The reflection, absorption and emission features form spectra that are closely related to the molecular composition of the material being examined. Table 1. is well known and it highlights the spectral bands in which each of the most common molecules has its electromagnetic resonance overtone.

So far, we’ve been discussing lens distortion which is an optical flaw. However, I absolutely have to mention fisheye lenses. They intentionally distort an image because they use optical distortion to simply fit as wide of a view as possible into the image frame. In other words, if you let straight lines curve, you can have a diagonal (corner-to-corner) angle of view of fisheye lenses that is often a complete 180 degrees, or sometimes more.

In fact, fisheye lenses often do a better job than corrected “rectilinear” ultra-wide lenses at rendering a subject exactly the same in an extreme corner of the frame and in the dead-center of the frame. Ordinary ultra-wide lenses may keep straight lines straight. However, they still optically warp other subjects in order to do so.

Both multispectral and hyperspectral imaging are techniques used to capture and analyze the electromagnetic spectrum for various research, industry, and remote sensing applications. Choosing between these two complementary technologies comes down to the application requirements and the level of data already existing.

This is a very simple trick that you can easily demonstrate yourself: using a telephoto lens, align a distant subject with an even more distant, larger subject. If you get the distances right, you can appear to make a smaller-but-closer subject appear equal in size, or even larger than, a bigger-but-farther subject.

It may sound advanced and complicated at first, but it’s really quite simple! Let’s dive in and break it down. Ultimately, you will even learn how to use certain types of distortion creatively in your photography!

*The full width half maximum defines the spectral resolution of a system, more precisely, in this context, its ability to separate spectral peaks from each other.

First and foremost, I want to answer this most basic question: what is lens distortion? Generally speaking, when we use this term we are referring to distortion that literally distorts what a “perfect” image ought to look like, especially for a certain focal length and camera angle.

Hyperspectral vs multispectral

Lastly, one aspect to consider when choosing between hyperspectral and multispectral cameras is the price. Typically hyperspectral cameras are more expensive and require more processing power than multispectral cameras. However, this is not always the case, especially if the multispectral camera needs to be custom-made with a relatively large number of bands.

Now that we have learned about virtually every form of lens distortion that photographers will encounter, let’s talk about how to “fix” undesirable distortion. Stay tuned, because then we are going to talk about how to use lens distortion creatively, too.

Multispectralimagingsatellites

Lens compression is basically the same thing: using your perspective to create an image where two very different-sized subjects appear more similar in size than they naturally would.

A standard definition states that hyperspectral cameras have more than 100 bands, whereas multispectral ones have fewer. But this definition does not consider the width of the spectral range or the sampling. That means that if a camera covers the spectral range 400 – 600 nm with 50 bands, it would not be hyperspectral, whereas if it covered 400 – 800 nm with the same sampling (meaning this time 100 bands), it would be hyperspectral.

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Difference between multispectral and hyperspectral remote sensing

The main difference between multispectral and hyperspectral cameras is the number of bands they record and how narrow the bands are (i.e., the spectral resolution).

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For most of the multispectral cameras on the market, the spectral range is limited to 400 – 1000 nm and the typical number of bands is between 4 – 5. Those are crucial limitations for many applications.

Last but not least, you have numerous options for manually correcting lens distortion. First and foremost, if your lens does have a correction profile available, you can manually load it into your RAW conversion software. This is common for all-manual lenses that have no electronic contact with the camera.

Also, you can use brute force: most RAW photo editing software has a slider that allows you to manually correct barrel or pincushion distortion, though not mustache distortion.

In other words, because the number of spectral channels captured is limited to 28, the spectral information required to tell the difference between the almond and shell is lost.

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Be careful, though. An in-your-face view of an actual face can create distortion that looks really weird and sometimes downright unflattering.

In addition, some pre-processing methods are not suitable for multispectral data. For instance, derivative or smoother such as Savitzky-Golay, require continuous spectra to perform well, which are not provided by multispectral sensors.

