During the analysis of the structure of chemical substances, the process determines the structure of the molecules in a chemical substance. This process, important in chemistry and pharmaceuticals, determines the polarisation of the Raman scattered light. If this light is completely polarised, the molecules are isotropically polarised; on the other hand if the polarisation of the scattered light is incomplete, the molecules are anisotropically polarised. The exact degree of depolarisation is determined by placing various polarisation filters in the beam path.

Using Raman spectroscopy, it is possible to draw conclusions about the following material characteristics, among others:

Zeiss has a pretty good paper on how to read MTF charts. It is rather detailed and extensive, but if you are interested in fully understanding how an MTF represents a lenses quality (and how accurate the MTF may be), it is an excellent read.

On a Modulation Transfer Function, the horizontal axis denotes the distance from the center of the lens, so the zero point on the left is the performance of the lens at center, and the far distance on the right denotes corner performance. Note also, this way you can see the difference in corner performance between use on a crop and on a full frame sensor.

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This is the Raman spectrum for a particle of polypropylene (red) compared to a reference from a spectral database (blue). The identification is unambiguous.

However, keep in mind that these particular conventions only apply to Canon MTF charts. Other lens makers may have different ways to denote these things, and may or may not show all the same information. And, as Reichmann says in that article:

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For years we have used Raman spectroscopy reliably and routinely to analyse samples for our customers. For this reason, we are also able to obtain exact measurement results in challenging conditions. The spectrometers and databases we use are from renowned brand-name manufacturers and as such guarantee not only precise results, but also maximum protection for the material.

Light incident on a non-transparent medium is predominantly scattered without changing its wavelength. This effect is termed Rayleigh scattering. A small part of this visible light is, however, scattered in a different wavelength. This phenomenon is called Raman scattering or the Raman effect, after the Indian physicist and Nobel laureate C. V. Raman. But what exactly is Raman scattering?

The physics of visible light itself, and how different wavelengths are refracted differently by the same lens elements is what leads to less than perfect MTF charts, not the imperfections of a manufacturing process.

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Depending on the specific characteristics of the material to be analysed (e.g. the area of the excitation wavelength), Raman spectroscopy also has disadvantages. In particular, these include:

The vertical axis denotes the amount of contrast, on a scale from 0 to 1 (I.e., you can think of it as the scale of 0% to 100). So, air, for example, would give you a straight horizontal line at 1. The flatter and nearer the top of the chart the line is, the better the overall performance.

The applications of Raman spectroscopy in medicine are very varied. In this way, for example, the chemical composition of kidney stones can be analysed immediately after their removal. Then the patient can be given tailored recommendations for the prevention of new stones without complex analyses in specialist laboratories. It is also possible to analyse a living biological sample using its Raman spectrum. Neither of these methods has become established as standard yet.

Incidentally, this is why the datasheets of films always also gave the resolving power for the low contrast of 1:1.6. The resolution figures for the contrast of 1:1000 can only be measured using contact exposure. For the finest structures (i.e. very high spatial frequencies), no lens in the world is capable of producing a contrast of ten aperture stops. Estimating the amount of information of film images based on this higher resolution value is thus too optimistic.

If a material is treated thermally or mechanically, its internal stress can change. If you now compare the Raman spectra of a sample of the treated and untreated material, the changes in the stress can be detected in the form of frequency shifts. Higher frequencies are indicative of a compressive stress, while lower frequencies indicate an increase in the tensile stress.

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The thick lines are measurements taken at 10 lines per millimeter (low resolution). The higher up the chart these are, the better the contrast of the lens.

The basic prerequisite for Raman spectroscopy is a monochromatic light source. Because the light scattered during Raman scattering is of relatively low intensity, the light source must also have a very high radiation intensity. Lasers have both characteristics, lasers are available on the market with different fixed frequencies or as tunable devices.

At Quality Analysis we offer a broad spectrum of measuring and analytical services. These services also include the use of Raman spectroscopy to identify inorganic and organic samples as well as their composition and crystal orientation. Having spectroscopic analysis undertaken by us offers you a whole string of technical and commercial advantages.

Compared to other spectroscopic methods, for example FTIR spectroscopy, Raman spectroscopy offers a few advantages that result above all from the usage of different lasers in the visible to near IR range for a very wide range of materials. Specifically, these include:

— the higher up the chart the 10 LP/mm line is (the thick lines), the higher the contrast reproduction capability of the lens will be.

... be aware that an MTF chart doesn’t tell us everything that there is to know about a lens. Important variables such as vignetting, linear distortions of various sorts, and resistance to flare are among the things not measured.

