The Nikon D4 has quickly captured the attention of serious photographers and videographers, marking a significant leap forward in the world of digital SLR cameras. As a high-performance tool, the D4 builds upon the legacy of the D3 series, offering a host of advancements that make it an attractive choice for professional photographers across various disciplines. Aimed primarily at professionals who demand top-tier performance, the Nikon D4’s impressive specs push the boundaries of what a DSLR camera can achieve.
While the camera still excels at its primary role as a stills shooter, the D4 distinguishes itself by offering remarkable improvements in video capabilities, particularly in low-light performance. It stands head-to-head with its closest competitor, the Canon EOS 1DX, setting new standards in the industry. A defining characteristic of the D4 is its ability to perform at the highest level in almost any environment, making it particularly appealing for photographers who frequently work in challenging conditions, such as wildlife photographers, sports shooters, and event photographers.
One of the standout features of the Nikon D4 is its continuous shooting speed. With a burst rate of 10 frames per second and autofocus tracking, it excels in high-speed photography, such as capturing fast-moving sports action or wildlife moments. Its ability to focus at F8 also adds another layer of versatility, allowing photographers to achieve greater precision even with slower lenses, which is a key advantage when shooting in unpredictable settings. This unique combination of high-speed shooting and precise autofocus makes the Nikon D4 a must-have tool for those who specialize in fast-paced, dynamic photography.
Exceptional Performance in Low-Light and Harsh Environments
For many professional photographers, low-light performance is crucial, and the Nikon D4 excels in this area. With enhanced sensitivity and the ability to shoot at high ISO settings without introducing excessive noise, this camera is tailored to deliver exceptional results even in the most challenging lighting conditions. Whether shooting an evening sports game or working in a dimly lit event, the D4 ensures that every shot remains crisp and clear, making it the go-to option for those who cannot afford to miss a moment due to inadequate lighting.
This low-light capability also makes the D4 an excellent choice for photographers who specialize in night photography, astrophotography, or indoor events, where natural light is often scarce. Unlike its predecessors, the D4’s image sensor captures stunning detail without the usual drawbacks associated with high ISO settings, such as grainy or overexposed images. This makes the Nikon D4 especially attractive for those venturing into the underwater world of photography, where light can be especially limited and harsh conditions are common.
Underwater photography, in particular, demands more than just a good lens or waterproof housing. It requires a camera that can handle the unique challenges presented by submerged environments. The Nikon D4’s robust construction, which includes weather sealing and a durable magnesium alloy body, ensures that the camera can withstand the pressures and conditions encountered in underwater shoots. Additionally, the camera's exceptional performance in low light further enhances its appeal for underwater photographers who are often working in dimly lit or murky waters.
The Nikon D4's low-light prowess extends beyond just high ISO capabilities. The camera's autofocus system is highly refined and can function in light levels as low as -2 EV. This means that even in environments where most cameras would struggle to achieve proper focus, the D4 continues to operate with precision. The advanced autofocus system, combined with the camera's exceptional sensitivity to light, ensures that professionals in all fields can rely on the D4 to produce outstanding images, regardless of the environment.
The Nikon D4: Built for Professionals, Ready for the Future
The Nikon D4 isn’t just about the impressive technical specifications’s also about offering photographers the tools they need to push the limits of their craft. One of the most significant upgrades in the D4 is its use of XQD memory cards. These cards, designed for ultra-fast data transfer speeds, significantly enhance the D4’s performance, particularly when working with large files like RAW images or high-definition video. The XQD cards allow for rapid write speeds, ensuring that photographers can shoot continuously without the risk of buffer delays or lag. This is especially important when working in fast-paced environments or during critical moments when every frame matters.
Another aspect that professionals will appreciate is the Nikon D4’s ability to shoot video in full 1080p HD resolution. The addition of video functionality has become a must-have feature for many photographers, as it allows them to produce professional-grade video content alongside their stills. With the D4, Nikon has delivered an exceptional video recording experience that rivals that of dedicated video cameras. The camera’s ability to shoot at multiple frame rates, including 30p, 25p, and 24p, gives filmmakers the flexibility they need to achieve cinematic results. Furthermore, the camera’s video autofocus and audio controls ensure that the D4 is not only suitable for professional photographers but also for videographers who demand the best in both still and motion capture.