In other words, your perspective on the scene is distorting the scale of a 6-inch subject and a 6,000-foot subject, making them appear the same size.

If your camera and lens don’t have this option, don’t worry. Your next line of defense is your raw processing software. Most modern autofocus lenses will communicate their make and model information to the camera, so you’ll see it in your image’s EXIF data. RAW conversion software such as Adobe Lightroom can read this Exif data and, if you turn the option on, will automatically apply a lens correction profile.

Also, and most importantly, every photographer should learn how to use both perspective and compression to their creative advantage. Try zooming out to ultra-wide focal lengths, and get really up-close and personal with your subjects.

Every photographer has probably heard about distortion, or lens distortion. However, most have never actually learned what it is, and how it affects their photography.

Thankfully, today and beyond, optical distortions such as barrel, pincushion, and even the challenging mustache distortion, can all be corrected easily with most photo editing software.

Mustache distortion is most common at ultra-wide focal lengths, on both prime and zoom lenses. It is especially common on affordable, all-manual, ultra-wide lenses such as 14mm primes.

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Considering the previous point, we compared almonds and shells with Specim FX17 camera data. We covered the spectral range of 900 – 1700 nm in the first data set with 224 bands. In the second data set, we used only 28 binned bands (i.e., merging consecutive spectral bands) to simulate a multispectral camera.

Figure 2. Specim FX17 spectrum of almond with 224 and 28 bands. Purple circles highlight the spectral differences due to the presence of oil in almonds but not in the shell.

We find this discrepancy irrelevant and prefer to speak about spectral resolution (FWHM, Full Width Half Maximum*), highlighting the ability of a camera to separate two consecutive spectral peaks from each other.

Simply put, the most common types of lens distortion are usually just the undesirable warping of lines. Thankfully, however, nowadays they are almost always removed automatically. When you understand all the different types of distortion, though, the basic concepts can become powerful creative tools.

Multispectralimagingin agriculture

The best option is to correct lens distortion before you even get to see it: in-camera corrections. Most modern mirrorless camera systems have electronic communication with each lens, and the lens actually tells the camera exactly how much distortion it has, and how to un-warp any curved lines so that they are perfectly straight. If you turn on Distortion correction in the menu of your camera, you’ll hardly ever see it.

Say you’re photographing any sort of architecture or structure that has a lot of straight lines. They ought to appear straight in your photograph. However, because of the imperfections in the optics of the lens, the lines appear curved instead.

Our article about Hyperspectral imaging vs. RGB cameras shows that RGB cameras do not perform well in sorting nuts and pistachios. The Specim FX10 provided better results, and the Specim FX17 had the most accurate sorting capability.

The hyperspectral camera also offers more flexibility as the user can later upgrade the machine to sort new contaminants or materials. With the Specim FX cameras, users can freely select relevant bands. In practice, the Specim FX hyperspectral cameras can transform into multispectral cameras, whereas a multispectral camera could never become hyperspectral.

Barrel distortion is when any straight line that is parallel to the edge of your frame is curved outward. If your composition has straight lines around all four edges, they would appear like a barrel — hence the name. Barrel distortion is most common on normal and medium-wide focal length lenses, especially zoom lenses.

Most of the time, you’re going to want to fix things like barrel, pincushion, and mustache distortion, because they are simply a distraction that hinders you from capturing the real world accurately.

Multi spectral imagingcamera

Although fisheye lenses radically warp straight lines around the edges of an image frame, any straight line crossing through the dead-center of the frame will still be rendered perfectly straight.

Multispectral images in Remote sensing

Hyperspectral imaging involves capturing and analyzing data from a large number of narrow, contiguous bands across the electromagnetic spectrum, resulting in a high-resolution spectrum for each pixel in the image. As a result, a hyperspectral camera provides smooth spectra. The spectra provided by multispectral cameras are more like stairs or saw teeth without the ability to depict acute spectral signatures.

