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

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.

KBBFcrystal

Cleaning procedure: 1. Place a clean tissue on the surface to be cleaned 2. Apply a small drop of: 2.1. (A) Mixture to the tissue 2.2. (B) Pure Isopropyl Alcohol (IPA) to the tissue 3. Drag the tissue in a single, continuous direction across the surface 4. When using lens tissue, avoid reusing it Exercise extreme caution when cleaning thin or small BBO crystals, as the material is delicate and easily damaged.

Handling Never handle BBO crystals with bare hands! 3photon advises using forceps or plastic tweezers for handling optical components. If these are not available, ensure you wear powder-free gloves or finger cots.

Beta-barium-borate (BBO) is a nonlinear optical (NLO) crystal with a unique set of features: a large nonlinear coefficient for ultraviolet (UV) applications, a high damage threshold (LIDT), broad phase matching, low group delay dispersion (GDD) for ultrafast applications, and a wide transparency range.

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

3photon strongly advises keeping humidity as low as possible during this process. When using the gas-blowing method on thin or small BBO crystals, maintain a distance of 10-20 cm to prevent accidentally dislodging or damaging the crystal.

Transporting 3photon BBO crystals are packaged in transparent membrane boxes and vacuum-sealed plastic bags to safeguard them from mechanical shock and humidity. For BBO crystals with complex dimensions (such as thin or small crystals), custom mounts or holders can also be supplied by 3photon.

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.

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BBO is one of the best crystals for picosecond SHG (532 nm) and THG (355 nm) generation, or 4th (266 nm) and 5th harmonic generation (213 nm) with any pulse duration.

Second harmonic generation

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…

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…

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.

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.

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.

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.

BBOrefractive index

Although both α-BBO and β-BBO phases are birefringent, the α-BBO phase exhibits centric symmetry, resulting in different nonlinear properties compared to the β-BBO phase.

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.

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.

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

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.

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.

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.

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.

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.

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.

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.

OPO/OPA Applications: BBO OPO/OPA pumped: 355 nm for 400-2400 nm generation 532 nm for 650-2400 nm generation 800 nm for 1200-2400 nm generation

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.

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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He has also written for a wide range of publications including T3, BBC Focus, Empire, NME, Radio Times, MacWorld, Computer Active, What Digital Camera and the Rough Guide books.

BBO crystals are among the best electro-optical (E-O) and top-tier nonlinear optical (NLO) crystals. They are widely used for 2nd to 5th harmonic generation in many popular lasers and optical parametric oscillators (OPOs), especially (but not limited to) Ti: Sapphire, Yb: KGW/KYW laser systems, Pockels cells and other applications.

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.

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.

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.

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…

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.

BBO crystals are moderately hygroscopic, which is why thin film coatings are applied after polishing. Protective coatings (P-coating) or anti-reflective coatings (AR coating) provide some protection against humidity. However, even the smallest contaminants (dust particles, stains, fingerprints) can degrade the crystal’s performance, impacting transmission or reflection. Additionally, if contaminants come into direct contact with laser radiation, the BBO crystal is susceptible to damage.

Type 2 BBO is used for further Double-Frequency-Generation (DFG) mixing of signal and idler. Non-collinear BBO OPA, pumped at 400 nm, generates pulses with wide spectral width to achieve ultrashort pulses of <4 fs.

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.

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.

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.

The BBO crystal is widely used for temporal walk-off compensation, synchronizing different wavelength pulses after harmonic or other optical setup stages. It can also be used to compensate for temporal differences in very short pulses, ranging from femtoseconds to picoseconds. Due to its high spatial walk-off, the BBO crystal can separate beams of different polarizations spatially, which makes it suitable for custom prism or polarizer production.

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…

Storing Store the BBO crystal in a clean, dry environment. For storage periods exceeding 1-2 weeks: • Ensure the crystal is placed in a clean and dry space • Avoid reusing the original plastic membrane box for extended storage • 3photon recommends using a desiccator for optimal storage conditions

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.

Inspecting Open the BBO crystal’s membrane box only in a clean environment. 3photon recommends inspecting BBO crystals in a clean room, or at the very least in a controlled environment with relative humidity between 20% and 30%. Ensure the crystal’s temperature is equal to the inspection room’s temperature, as any difference can cause condensation, compromising the crystal’s performance and lifespan.

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.

Cleaning Never use acetone to clean the BBO crystal, as it will cause damage. Also, avoid using breath to remove contaminants, as the crystal’s hygroscopic properties could result in damage. Drop and drag method: A. Silk tissue with a mixture of high-purity alcohol and ether in a 1:1 ratio (50%-50%) B. Lens tissue

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

BBO crystal has moderate hygroscopic properties, so thin film coating is highly recommended to prevent the crystal from absorbing moisture from the air.

KTPcrystal

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.

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!

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