The D4 also excels when it comes to ergonomics and ease of use. Its design prioritizes comfort during extended shoots, with intuitive controls and a user-friendly interface. Whether in the studio, on location, or underwater, the camera’s robust build quality and weather-sealed body ensure that it can endure the demands of professional photographers across various shooting environments. The inclusion of a large, high-resolution LCD monitor allows for precise framing and image review, further enhancing the photographer’s ability to capture the perfect shot.
With all these attributes, it’s clear that the Nikon D4 is not just a camera’s a versatile tool built to meet the ever-evolving needs of professionals. Whether capturing breathtaking landscapes, high-speed action shots, or stunning underwater imagery, the D4 offers the performance and durability required by photographers working at the top of their game. While its $6,000 price tag might seem steep to some, for serious professionals, the investment is justified by the unparalleled quality and reliability the D4 provides.
Nikon D4: A Leap Forward in Imaging Technology
The Nikon D4 brings a host of exciting upgrades and improvements over its predecessor, the Nikon D3s, making it an even more powerful tool for professional photographers and videographers alike. Among the most notable advancements is the introduction of a 16.2-megapixel full-frame sensor, which provides significantly better image quality, particularly in low-light conditions. This enhanced sensor ensures that photographers can continue capturing fine details with minimal noise, even when working in challenging lighting environments. The ability to capture more detailed images with high fidelity makes it a highly desirable camera for those who demand top-tier performance across a range of shooting scenarios. Whether in dimly lit indoor spaces or shooting under the moonlight outdoors, the D4 rises to the occasion, offering remarkable clarity and preserving minute details that lesser sensors may struggle to capture.
With the Nikon D4, photographers can take advantage of an impressive ISO range, extending up to 204,800. This incredible sensitivity allows for shooting in extremely dim settings while maintaining sharpness and clarity. For professionals in fields such as event photography or low-light landscapes, the expanded ISO range is a game-changer. The camera's performance at these high sensitivities significantly reduces the noise that typically arises in such challenging environments. When capturing events like weddings or concerts, where lighting can often be unpredictable, the ability to shoot at these extended ISO settings offers unparalleled flexibility. Photographers can shoot without worrying about the trade-off between exposure and noise, making the D4 ideal for spontaneous, fast-paced scenarios.
The boost in resolution from the D3s’ 12.1 megapixels to the D4’s 16.2 megapixels brings significant improvements in image detail, providing more room for cropping, enlarging prints, or capturing minute textures that were previously out of reach. This higher resolution ensures that every shot is imbued with a greater sense of clarity, allowing professionals to showcase their craft in the finest detail. For commercial photographers, the increase in megapixels also expands the creative and practical possibilities, making it easier to produce large-scale prints with exceptional precision and depth. Whether it's capturing the intricate textures of a luxury product, the fine details of a landscape, or the subtle nuances in a portrait, the D4 ensures that every pixel is utilized effectively.
The enhanced sensor also contributes to a greater range of possibilities for high-resolution commercial photography, where every pixel matters. The improved image quality is complemented by a suite of processing power upgrades, ensuring that the D4 produces results that stand up to rigorous commercial standards. This makes the D4 not only a robust camera for professionals working in standard photography fields but also an excellent tool for industries demanding higher levels of detail and precision in their images. For photographers specializing in fine art, fashion, or even scientific imaging, the D4's image quality and processing capabilities allow them to consistently meet the demands of the most discerning clients.
Beyond the technical upgrades in resolution and ISO performance, the D4’s overall design makes it a more versatile tool for a wider variety of shooting environments. With its rugged, weather-sealed body, it is built to withstand the rigors of shooting in less-than-ideal conditions, whether it's a rain-soaked outdoor shoot or a dusty desert scene. The D4’s ergonomic design ensures that photographers can hold and operate the camera for long periods without fatigue, which is a crucial aspect for professionals engaged in lengthy assignments. Its intuitive control layout allows for quick adjustments, enabling photographers to focus on their creative process without constantly having to fumble through settings. Additionally, the camera’s continuous shooting speed of 10 frames per second ensures that photographers can capture fast-moving subjects with ease, whether it's an athlete in motion or a fleeting moment in a wedding ceremony.
