An obvious byproduct of the correction is that the detail that was previously visible at the edges of the image has now gone out of the image canvas. To address this, we can switch back to the Settings tab of the Undistort filter settings. From there, we can increase the size of the canvas to bring back the detail. This will also increase the resolution of the processed image.

The best protection filters don't need to do anything at all except take the bangs, knocks, and scratches that might otherwise wreck a very expensive lens!

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Whether or not this warrants consideration if you’re shooting with a digital camera – as opposed to film – is up for debate, as we noted in our introduction, but at least here we have a ready-made and inexpensive disc to protect our expensive lenses into the bargain.

Protective filters can serve other purposes too. Some are simply clear pieces of glass that have no other function, but you can also get a UV filter, which is designed to block out unwanted ultraviolet light to reduce the level of haze in an image. This is something of a holdover from the days of film, and is considerably less necessary when using digital sensors, but can still come in handy and certainly doesn't hurt.

When using Undistort and Correct Fisheye, we need to take into account the question that is being asked of us. For sure, we will need to undistort imagery if we are commenting on or comparing shapes and sizes of subjects or objects depicted in the scene.

This precision-engineered filter uses high-quality materials, including a titanium frame and Water White glass. Multiple coatings are applied to increase strength and durability, with a 10-year warranty as reassurance. It’s also scratch-resistant and easy to clean.

Crucially, during our testing we were unable to detect any color cast or loss of contrast in the shots we took with the Digital HT Ultra Clear attached, even when we zoomed in to 100% in our image editor. We also appreciated the ability to attach other Tiffen filters, such as a polarizer.

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Notice how the distortion is corrected. But the majority of the original image content is no longer visible inside the canvas. This is because of the greater amount of distortion inherent in fisheye lenses. You can still register Lines with the Curve tool, just as we did previously with the Undistort filter, by clicking on the Line tabs.

Gavin has over 30 years’ experience of writing about photography and television. He is currently the editor of British Photographic Industry News, and previously served as editor of Which Digital Camera and deputy editor of Total Digital Photography.

However, an issue would arise due to the higher amount of distortion. This would result in a great part of the image extending beyond the original image canvas. So, to make the most of all detail present in the original image, we will use the Correct Fisheye filter, instead. This can also be found in the Edit group of filters. The advantage of using Correct Fisheye is that we will be able to change the virtual view of the camera to focus on a particular area of interest.

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If the presence of anti-reflective and water-repellent coatings is a deal-breaker for you, then you might want to consider investing a bit more in a premium UV filter. However, if you’re not particularly concerned about these coatings, or if you’re operating on a tight budget, then the Kenko AIR II UV filter is an excellent option.

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There are three types we cover in this guide: clear filters, which ideally make no difference optically, but protect the lens’s front element; UV filters, which add the ability to block the transmission of ultraviolet light; and skylight filters, which do the same but with blue light.

The Urth UV Filter Plus+ is a premium product, capable of cutting out up to 99.6% of ultraviolet light. Its Schott B270 glass is protected by 30 layers of nano-coating, meaning it's easy to clean and is well protected against water, dirt and oils, while also providing top-notch light transmission. The size range, from 37mm to 127mm, also means you're all but guaranteed to find a size that fits your lens.

Conversely, in Pincushion distortion, curved lines will diverge from the central aspect of the image. This type of distortion is less commonly observed and it applies to convex lenses.

When we test filters, we look at both optical performance and general build quality – how robust the filter feels, how easy it is to use and how securely it attaches to the lens. We use a combination of real-world and lens testing to inform our comments in reviews and buying guides. For filters, we test light transmission levels, colour neutrality (i.e. if the filter introduces a cool or warm cast) and the impact a filter has on image quality.

Due to this shape, we may encounter an optical aberration when capturing a wide area. This can cause straight lines in the real world to appear curved in the resulting image.

Correcting distortion in Amped FIVE requires the use of the Undistort filter. It can be found in the Edit group of filters. We need to register the amount of distortion in an image. This can be done by drawing a pattern on a feature, which we know is completely linear in the scene, but that appears curved in the image.

Lens distortion can appear in the form of two defined curvatures. In Barrel distortion, curved lines will converge toward the central aspect of the image, as shown below. This is the most common type of lens distortion we will encounter. It applies to concave lenses.

Another bonus is that since the filter features no additional coloration or contrast, this Tiffen option can be paired with other filters if so desired. It’s an inexpensive option that many may consider good value.