Since the invention of more powerful lasers that are at the same time less aggressive on the material, Raman spectroscopy has become established in almost all areas of chemical analytics. Thanks to the high information density, chemicals can not only be reliably identified, pure material concentrations can also be assessed in complex mixtures. Raman spectroscopy offers many other possible applications.

Another excellent resource for how to read MTF charts can be found in the last parts of Canon's book "Lens Works". The part "Optical Terminology and MTF Characteristics" provides an EXCELLENT overview of lens types and capabilities, and provides a wonderful, visual review of MTF charts for many Canon lenses.

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The black lines indicate performance wide open. The blue lines indicate performance stopped down to f/8 (I think Nikon doesn't bother to show this on their MTFs).

Solid lines are meridonial (i.e., the test chart lines are slanted 45° from upper left to lower right). And the dashed lines are sagittal (the lines are slanted from upper right to lower left). They evaluate astigmatism and field curvature, and the closer these two lines are to each other, the smoother the bokeh will tend to be.

For the analysis of the reflected, scattered light, first all the light at the excitation wavelength (that is the Rayleigh scattering) must be removed using an optical filter. The remaining scattered light (the Raman scattering) is guided to an optical grid and split into its individual wavelengths. A CCD sensor produces a spectrum from this light.

The thin ones at 30 lines per millimeter (high resolution). The higher up the chart these are, the better the perceived sharpness of the lens.

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— generally speaking a lens whose thick lines (10 LP/mm) are above .8 on the chart should be regarded as having excellent image quality. Above .6 is regarded as "satisfactory". Below .6 is, well, below.

That something special about Quality Analysis: in our organisation you will find the right experts and the right analysis methods for all materials and every requirement.

— keep in mind that the black lines show the lens wide open while the blue lines show the lens stopped down to f/8, so the closer these sets of lines are to each other the better the performance of the lens when used wide open. The very best lenses will have the black and the blue lines close together.

Raman scattering describes the interaction of the photons (particles of light) with the molecules of the medium. The photons create molecular vibration in the sample. During this process the photons lose energy. Because the wavelength of the light is dependent on its energy, the wavelength is reduced by the loss of energy, in other words: the frequency changes compared to that of the incident light. The frequencies produced by Raman scattering are dependent on the material on which the light is incident. The frequency differences are dependent on various energies in the material such as the rotation, spin-flip and vibration processes. Part of this energy is transferred from the material to the light and changes the frequency of the light. This is the so-called Raman effect.

The source we all quote on how to read an MTF chart is Michael Reichmann's article at Luminous Landscape. Most of the following information is cribbed from that article.

What none of the existing answers mention that is critically important is that MTF charts supplied by manufacturers are not measurements of actual lenses. Rather, they are the theoretical limits of a perfectly executed example of the lens design.

The Raman spectrum is characterised by the bands mentioned. These areas of higher Raman intensity are characteristic for every substance. In this way the spectrogram of an unknown substance can be compared to samples from a spectral database. If the bands are in the same places, the classification is unambiguous, as in our example for the comparison of a sample to the spectrum for polypropylene.

The Raman spectrum of each substance has certain areas with higher and lower areas of Raman intensity (so-called bands); these areas produce a characteristic image. This image can be compared to known patterns in a spectral library and the type of sample and its characteristics determined beyond doubt.

Raman spectroscopy is suitable for the analysis of a large number of substances. It is possible to analyse liquids, gases and solids.

Here is a sample MTF Chart for the 16-35 f2.8 L II (one of my favorite lenses for walkabout photography). What do the various lines mean? What are the axes?

However, these disadvantages are minimised to a large extent by using modern lasers such that they are increasingly irrelevant for the practical use of Raman spectroscopy.

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In our modern laboratory, we use Raman spectroscopy for residual dirt analysis for the assessment of fibres, plastics or salts, for the verification and the identification of filmic contamination as well as particulate contamination and in chemical analytics, in particular in plastics analytics. There the method is used, e.g. for the identification of deposits, residues, inclusions, media, substances, additives and materials (plastics).

As part of how Canon (and other lens makers) give technical information about their lenses, they supply an MTF (Modulation Transfer Function) chart. How do I read and interpret what the chart is telling me?

Raman spectroscopy is a method for the analysis of the inelastic scattering of light at molecules or solids and is used for the analysis of material characteristics, among other aspects.

— the higher up the chart the 30 LP/mm line is (the thin lines), the higher the resolving power and thus subjective sharpness of the lens will be.

A very interesting little facet of the article denotes three important properties of MTF, which lead to an intriguing conclusion about the maximum contrast range of a camera lens. I found this to be interesting not only in the context of understanding lens MTF, but also as an important factor in the camera film/sensor dynamic range argument.