When paired with the Nikon D4’s advanced autofocus system, which includes 51 focus points, photographers can achieve precise focus in a wide variety of situations. Whether working with fast-moving subjects, shallow depth of field, or unpredictable lighting conditions, the D4’s autofocus capabilities deliver consistent and reliable results. This is particularly useful for action-oriented fields like sports or wildlife photography, where split-second timing can make all the difference between getting the shot or missing it entirely.
The video capabilities of the D4 are also impressive, with full 1080p HD video recording at 30 frames per second, making it a fantastic choice for professionals who need both stills and video capabilities in a single package. The ability to record in multiple frame rates, such as 24p for cinematic looks or 60p for smooth motion, gives videographers the creative freedom to experiment and achieve different visual styles. With the Nikon D4, videographers have access to rich color reproduction and detailed footage that can be easily integrated into any post-production workflow. Whether documenting a documentary, capturing a live performance, or creating high-end promotional content, the D4’s video features offer the versatility needed to meet the demands of modern filmmaking.
The Nikon D4 not only improves on the strengths of its predecessor, the D3s, but also pushes the boundaries of what a professional DSLR can do. Its enhanced sensor, extended ISO range, higher resolution, and improved autofocus system make it a powerful tool for a wide range of applications, from event and commercial photography to video production. With its impressive image quality, robust build, and versatile features, the D4 sets a new standard for professional imaging, ensuring that photographers and videographers alike can capture every detail with confidence and creativity.
Unmatched Autofocus System and Performance
One of the standout features of the Nikon D4 is its upgraded autofocus system, which now boasts 51 autofocus points, a significant improvement over the D3s. This expanded autofocus coverage allows for faster and more accurate focusing, ensuring that even the most dynamic scenes can be captured with crystal clarity. Whether you are photographing a fast-moving athlete in a sports setting or tracking wildlife in nature, the D4’s autofocus system delivers the kind of precision that professionals depend on.
This improvement is especially important for photographers working with moving subjects. The 51 autofocus points provide excellent coverage across the frame, making it easier to lock focus on subjects without having to manually adjust the point or recompose. Additionally, the D4 introduces the ability to autofocus even with lenses up to F8, a feature that greatly enhances the versatility of the system when used with teleconverters or long lenses. This means that photographers can achieve quick, reliable focus even when using high-magnification lenses, a scenario where many other cameras might struggle.
For wildlife photographers or those who specialize in action shots, the enhanced autofocus system allows for precise tracking of fast-moving subjects. The D4 ensures that even when shooting at long distances or with extreme zoom, focus remains locked on the subject, avoiding the blurring that can occur when using less capable autofocus systems. Whether capturing a bird in flight or a race car speeding by, the Nikon D4's advanced autofocus system ensures that every shot is in focus, bringing the subject into sharp detail.
Faster Workflow and Video Capabilities
Beyond the exceptional image quality and autofocus performance, the Nikon D4 introduces significant upgrades to its overall workflow. The camera’s large RAW buffer size allows photographers to shoot up to 98 frames in 12-bit RAW mode, a considerable improvement over its predecessors. This enhancement allows for more continuous shooting, particularly during fast-paced events or sports photography, without the camera slowing down as the buffer fills up. Whether you are photographing a sporting event or capturing a fleeting moment at a wedding, the D4 enables an uninterrupted shooting experience, enhancing workflow efficiency.
Additionally, the camera’s switch to XQD memory cards, replacing the CF cards used in the D3s, marks a major advancement in data storage. The XQD cards offer faster write speeds, allowing the camera to handle larger file sizes with greater ease. This ensures that photographers can shoot at higher frame rates without worrying about data bottlenecks, resulting in smoother performance when capturing sequences of high-resolution images. This faster memory card system also aids in faster read times, allowing images to be transferred more quickly, which is especially important for professional photographers working with tight deadlines.
For videographers, the Nikon D4 offers a suite of impressive video features that make it stand out among other DSLR cameras in its class. The camera is capable of recording full 1080p video at 30 frames per second, providing filmmakers with high-quality footage for various production needs. Additionally, the camera supports shooting at 720p at 60 frames per second, enabling smoother video playback for action-packed scenes or slow-motion sequences. This flexibility in video resolution gives users more options when creating video content, whether for film, television, or online platforms.