A general means of protecting your lens from filter stalwart Tiffen, this one comes with a ten-year warranty and is designed to reduce the blue-ish cast of daylight, while helping to absorb ultraviolet light. Tiffen adds that this filter, in particular, is useful if you’re taking photographs at high altitudes or around bodies of water.

Simply put, they're one of the most affordable ways to give your lens an extra layer of protection from damage. Lenses are precious, fragile, and often expensive – and if something has to get cracked against the ground, wouldn't you rather it was a $20 filter than the front element of a $1,000 lens? Yep.

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Let’s apply the filter to our chain. This time we will drag the Focal Length slider in the filter settings towards the value “0” to straighten the lines in the scene. This is a similar process to what we previously did with the Undistort filter.

There is also an Urth Magnetic UV Plus+ filter - but we were not very impressed with this in our tests, as the magnetic attachment left the filter rattling, and there was a tendency for the filter to drop off.

Before clicking the Apply button, you can register other lines (for a maximum of three), using other references in the image. This will result in an even more accurate correction.

Using the parking lot image at the top of this article as an example, let’s put the Undistort filter in our chain. Switch to the Line 1 tab and notice how your Curve tool is now activated in the Tools panel.

If you own new or relatively modern lenses, the coatings applied to these lenses are generally sufficient to stop UV light compromising image quality. But there are still situations where the effect strong sunlight can generate haze and affect color balance.

The imagery must be corrected when performing measurements, such as the speed of a vehicle or the height of a suspect, for example. Although it is true that any correction using these filters will never be 100% perfect, measurements will be severely compromised if performed on distorted images.

The selection of sizes reaches from 37 to 127mm, and the quality of the filter blocks UV rays while maintaining light transmission. Read more below…

The image will be corrected and the curved lines should now have a straight appearance. Notice how the appearance of the gray hatchback vehicle has considerably changed, despite its central position within the image.

Barreldistortion correction

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This filter is available in a broad range of thread sizes, enabling you to maintain light transmission and suppress reflections while protecting precious and expensive lenses from dust, moisture, and scratches. Here we also get multi-coating to help protect against lens flare and ghosting, while Hoya claims it can reduce the excessive bluish-ness that can occur in outdoor color photography – especially with a clear blue sky.

Imagine a vehicle going through the middle line of these distorted images, as illustrated below. Time and spatial measurements will be affected quite considerably by the distortion. Furthermore, the optical distortion also affects the perspective of an image. Lines that are parallel in the real world would not converge anymore to a vanishing point. For these reasons, we must correct this issue before we perform any measurements on images.

K&F manufactures a range of different UV filters, and the Nano-X is one of its premium options, using Japanese optical glass and an aluminum frame. Nano-X offers a 28-layer multicoating, which means this model can boast that it is scratch-resistant. This is a very affordable option, available in an impressive range of sizes right up to 112mm, that performed really well in our field tests

Switch to the Post Projection tab. Move the three sliders in this tab to reposition the view of the camera in a virtual three-dimensional environment, as shown below.

The Quick List ↩ 1. Best premium UV 2. Best budget UV 3. Best mid-price UV 4. Best UV for size choice 5. Best skylight 6. Best premium clear 7. Best mid-price clear 8. Best for a bargain How to choose How we test

Once the filter has obtained this information, it can then apply a mathematical process. This will interpolate pixels in the whole image to re-establish the correct position and shape of elements depicting the scene. Simple, right? Let’s give it a go.

That depends on the filter. The minimum should be protective coatings to guard against scratches; some filters use strengthened glass that is able to withstand tough knocks without shattering.

Using high-grade materials that include a brass frame and Schott optical glass, this offers protection while staying optically neutral. Read more below…

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Owners of film cameras or older lenses could benefit from this filter, which reduces the amount of blue light in landscape and outdoor shots. Read more below…

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A highly effective UV-blocking filter that has the added bonus of shatter-resistant glass to protect the front of your lens against knocks. Read more below…

As well appreciating its optical qualities during our testing, we also liked the knurled front edge to aid grip during fitting and removal.

While UV filters could be thought of as cheap, disposable items, at least one  company is choosing to think of them from a more sustainable perspective. Urth makes a variety of premium, high-grade filters, and its protective UV filter is a great choice for those who want the UV component of the filter as much as the protective aspect of it.

This Hoya UV filter achieves the delicate balance of blocking UV light effectively for those cases when it’s needed, while not causing any additional ghosting or flare by its presence. Crucially, it’s also extremely strong, using shatter-resistant glass to stay intact even if it takes a significant enough blow to damage it.

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The Kenko AIR II UV might be one of the cheapest protection filters you are going to find from a well-known brand. It doesn't have the prestige of its Japanese filters, but does an adequate job of protecting the front of your lens.