As the table shows, the spectral range of 700 – 2500 nm is necessary for many applications. Especially for those related to food quality assessment and plastics sorting, the spectral range of 1100 – 1700 nm is mandatory. Multispectral cameras limited to 400 – 1000 nm are not applicable for those applications.

For applications that require more spectral bands and higher spectral resolution than multispectral imaging can provide, a hyperspectral camera is the natural solution.

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To illustrate the advantages of a hyperspectral camera compared to a multispectral one, we studied the sorting of shells among almonds. It is a typical application that requires highly accurate identification between materials that look very similar.

Hyperspectral imaging provides more detailed data than multispectral imaging, allowing more specific analysis and accurate identification of materials and substances. Multispectral imaging may not be able to distinguish between closely related materials due to the limited spectral resolution.

Multispectralimagingarchaeology

Spectral channels are distinct wavelength ranges within the electromagnetic spectrum that sensors in multispectral or hyperspectral imaging systems capture. Each channel represents a specific band of wavelengths, and the collection of these channels helps to identify objects or materials based on their unique spectral signatures.

Pincushion distortion is when the opposite is the case. Instead of bending outward, a straight line at the edge of the frame is curved inward. This type of lens distortion is common at telephoto focal lengths. Modern lenses have very little. It’s more common on older lenses that cross from wide-angle to telephoto focal lengths.

For applications where the entire spectral range is unnecessary, custom-made multispectral cameras with well-chosen bands can perform equally well as hyperspectral cameras. However, this requires the user to know the selected number of spectral bands that are necessary for the inspection or analysis. If the user does not know the spectral requirements of an application or they are very complex, a hyperspectral camera provides an ideal solution to gather the data for analysis.

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In this article, we will explore all the different types of lens distortion. We’ll go over the various optical flaws in a lens, as well as types of natural distortion that our own eyes experience.

As mentioned above, we built two models to illustrate these points related to the ability to depict acute spectral features. The hyperspectral model covering 224 bands is more accurate than the multispectral model covering only 28 bands. With the hyperspectral data, the edge effect disappears, and we can classify even the smallest pieces of shell correctly.

In this common category of lens distortion, there are three main types of optical issues that lenses experience. Barrel distortion, pincushion distortion, and mustache distortion.

We selected 28 spectral bands in this study to simulate a multispectral camera. However, typical multispectral cameras can have notably fewer spectral bands, making their ability to depict fine spectral features even lower.

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This is what people are usually referring to when they talk about lens distortion. It’s when a line that is clearly straight, in reality, becomes curved when photographed with a “flawed” lens.

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Have you ever looked at a photo and thought, “Why does that tiny little barrel cactus appear as large as that distant mountain?” It was simply because you physically moved your camera very close to a small subject, while an enormous subject was far off in the distance.

Figure 3. RGB, multispectral (28 bands), and hyperspectral (224 bands) images of almonds and shells (green is almond and blue is shell).

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Multispectralimagingskin

Simply put, the term distortion is broadly used. Sometimes people use it to describe any time reality appears weird, or inconsistent with how we think it ought to.

This type of distortion is difficult to correct because it actually features both pincushion distortion and barrel distortion. It’s a rather diabolical and visually distracting phenomenon.

The best part is that you don’t need an expensive, exotic lens to avoid unwanted lens distortion. You can use whichever camera and lens you already own to get started.

Most of the time, your eyes will see such things and your brain won’t even notice because you’re naturally accustomed to some level of perspective distortion. Other times, when a viewer’s angle exaggerates this perspective distortion, it can really catch a viewer’s eye, or even be unsettling.

As can be seen in Figures 1. and 2., spectra related to the 224 bands dataset are much smoother than those depicted with only 28 bands. From the hypersepctral data, we can also pick the small but crucial spectral differences that enable us to separate the almonds from the shells.

Lens distortion comes in all different shapes and sizes — literally. Most photographers use the term rather loosely, so we hope this article cleared everything up for you.

That just about covers it in terms of lens distortion. However, this article would be incomplete without mentioning the types of natural distortion that are always visible in reality.

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