Another significant feature for videographers is the D4’s clean HDMI output, which allows for uncompressed HD video streaming. This is a crucial tool for professionals who need to deliver pristine, high-quality footage for post-production or live streaming. The uncompressed HDMI output ensures that every frame of video maintains the highest level of detail, giving videographers full control over the video quality. This feature opens up opportunities for filmmakers and content creators to use the D4 as a key tool in their video production workflow, further solidifying its reputation as a versatile and professional-grade camera.
The D4 also includes a 20-minute video recording limit, which ensures that filmmakers can capture extended footage without worrying about the camera cutting off prematurely. This feature makes the D4 ideal for events, interviews, or documentaries where long, uninterrupted takes are essential. The ability to record for longer durations without interruptions is a significant improvement over previous models, providing videographers with the flexibility to capture detailed and continuous video footage without limitations.
Nikon D4 vs Nikon D3s: Evolution in Image Quality and Performance
When comparing the Nikon D4 to its predecessor, the Nikon D3s, the advancements in both image quality and overall performance are immediately noticeable. The D4 takes a significant leap forward in terms of resolution, boasting a 16.2-megapixel sensor, which offers a considerable upgrade over the D3s's 12.1-megapixel sensor. This increase in megapixels results in more detailed images, making the D4 a better option for professional photographers who require high-resolution images for large prints or significant digital cropping. The higher pixel count on the D4 is particularly beneficial when working in commercial photography, landscape photography, or any genre that demands sharp, high-quality images.
One of the standout features of the Nikon D4 is its expanded ISO range, which extends all the way up to 204,800. This is a major improvement over the D3s, which maxes out at ISO 102,400. For photographers who work in challenging lighting conditionswhether in low-light environments or at nightthe D4 offers a significant advantage. The higher ISO range helps to maintain image clarity and reduce noise, even in dimly lit settings. This expanded sensitivity range makes the D4 an ideal choice for event photographers, wedding photographers, and those who specialize in nighttime or indoor photography where lighting might be unpredictable.
The autofocus system in the D4 also surpasses the D3s, enhancing its usability for a variety of shooting conditions. Both cameras share the same number of autofocus points, but the D4 offers a more refined experience due to its ability to focus at f/8. This is particularly valuable for wildlife and sports photographers who often use telephoto lenses with extenders. The D4's autofocus system ensures faster and more accurate tracking of subjects, even in challenging situations. Additionally, the D4's processing power and larger buffer size enable it to handle rapid bursts of continuous shooting, making it ideal for fast-moving subjects. In contrast, the D3s has a smaller buffer and slower write speeds, which can limit the number of frames it can capture before hitting a bottleneck.
Nikon D4 vs Canon EOS 1DX: A Close Call in Performance
When pitting the Nikon D4 against the Canon EOS 1DX, the competition intensifies, as both cameras are designed for high-end professional photographers. The Canon 1DX holds a slight edge in terms of megapixels, offering an 18.1-megapixel sensor. While this might appeal to photographers who need more resolution for fine detail, the difference in pixel count is relatively minor in most real-world scenarios. The Nikon D4's 16.2-megapixel sensor still delivers incredible image quality, and its superior low-light performance might make it the better choice for those who prioritize clean, noise-free images in dark conditions.
One of the primary factors that could influence a photographer’s decision between the two cameras is their respective prices. The Nikon D4 is priced at approximately $6,000, while the Canon EOS 1DX comes in at a higher price point of $6,800. For many professionals, the Nikon D4 presents a more affordable option that still offers exceptional performance. This lower price tag could be a key consideration for photographers who are looking for professional-level features without breaking the bank.
In terms of continuous shooting speed, the Canon 1DX has a slight advantage, capable of reaching up to 12 frames per second. The Nikon D4, while still impressive, can shoot at 10 frames per second. For photographers specializing in fast-paced sports or wildlife photography, the extra two frames per second offered by the 1DX might make a noticeable difference in capturing decisive moments. However, the Nikon D4 compensates for this by offering faster autofocus, particularly in low-light conditions. This gives it an edge when photographing moving subjects in environments with limited lighting.
Both cameras also offer outstanding video capabilities, with each providing full 1080p resolution at 30 frames per second. However, the Nikon D4 sets itself apart with its superior video features. It includes a time-lapse function, which is ideal for photographers and videographers who want to create stunning time-lapse sequences. The D4 also offers clean HDMI output, which allows for high-quality video capture without any on-screen overlays, making it a strong choice for videographers who need top-tier video performance. Moreover, the D4 includes built-in microphone and headphone inputs with adjustable levels, offering more control over audio during video shoots. This makes the D4 a particularly attractive option for professional videographers who need to manage both audio and video in real time.
