MTF information is plotted onto MTF graphs for easy visualization when assessing the resolution of a lens, and it can be presented in different ways. Generally, MTF graphs are broadly categorized into two main types. The first type (MTF Graph by Frequency) presents the X-axis in terms of lp/mm (line pairs per millimeter). However, this type of graph has the drawback of not accurately depicting a lens's MTF performance across the entire sensor. To address this, the second type of graph (MTF Graph by Image Circle) has been introduced. In this graph, the X-axis represents the position (beginning at the center of the sensor and moving toward its edges), allowing for a more precise assessment of the lens's performance across the entire sensor. However, it has the limitation of only examining specific lp/mm values. For this reason, in most cases, these graphs display representative lp/mm values simultaneously.

Objective lenses are used to magnify an image. In addition to numerical aperture, magnification is also an important parameter. The objective magnification typically ranges from 4X to 100X. As the image sensor size or eye observed area is fixed, the field of view of a microscopy system changes with the magnification of the objective lens. Typically a lower magnification objective lens will have a larger field of view and lower resolution, and a higher magnification objective lens will have a smaller field of view and higher resolution. The diameter of the FOV can be calculated by using the following formula: FOV= FN/Mag The field number (FN) in microscopy is defined as the diameter of the area in the image plane that can be observed through the eyepiece or image sensor.

In reality, the smaller the spacing of the stripes, the harder it becomes for optical systems (like lenses and cameras) to express them—as shown in the illustration below. Lenses capture these line patterns and project them onto a sensor, and as the line spacing decreases, it becomes increasingly challenging to accurately reproduce them. Consequently, as the line spacing becomes tighter, the contrast of the black and white lines represented through the lens decreases. The MTF (Modulation Transfer Function) graph visually represents this phenomenon, helping us understand how well a lens can reproduce fine details in terms of contrast.

Alpha Industrial Park, Tu Thon Village, Ly Thuong Kiet Commune, Yen My District, Hung Yen Province Vietnam 17721 +84 221-730-8668 rfqvn@shanghai-optics.com

MTFcamera

In general, similar sagittal and tangential characteristics create a more uniform image. This means that if the sagittal and tangential lines are closer to each other on an MTF graph, the images produced by the lens will have more uniform image performance on both the X-axis (horizontal) and Y-axis (vertical). On the other hand, if the sagittal and tangential lines diverge across an image, it indicates an uneven image with aberrations—meaning the lens is astigmatic. Ideally you want the curves to be closer to each other for more uniform image performance.

NA is commonly expressed as NA = n × sinθa where θa is the maximum 1/2 acceptance angle of the objective, and n is the index of refraction of the immersion medium. The limit of resolution of a microscope objective refers to its ability to distinguish two closely spaced Airy disks. Resolution (r) = λ/(2NA) Where r is resolution (the smallest resolvable distance between two objects), and λ is the imaging wavelength. The higher the NA, the better the objective resolution.

Take advantage of our broad portfolio and choose the right product for your vision system. Ourlens selector helps you to easily choose the right lens for your overall system.

The X-axis (horizonal) refers to the Spatial frequency in cycles per mm by indicating the number of line pairs (i.e., one black and one white line) per millimeter (lp/mm)

The most common immersion media are air, water, oil, and silicone. Choosing the appropriate objective designed for your immersion medium will result in higher resolution images.

The most important parameter of a microscope objective is the numerical aperture (NA). NA measures the microscope objective’s ability to gather light and determines the resolution of a microscopy system.

Line pairs per millimeter (lp/mm), a unit of measurement that refers to pairs of alternating dark and white lines (line pairs) that can be distinguished in one millimeter of an image.

Multiple lines plotted in different colors represent different points along the curvature of the lens (i.e., 0.0000 mm is image center while 5.5000 mm is 5.5 mm from the image center)

By comparing MTF graphs of different lenses, you can make informed decisions based on the optical characteristics that matter most for your specific needs. In a real application, there may also be additional factors, such as sensor image size, magnification, and F/# sensor specifications to be considered.

Modulation transfer function

The Y-axis (vertical) represents the MTF on a scale from 0 to 100% or 1 and is marked by a scale of 10% or 0.1, respectively.

To get a sharp image, you not only need the right camera, but also the right lens to go with it. Our beginner's guide to lens selection will help.

Objective lenses are used in microscopy systems for a range of scientific research, industrial, and general lab applications. A microscope objective is typically composed of multiple lens elements and located closest to the object. There are so many types of microscope objectives available, choosing the right objective can help you produce good quality images at a reasonable cost. When choosing a microscope objective, we will need to consider a number of factors including conjugate distance, numerical aperture (NA), magnification, working distance, immersion medium, cover glass thickness, and optical aberration corrections. In this article, we will discuss how to choose the right microscope objective.

Contrast Performance: MTF graphs show contrast levels at various spatial frequencies. High contrast is crucial for image clarity, and MTF graphs help you to evaluate a lens's contrast performance under different conditions.

Many objective lenses are corrected for infinite conjugate distance, while others are designed for finite conjugate distance applications. Compared to infinite conjugate objectives which need a secondary lens (also called tube lens), a finite conjugate objective can generate an image of a specimen by itself. A finite conjugate objective, as shown in Figure 1, is a good, economical choice for a simple microscopy system.

