I. What is MTF?

MTF is short for Modulation Transfer Function, an important indicator widely used for qualitatively and quantitatively expressing the performance of optical imaging systems. MTF curves simultaneously display information on resolution and contrast. It can also reflect the imaging quality of the imaging system across different frequency bands. Lenses can be evaluated based on the specific application requirements, and it can be used to compare the performance of multiple lenses. Typically, professional lens manufacturers measure in extremely objective and rigorous testing environments using specialized instruments, providing authoritative and objective technical references for lens imaging quality.

To ensure the accuracy and scientific nature of the measurement data, MTF data from the Thypoch brand comes from the company's TRIOPTICS ImageMaster HR series products. As of 2024, this equipment will still be the top measuring instrument in the industry.

--What can the MTF curve tell you?

1. Image surface (frame) size

2. Resolution

3. Contrast

4. Transverse Chromatic Aberration

5. Defocus state

--What can the MTF curve tell you?

1. Glare

2. Distortion

3. Longitudinal Chromatic Aberration

4. Color Reproduction

* This refers to the quality of the lens that we can and cannot see on the MTF curve released by the manufacturer. This is not data that the manufacturer cannot detect.

II. How to analyze MTF charts?

1. First, we need to understand the meaning of the X-axis and Y-axis:

The X-axis represents spatial frequency, usually measured in line pairs per millimeter (lp/mm), representing the size of the details in the image. For example, the curve in the image below ranges from 0 to 22 on the X-axis, indicating the range from the center of the measured image to the corners. 0 represents the center of the image, and 22 represents the edge of the image, where 22 is the radius of the imaging element. From this, we can also understand that the imaging diameter of the measured lens is 44mm, which can cover a full-frame sensor.

Note: Spatial frequency refers to the number of identical geometric structures appearing within a unit length. It is used to describe the degree of detail or variation in an image. The higher the spatial frequency, the smaller the details in the image; the lower the frequency, the larger the details in the image.

The Y-axis represents contrast transmission, usually expressed as a percentage between 0 and 1. It indicates the degree to which the lens maintains clarity and contrast of details. The higher the percentage, the stronger the lens's resolution.

Note:

Resolution - the ability to reproduce fine details;

Contrast - the ability to distinguish between light and dark levels.

2. Second, we need to understand the parameters below the MTF curve.

Before discussing the parameters, let's first understand line pairs.

Line Pair (LP): simply put, it is a pair of lines, usually represented as follows:

The logo in the lower right corner of the image indicates T for Tangential, representing the meridional direction (tangential), and S for Sagittal, representing the sagittal direction (radial), which are line pairs in two different directions.

When the two lines on the MTF curve are closer together, it means that the lens imaging is more scientific, and the transverse chromatic aberration (TCA) will be smaller; when the distance between T and S is greater, it indicates that when the lens approaches the edge, the color aberration (astigmatism) will vary in all directions. Understanding this, if your image is slightly blurred, scientifically speaking, the degree of blurriness is the same in all directions; but the so-called characteristic defocus is precisely caused by different degrees of blurriness in different directions. This is also why the defocusing effect can be so magical.

10 represents 10 pairs of lines within 1 millimeter; 30 represents 30 pairs of lines within 1 millimeter, which is denser. The closer the 10 lines/mm curve gets to 1 (the maximum value), the better the lens imaging contrast. The closer the 30 lines/mm curve gets to 1, the higher the lens resolution.

F corresponds to the aperture value we are familiar with. Typically, the imaging quality of the lens at its maximum aperture is not the best, so manufacturers usually provide two reference values, such as F/1.4 and F/5.6.

Here, we can briefly summarize the method for viewing MTF curves

1. The higher any line is on the Y-axis, the better the contrast and resolution;

2. The flatter the downward trend of any line, the smaller the gap between the center of the image and the edge of the image;

3. The closer the dashed line is to the solid line, the more scientific the lens's defocus performance, and the less pronounced its style;

Analysis of the imaging quality of Simera 35mm & 28mm f/1.4 through the MTF chart:

It can be seen that at a spatial frequency of 10 lines/mm, whether at the wide open aperture F1.4 or the optimal aperture F5.6, the Simera 28mm f/1.4 performs excellently from the center of the image to the edge, very close to 1. The contrast performance is outstanding, and the dashed and solid lines are very close, resulting in a more scientific defocus performance.

Regarding Simera 28mm f/1.4, at a spatial frequency of 30 lines/mm, it can be seen that the best part is located at the center of the frame, displaying a downward trend gradually towards the edge, and the lens resolution will slightly decrease from center to edge. When the aperture is narrowed to f/5.6, the resolution at the edge improves compared to the fully open aperture f/1.4.

MTF charts can provide us with a preliminary impression of the sharpness of the lens. For photography, it can only give us a rough impression that can be referred to as "quality." Our preferred imaging effects and styles do not have specific definitions or measurable units. It is not solely composed of sharpness. Vignetting, glare, color reproduction, and distortion all collectively contribute to the overall style of the lens. Whether a lens is suitable for you depends not just on what you see on the MTF chart, but also on other factors that often need to be discovered through actual shooting samples.