered excellent. In this example, wide open (the black lines) the lens is excellent, with both black lines being well above the 0.6 mark. When stopped down to f/8 (the blue lines) the results are even better.
What is also evident here is how the lines are relatively flat from left to right, especially the solid lines. This indicates the lens’s performance is consistent all the way to the edge. In other words, little deterioration in sharpness and contrast towards the outer edges.
What is also immediately evident, is that a Full Frame sensor will be more prone to differences in edge-sharpness than smaller sensors such as the APS-C sensor found in the Canon Rebel and Canon 30D/40D lines.
A Full Frame sensor is 36x24mm. Measured from the center, the longest side is therefore 18mm. Looking at the MTF chart above, one can see this puts the “edge” close to the outer edge of the chart. Here we see the curves starting to trail downwards, a sign of lesser performance, indicating a “softer” picture.
The APS-C sensor is 15×22.5mm. Here, the longest side is only 11.25 when measured from the center. That puts the edge of that sensor in the middle of the MTF chart. Usually at a spot where the curves have not yet trailed downwards, or only very slightly. Therefore, you’ll find the cameras with this size sensor to have better edge-sharpness characteristics than Full Frame cameras.
This lens used in the example above (the Canon EF 300mm f/4 L IS lens) is a very sharp lens with excellent contrast.