Key Advantages of the Nikon D4 Over Its Rivals
The Nikon D4 stands out in a number of key areas that make it an attractive option for professional photographers who demand exceptional performance across the board. One of the primary advantages of the D4 is its superior low-light capability. While both the D3s and Canon 1DX are excellent cameras in their own right, the D4's expanded ISO range and its ability to maintain image quality at higher sensitivities put it in a league of its own. The D4’s ability to shoot at ISO settings up to 204,800 is unparalleled in terms of preserving detail and reducing noise at extreme settings.
Another area where the Nikon D4 excels is in its autofocus system. While the D3s and Canon 1DX both have advanced autofocus features, the D4’s ability to autofocus at f/8 is a major benefit for photographers who use telephoto lenses with extenders. This makes the D4 a more versatile option for wildlife and sports photographers, who often rely on long lenses and need the camera to maintain fast, accurate autofocus, even at wide apertures or in challenging conditions.
The D4’s enhanced processing power is also worth noting. The camera’s larger buffer and faster write speeds allow photographers to shoot longer bursts without worrying about slowing down or filling the buffer. This is especially useful for capturing fast-moving action, as photographers can focus on their subjects rather than being limited by the camera’s capacity to store images. While the Canon 1DX also offers high-speed shooting, the Nikon D4’s faster processing speed allows it to maintain its performance over extended periods, especially when shooting in RAW format.
Video enthusiasts will find that the Nikon D4 offers more comprehensive video features compared to the Canon 1DX. The inclusion of clean HDMI output, time-lapse functionality, and high-quality audio control via built-in microphone and headphone inputs make the D4 a more robust tool for professional video work. The Canon 1DX does offer excellent video performance, but the D4’s added versatility with video settings and audio management make it a stronger choice for those who are serious about integrating high-quality video into their photography workflow.
For photographers seeking a balance between resolution, low-light performance, autofocus speed, and video capabilities, the Nikon D4 offers an exceptional package. Its combination of powerful features, including the large sensor, fast autofocus, expanded ISO range, and advanced video capabilities, makes it one of the best cameras available for professionals who need a reliable, high-performance tool. Whether you’re shooting sports, wildlife, events, or commercial work, the Nikon D4 offers the versatility and performance that high-end photographers demand, all at a competitive price point.
Nikon D4: A Cutting-Edge Camera for Professionals
The Nikon D4 has earned a reputation as one of the most powerful and impressive professional cameras in the market, blending advanced technology with a robust set of features that cater to the specific demands of professional photographers and videographers. This flagship DSLR is designed to provide exceptional performance, with state-of-the-art technology that ensures every shot is captured with unparalleled clarity and precision. Whether you are a photographer specializing in fast-action sports, wildlife, or even portrait photography, or a videographer focused on producing high-quality footage, the Nikon D4 delivers results that exceed expectations.
At the heart of the Nikon D4 is its exceptional autofocus system, which provides a seamless and precise focusing experience, even when dealing with fast-moving subjects. This makes it particularly suitable for photographers in need of speed and accuracy, such as those covering high-speed sports or wildlife scenes. The camera’s ability to focus quickly and accurately in a variety of lighting conditions, including low light, is a major asset for professionals who work in environments where lighting can be challenging. In addition to its powerful autofocus system, the Nikon D4’s large buffer size ensures that the camera can handle rapid bursts of shots without lag, making it ideal for photographers who need to capture a series of images in quick succession.
Another standout feature of the Nikon D4 is its remarkable low-light performance. With the ability to capture sharp and clear images even in near-dark conditions, this camera is the perfect choice for those who often shoot in dimly lit environments. Whether you are shooting indoor events, concerts, or night photography, the D4 ensures your images will remain crisp and noise-free, allowing you to work in challenging conditions without compromising on image quality.
The Nikon D4: A Versatile Tool for Video Professionals
In addition to its exceptional still photography capabilities, the Nikon D4 stands out as a powerful tool for videographers. With a range of video features designed to meet the needs of professionals, this camera is a go-to choice for high-quality video production. The D4’s ability to shoot full HD video at 1080p resolution at 30, 25, or 24 frames per second makes it a versatile option for various video projects, from documentaries to cinematic productions.