It is not possible to draw one single curve. However, this can be done by registering some points in the image. To replicate the curvature of your reference object accurately, simply left-click in the viewer to mark these points. In this case, the water pipe. You can add as many points as you like to replicate this curve the best you can. You can also right-click on any point you’ve drawn to edit its position. When done, simply right-click and click Apply on the filter settings.

Reading time: 30 min Codecs and formats form the bulk of our source material, from both proprietary and standard files.Our Video Formats and Conversion blog series continues with a

You could also consider using one of the best polarizing filters as a protective filter, though this will cost more than a clear or UV filter.

With his wealth of knowledge, Gavin is well placed to recognize great camera deals and recommend the best products in Digital Camera World’s buying guides. He also writes on a number of specialist subjects including binoculars and monoculars, spotting scopes, microscopes, trail cameras, action cameras, body cameras, filters and cameras straps.

By doing so, however, we are also bringing back areas of the original image that are now unnaturally corrected. Look at the porch entrance at the bottom right of the image with increased canvas. That’s surely not what it would look like in real life! But the detail nearer to the center of the image has had its realistic appearance re-established.

The quality build of this filter uses an aluminum frame and Japanese optical glass, plus no fewer than 28 layers of protective coatings. Read more below…

With a 10-year warranty, this filter combats ultraviolet light and reduces daylight color casts that can affect long-distance scenes. Read more below…

Camera lenses are curved in shape for a specific reason: they have to ingest a large and wide quantity of light from the outside world, a much bigger area than the shape of the lens itself. The use of transparent curved glass surfaces will permit just that.

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When you purchase the Kenko AIR II, you’re getting exceptional value for your money. Our tests revealed that its image quality is quite impressive, and it offers substantial protection for your costly lenses without requiring a significant investment. This makes it an excellent choice for those who want to safeguard their equipment without breaking the bank.

Another alternative is a skylight filter; these also date back to the film era and were originally used to warm up images with a pink, orange, or magenta color cast, to prevent them from coming out too blue. Significantly less necessary in the era of custom white balance presets and RAW photo editing, skylight filters now also make great protective filters.

We were impressed by this clear flter’s ability to leave color and contrast untouched while offering durability and scratch protection. Read more below…

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Some camera lenses can capture an even wider range of light than conventional cameras. Because of the wide angle of view which resembles the vision of a fish, these types of lenses are amicably referred to as “fisheye”. Their advantage is the ability to capture events on the side of the recording camera. However, as we would expect, the wider the range, the more distorted the image. Below is an example of this type of lens.

The Kenko AIR II is a UV filter that embodies simplicity and functionality. It doesn’t come with an array of fancy features, but it does its job well. We would have been thrilled to see it equipped with anti-reflective and water-repellent coatings, but given its incredibly affordable price point, we can’t really hold this against it.

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If you wish, you can also further adjust the distortion manually by moving the Distortion slider in the filter settings. This slider also allows correction without the registration of the curved lines which we previously performed. You may wish to bypass that workflow and simply use this setting in conjunction with a visual aid. For example, in the viewer, you can easily draw a straight line using the ruler tool.

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The best protection filters may have no optical effect at all – but it's cheaper to replace a cracked filter than a cracked lens!

Have a look at the image below. The water pipe and parking lines are straight lines in the scene but have a clearly defined curvature in the image. This is what we call Barrel distortion.

In the image above, we observe that not only the appearance but also the size of lines gradually change due to lens distortion. As we move away from the center of the image, the lines progressively increase in size.

Gavin has over 30 years of experience writing about photography and television. He is currently the editor of British Photographic Industry News, and previously served as editor of Which Digital Camera and deputy editor of Total Digital Photography.

To deal with fisheye distortion, we could, of course, use the Undistort filter. Technically, this is the same aberration that we encountered previously.

B+W makes two different protection filters in its T-Pro range: this Clear version, which is optically neutral, and a UV-Haze option that can help you improve the clarity of some landscape images, by removing the blue cast on the horizon. The filters are made from brass and with Schott glass, promising great performance in the field. This is an established line of filters - and has an impressive range of sizes on offer, so can be a particularly useful option for XS and XXL filter mounts.

G’day folks and thank you for coming back to our blog and checking out another article of our “Learn and solve it with Amped FIVE”. Today we are going to talk about issues deriving from the curved shape of camera lenses and how to correct optical distortion. If you analyze shapes and sizes of subjects and objects on video or, indeed, if you conduct measurements on surveillance cameras, then you will want to read on!

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