MTFOptics

For industrial lens users, it is highly recommended to utilize the MTF (Modulation Transfer Function) graph more proactively for a better assessment of the lens's resolution, contrast performance, and consistent image quality. In this article, we will to go into more detail about MTF graphs and share how you can use them for better lens comparisons.

In general, MTF graphs with high values indicate good contrast and resolution, meaning that the optical system is capable of reproducing fine details in the image. On the other hand, a low MTF value at a specific spatial frequency indicates a loss of contrast and resolution at a specific level of detail.

Room 609, 6/F, Global Gateway Tower, No.63 Wing Hong Street, Cheung Sha Wan, Kowloon, Hong Kong +852-54993705 info@shanghai-optics.com

MTF lenstest

MTFimage quality

These graphs are not different MTFs, they just present the information in different ways. MTF Graph by Frequency indicate how much of the object's contrast is captured in the image as a function of spatial frequency. MTF Graph by Image Circle shows the value changes from the center of the lens's image circle to the outside at a specific lp/mm. This can be extremely useful, especially when large fields of view are in play. Now, let’s have a closer look at both types of MTF graphs.

Lens MTFdatabase

A dry objective is designed to work with the air medium between the specimen and the objective lens, while an immersion objective requires a liquid medium to occupy the space between the object and the front element of the objective for enabling a high NA and high resolution. Figure 4 shows the oil immersion objective, which can collect more light (i.e., have a higher NA) compared to a dry objective.

How do you find the right lens for a camera? What roles do resolution, sensor size, image circle, focal length, and lens size play in creating optimal image quality?

The MTF (Modulation Transfer Function) graph is useful not only for industrial users, but also for commercial users. The following illustration is an example of the MTF graph typically used by commercial customers, including those who engage in photography as a hobby or profession. These users typically utilize the MTF graph for a quick assessment of a lens's contrast and sharpness rather than for precise measurement.

Now, with more understanding of MTF graphs, you will be able to perform quality assessment and comparison across different manufacturers' products or compare different models from the same manufacturer.

Fixed focal lenses are a perfect fit for Basler cameras. We always offer the optimal lens for different vision requirements.

The optical aberration corrections determine the optical performance of an objective lens. According to the degrees of the aberration corrections, objective lenses are typically classified into five basic types: Achromat, Plan Achromat, Plan Fluorite (Plan Semi-Apochromat), Plan Apochromat, and Super Apochromat. Choosing an objective with a proper aberration correction level will help you build a microscopy system at a reasonable cost.

Performance across the Image Sensor: MTF graphs provide insights into a lens's performance across the entire image frame. This helps you select lenses that suit your requirement for consistent image quality from the center of the frame to the corners.

Lens MTFchart

In the context of resolution, higher values of lp/mm indicate a finer level of detail that the optical system can capture. Therefore, the number of lines that can be resolved becomes an important performance criterion for the optical system.

If you have 1 lp/mm, it means there's 1 pair of lines within a 1mm distance, with each line having a spacing of 0.5mm between them.

Lens MTFcomparison

It is often a challenge to pick the right lens, since a lens's performance is not easily determined by specific measurements like camera or computer specifications. For this reason, lens manufacturers created standard methods of measuring lens performance under a controlled environment and offer the testing results as MTF information for their products.

SO offers a wide range of objective designs, which provide various degrees of optical aberration corrections for supporting different needs, such as achromatic objectives (the cheaper objectives) for laboratory microscope applications and long working distance apochromats (expensive objectives) for biological and scientific research applications. We can help you choose or design a properly corrected objective lens for meeting your application requirements.

Infinity-corrected objectives are ideal for research-grade biomedical industrial applications especially when additional components (such as filters, dichroic mirrors, polarizers) are needed in the microscopy system. Adding optical plate components in the infinity space (shown in the Fig.2 labelled as “Parallel Optical Path) between the infinity-corrected objective and tube lens will not introduce spherical aberration, or change the objective’s working distance.

Usually the working distance (WD) refers the distance from the front lens element of the objective to the observed object when the object is in sharp focus. Objective lenses with long working distance are needed for many scientific research applications such as atom trapping and analyzing fluid samples that require putting an object in a chamber. The resolution of a microscopy system can be significantly affected if the observed object is not placed on the designed object plane, especially for an objective with high NA.

MTF (Modulation Transfer Function) is a measure of the ability of an optical system to transfer contrast from the object to the image. It describes the level of contrast a lens delivers at a specific resolution, e.g. Y % at X lp/mm.

Resolution Assessment: MTF graphs provide a quantitative measure of a lens's ability to reproduce fine details. Higher MTF values indicate better resolution, helping you select lenses that suit your needs for sharpness.

A typical MTF graph includes both Tangential Lines (T) and Sagittal Lines (S). The Tangential Line (T)—also known as Meridional—is displayed as solid lines showing the tangential MTF measurements, while the dotted lines show Sagittal MTF. Sagittal means that the test pattern lines are parallel to a line running from the center of the image to the corner, while tangential lines are perpendicular to that diagonal. Usually, the MTF value of the solid line is higher because the lens is generally manufactured based on the tangential direction.