The camera’s advanced video features don’t stop at resolution; the D4 also offers precise control over exposure settings, ensuring that videographers can achieve the exact look they are aiming for. Whether you are shooting in bright daylight or under artificial lighting, the Nikon D4 provides the flexibility needed to produce flawless video footage. Its improved video autofocus system also ensures smooth, sharp focus transitions, allowing for cinematic-style video captures, even in dynamic shooting environments.
For those looking to produce high-quality video with professional-grade sound, the Nikon D4 supports external microphone inputs, giving users the ability to record audio in high fidelity. Combined with the camera’s excellent low-light performance, this feature makes the D4 an ideal option for videographers who are shooting in challenging conditions or on location in low-light environments.
Moreover, the Nikon D4’s rugged build and weather-sealed body are perfect for videographers working in harsh environments, whether shooting in extreme weather or on challenging terrains. The camera’s durability and reliability make it an excellent choice for professional video production, where maintaining top-quality footage, regardless of the conditions, is a top priority.
The Nikon D4 for Underwater Photographers: Performance and Durability
For underwater photographers, the Nikon D4 offers the perfect balance of performance and rugged durability. When shooting in underwater environments, photographers face a unique set of challenges, from harsh lighting conditions to the risk of camera damage from water exposure. The Nikon D4’s robust construction ensures that it can handle the extreme conditions of underwater photography, making it a reliable choice for professionals in this specialized field.
The camera’s exceptional image quality remains intact even when shooting underwater, ensuring that each shot is captured with remarkable clarity and detail. Whether capturing vibrant marine life or dramatic underwater landscapes, the D4 provides the sharpness and depth of color needed to showcase the beauty of the underwater world. Additionally, the D4’s impressive autofocus system allows photographers to capture fast-moving marine subjects with ease, making it an ideal tool for capturing everything from schools of fish to fast-swimming sharks.
The Nikon D4’s ability to handle demanding environments extends beyond its physical construction. The camera’s advanced sensor technology ensures that even in low-light situations, photographers can still achieve high-quality results. This is particularly important for underwater photographers, where natural light is often scarce at deeper depths. The D4 excels at capturing sharp, vibrant images in these challenging lighting conditions, thanks to its superior low-light performance and noise reduction capabilities.
One of the standout features of the Nikon D4 is its compatibility with the new XQD memory cards. These memory cards offer faster read and write speeds, which is a significant advantage for underwater photographers who need to capture high-resolution images and video footage quickly. The D4’s ability to handle large files with ease ensures that photographers can continue shooting without worrying about running out of space or experiencing slow processing times. This is particularly important when shooting fast-moving underwater subjects, where every moment counts.
When considering the Nikon D4 for underwater photography, it is also essential to highlight its versatility. The camera can be used in a wide range of underwater settings, whether shooting in a shallow reef or at greater depths. The D4’s durability and high-performance features make it an excellent choice for underwater photographers who need a reliable camera that can handle the toughest conditions and deliver exceptional results every time.
Conclusion
In conclusion, the Nikon D4 sets a new standard for professional cameras, offering a unique combination of power, reliability, and innovation that few other cameras can match. Whether you are a photographer specializing in fast-paced sports, wildlife, or underwater photography, or a videographer looking for a tool that can handle high-quality video production, the D4 stands out as the ultimate choice.
Its powerful autofocus system, superior low-light performance, and large buffer size make it the ideal choice for fast-paced photography, allowing you to capture each moment with precision and clarity. Meanwhile, its exceptional video capabilities cater to the needs of professional videographers, ensuring that you can create high-quality footage in a range of settings.
For underwater photographers, the Nikon D4 offers the rugged build and performance necessary to thrive in challenging underwater environments. Its ability to capture stunning images and video even in low-light conditions, coupled with its high-speed memory cards and versatile design, make it the perfect companion for underwater expeditions.
As the successor to the D3s, the Nikon D4 raises the bar for DSLR cameras, offering a camera that combines cutting-edge technology with unparalleled durability. No matter the conditions or the type of photography or videography you specialize in, the Nikon D4 is designed to deliver exceptional results that meet the highest professional standards.