In general, CCD technology has better MTF in NIR because of higher electric fields in the depletion layer, and deep-P diffusions for isolation of the columns in the Interline Transfer (IT) structure (compared with standard CMOS pixel design).  Click here for more information on CCD vs. CMOS in defense applications.

The Optical Transfer Function (OTF) is an important specification of lens performance. The OTF describes both the amplitude and the phase of a signal. In most cases the former, most times indicated as is Modulation Transfer Function (MTF), suffices. Modulation, as it is a measure of the modulation depth (contrast) of an optical signal. The MTF is for that reason sometimes also called Contrast Transfer Function (CTF).

But, if you want to “zoom in” on a certain part such as the wing, you need to increase the magnification, and move the specimen slide slightly, until the part you want to see is centered on the viewing field.

The field number of the microscope eyepiece is typically restricted by the size of its field diaphragm and its magnification, but this can be somewhat affected if there are any auxiliary lenses with their own magnification placed in between the objective and ocular lenses.

mtf光学

Besides the spatial frequency, the MTF strongly depends on the lens aperture, the light spectrum, the optical conjugation (e.g. magnification), field position, temperature and last but not least the level of optical correction of the lens itself (its quality so to say).

When it comes to the human eye, field of view is often referred to as the visual field, and it’s usually a horizontal arc measuring a little over 210 degrees, meaning, we can see everything in front of us as long as it is within this range.

The MTF can be measured in various ways, either directly or indirectly. In case of direct measurement, a sine- or square-wave signal is used. In case of indirect measurement, a step, line or point source, of in principle infinite steepness is used. From the spatial light distribution after passing the optics, which will no longer be infinite, one can obtain the MTF via Fourier transform.

MTF

From the example above, we can conclude that the microscope’s magnification conversely affects its field of view- the more times the specimen is magnified, the less of it you can see. Or at least, less in terms of area size, but more in terms of detail.

The first number, ending with an X, is the magnification, while the second number is the diameter. This is called the field of view number, or simply field number, and it’s expressed in millimeters.

The field of microscopy can be fun and exciting, as you get to explore many different possibilities in the world around you. But, to fully understand how microscopy works, it’s important to learn about its basic principles and underlying concepts.

Experienced camera manufacturers will have measured the MTF of the implemented sensor for the full wavelength range and can make the appropriate recommendations for your application.

The MTF of the image sensor should not be overlooked when comparing image sensor technologies, especially when working in the Near Infra Red (NIR) spectrum where there are big differences.  The MTF of the lens and sensor varies with wavelengths which can be characterized using MTF-curves. The problem is that sensor manufacturers do not always (read “hardly ever”) specify MTF or when they do, it is only at a wavelength where QE is at maximum value (is close to the best case scenario).  The MTF of all sensors at 550 nm is similar, but at longer wavelengths, the MTF of a sensor can be lower compared to other sensors.  For some sensors, for instance, the MTF is reduced by a factor of 2 or 3 at 850 nm.

Modulation transfer function

The microscopy field of view is what determines how much of the specimen we can see. Understanding how it works means you can set your expectations on what you can view under the microscope as you manipulate the magnification power.

Lower MTF limits the resolution of the system as a result of which small details of objects are no longer discernable, and this is wavelength dependent.  In some cases this can be overcome to some extent by decreasing the noise level, which additional to MTF also causes a reduction in contrast, by increasing the light level.

In the case of optical microscopes, such as light microscopes, the field of view is determined by the diameter of the opening of the eyepiece field diaphragm, which can either be in between the objective lens and the ocular lens, or before the two lenses.

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So while MTF is widely accepted as an important factor with lens selection, it is often not considered with the sensor/camera selection.  MTF provides an indication of the sharpness of the image or image quality and is determined by both the lens AND the sensor.

As mentioned above, the field of view is determined by the diameter of the diaphragm, and the magnification of the lenses. You can usually find these numbers imprinted on the side of the microscope’s eyepiece.

modulation transfer function中文

If this is the case, the compounded magnification of the objective and auxiliary lens should be calculated by multiplying the two together, and this total magnification is what should be used in calculating the field size.

Alternatively, if the total magnification is only 20x, then the field size becomes 1 millimeter, which means you can see a bigger portion of the specimen. But, you can’t see as many minute details, since the magnification is low.

The microscopy field of view is the total visible area of the specimen plane, which is determined by the field number or the diameter of the diaphragm, and the magnification of the lens. This viewing field size is conversely affected by the magnification level of the microscope.

In a microscope, the microscopy field of view is the diameter of the viewing field measured at an intermediate plane of angle. To put it simply, it’s the diameter of the circular area you see when you look through the eyepiece of the microscope.

Field of view is defined as the extent of the observable world at any given point in time. It’s the range of visibility, typically measured as an angle, that the eye or an optical instrument is capable of.

The maximum field diameter typically falls within a range of 18 to 28 millimeters (or more), depending on how advanced the objective lens is, such as whether the lens is a special type of a flat field objective.

To help you visualize, let’s say you are looking at an insect specimen. If you want to be able to see the entire insect under the microscope, you need to use a low power lens.

LensMTF

SensorMTF

And, it’s also dependent on the magnification of the objective lens, or more accurately, the total compounded magnification of the objective and ocular lenses.

See the websites https://www.edmundoptics.eu/ and https://www.trioptics.com/ that provide a more detailed explanation of the MTF.

To give you an idea, an ocular lens or eyepiece with a magnification of 5x normally has a 20 millimeter field number, while a 10x magnification eyepiece has a somewhat smaller field number of 16 to 18 millimeters. The difference becomes noticeable as the magnification increases to a hundred or more.

mtf群体

“The modulation transfer function is a measure of the transfer of modulation (or contrast) from the subject to the image. In other words, it measures how faithfully the lens reproduces (or transfers) detail from the object to the image produced by the lens.“

mtf是什么

For example, if the maximum field diameter of the diaphragm is 20 millimeters, and the microscope has two lenses with a magnification each of 20x and 10x respectively, we can calculate the viewing field size as 20 millimeters divided by 200, equating to 0.1 millimeter.

Talking about the MTF of optics, say a lens, it is the reduction in modulation depth, starting with a modulation depth of hundred percent, due to that optics. The MTF of an optic is usually depicted versus the spatial frequency. Generally speaking the MTF drops with increasing frequency, which frequency is indicated as cycles or line pairs per mm (square-wave signal).

This includes having an idea about how light refraction works, understanding the concepts of magnification and resolution, and many other principles, including knowing what the field of view is, and how to calculate it.

Based on the definition of the microscopy field of view above, we can then infer that the viewing field size of the specimen plane as the field number divided by the objective lens’ magnification, or Field size = field number/ objective magnification.

While the best way to increase the field of view is by lowering the magnification of the microscope, this can be inefficient and counter-productive. This is why more sophisticated lenses have been developed to offer a wider viewing plane without sacrificing magnification.

Different animals and optical instruments also have different fields of view. Below is an explanation of microscopy field of view, how it’s calculated, and the different factors that affect it.

This is why the field of view of a simple microscope is as big as a few centimeters wide, while the field size of a transmission electron microscope is only a single nanometer to a few picometers.