Filtering the Unfilterable
The best thing about ultra-wide-angle lenses is that they allow you to see so much. Coincidentally, that extreme angle and depth of field are also the worst things about them. Just try putting a filter on one, and you'll "see" what I mean.
It's nice to show people the world in a way they don't usually see it. Reviewing all my images, I find that I tend to gravitate to the extremes of focal length, with only a modest number shot in the "standard" 50mm focal length region that matches the way human eyesight works. Nothing does this better than the wide-angle lens; the shorter the focal length, the better.
I love ultra-wide-angle lenses. For as long as I can remember, I've been on a quest to use the widest lenses I can find. I can fill the frame with a tiny subject while still showing it in its environment due to the incredible depth of field. They're great for landscape work. All I need is a vast panorama of nature's beauty and something to serve as a foreground subject, and I'm in heaven. I can lay on the ground photographing the smallest clump of flowers or lichen-covered rocky outcrop high in the mountains, a single perfect sea star in a tidepool on the beach, or even a random gracefully curved twig. Almost any excuse for a subject will do if the surrounding scenic vista will support it.
But shooting with a lens that sees so much isn't always easy. Never mind the part about laying on the ground in the mud all the time. No, the problem I would like to address here is that light outdoors can be challenging to work with, and a lens that takes in half the sky can significantly compound that challenge. It's common to screw a filter to the front of a "normal" focal length lens. But in practice, that isn't so easy with an ultra-wide.
Let's consider the problem. Since the 35mm frame is rectangular, the angle of view will be more on the horizontal axis than the vertical. An ultra-wide lens is generally considered a lens with a focal length shorter than 24mm on a full-frame SLR. But things don't start getting that interesting until we go wider than 20mm. At the extreme, we enter fish-eye territory where even straight lines become curved, but you can find lenses down to around 14mm that still produce a rectilinear image. For comparison, a 20mm lens on a full-frame sensor will see about 84 degrees horizontally, while a 14mm will cover 104 degrees. Rather than extending straight out from the lens's front element, the angle of view for an ultra-wide expands rapidly. If you try to affix a filter to the front of such a lens, the thickness of the filter ring could become visible in the frame. Typically, it won't blacken the edges completely, but it's not uncommon for it to vignette enough to darken them.
Companies that make filters understand this and market thinner, "slimline" filters that can minimize the problem somewhat, but the wider the field of view, the harder it is to avoid the problem this way altogether. There are limits to how thin a filter can be while maintaining structural rigidity. At 14mm, you can see everything. For this reason, some ultra-wides entirely omit the means of attaching a filter. Having the threads at all could risk vignetting, even without a filter.
I used to hold the edges of graduated neutral density filters with my fingers in a frustrating effort to avoid the filter holder. Often, I only succeeded in photographing my fingertips in place of the filter holder. Yes, this is challenging stuff indeed.
But over the years, several creative innovators have developed answers to the problem that succeed to varying degrees. Most are built on a shared premise of allowing filters much bigger than the lens front via a custom adapter. But the adapter doesn't screw into the front of the lens. It fits over the body of the lens so that the filter sits flush and doesn't protrude. The filter ring sits far beyond the barrel of the lens, where it falls outside the lens's view. Some years ago, I invested in the WonderPana FreeArc system by FotodioX, but you can find competitive versions by NiSi, Lee, Hitech, and others. I chose WonderPana based on the design of their custom Nikon 14-24mm f/2.8G AF-S adapter and their extensive support for similar lenses. I wanted to invest in a system I wouldn't outgrow as my lens collection evolved. But don't just copy me. Do your research to see which company best meets your needs.
Before you do, understand that while these things may be helpful, they are also huge and expensive. The holder alone for the Nikon 14-24mm will run you around $125. Add a few filters, and you could easily top $500. FotodioX offers WonderPana filters in two formats. Round filters are 145mm in diameter (about 5.3 inches). Graduated Neutral Density and other rectangular filters are 168mm x 216mm (about 6.6 x 8.5"). Did I mention they were huge and expensive? When I'm using it, people sometimes ask what kind of camera I'm shooting with, assuming anything that big must be medium format. Based on the size of the lens, it couldn't possibly be a DSLR. Frankly, it's somewhat humorous. There's also a WonderPana XL for even more troublesome ultra-wides with even bigger filters.
You might be wondering how helpful a polarizer that big would be with a lens that covers such a vast angle of view. A polarizer works best at ninety degrees to the sun, and a lens that covers over a hundred won't be evenly polarized over the entire frame. Its use for darkening the sky is somewhat limited. But it can work well for more targeted purposes, such as removing reflections from foliage or water. The same reason you can't darken the entire sky means such glare won't exist across the whole image. You only need the polarizer to work over the portion where the problem appears.
Over the time that I've owned the WonderPana system, I find I don't use it as often as I thought I might. I've come to appreciate the wide exposure latitude and quality of results I get from the Nikon D850. I've also found that the power of Lightroom and Photoshop these days allows me to make full use of what the camera provides. As such, I trust the camera more to produce usable raw data without filtering in many cases. And when the situation permits, there's always HDR to merge more than one capture into a single photograph.
Do I regret purchasing the WonderPana? If I had it to do over again, I might give it more thought first. But in the end, it was still worth it. When the need arises, it's nice to have. It's one more tool I can use to get the best results I can.