A circular polarizer filter can be an extremely useful photographic tool, but they have to be used with care on an extreme wide-angle lens.
Polarizer filters (also known as polarizing filters or just polarizers) can work wonders in cases where cutting through glare will bring out deeper colors and help peer through transparent reflective surfaces. They’ll deepen the blues of a blue sky (and add texture to skies in black and white when it might otherwise have been blown out highlights); intensify the color of water, especially when there’s reef or sand not far underneath the surface, or even mountain streams; reduce reflections from a building’s glass windows; and rescue some highlight detail in sand, snow, or ice. It can also go a long way to salvaging shooting outdoors in the middle of the day when the sun is at its harshest. I’ve put together some side-by-side examples.
And a true polarizing effect is something that just can’t really be mimicked effectively by post-processing. All of this makes a polarizer a must-have for me. Having started my photo adventures in Australia, where the sun is just that much harsher and there’s lots of it to go with lots of water and sand, polarizing filters have long been a staple of my camera bag.
That said, polarizers behave a little differently to other filters, and there’s some technique involved in getting the best out of them. So I thought I’d put together some examples of what to do and what not to do when using them on a wide-angle lens.
The complication is that polarizers are directional. That is, they work best at fairly specific angles from the light source. If you point the camera right at the sun, you’re not going to see much effect (except maybe some unwanted lens flares) not matter how much you rotate front element of the filter. Point it at the sky at 90 degrees from the sun, especially on a clear day, and you can going to see maximum effect with the right rotation. But when you’re using a very wide-angle lens, that naturally incorporates a broad swathe of angles from the light source in the scene. And that’s the problem–some parts of the frame will get more polarizing effect than others. In some scenes, that’s not much of an issue. In others, it can lead to ugly bands in the sky.
The shot at left was taken without a polarizing filter, while the one at right was taken with one. Both shots were taken within seconds of each other with the same camera, lens, and aperture and processed with identical settings. The only difference in exposure was that the one of the right was exposed for just under 2 stops longer to compensate for the light drop off from the polarizing filter. The sun was directly behind me and quite high in the sky. If you look at the center area of the dome and immediately vicinity you can see why the meter settled on these settings.
The most obvious difference is that the sky is a more intense blue. But the colors in the tree and grass are a little richer, and the highlight details on the Capitol dome are slightly better (easier to see with the original images). The filter also boosts contrast, which helps intensify colors.
But there are some things to watch out for. You’ll lose about 2 stops of light. They work best with directional light, ideally at 90° to the sun, and don’t work so well outside a certain range. Shadow detail will be much diminished. And it means screwing and unscrewing a filter each time you want to use it. You’ll also need to adjust the filter for each shot since it consists of two layers of glass and the outside layer rotates. And adding extra layers of glass can increase the chances of lens flare. Ideally, you wouldn’t be shooting directly at light sources with a polarizer anyway, but the modern coated filters can help mitigate flaring.
And there’s another one to look out for with polarizers: they don’t work well on extreme wide-angle lenses. That’s not to say they don’t work at all, just that they need to be used with care.
It’s a problem of physics. The polarizing effect is directly related to the angle from the light source. Extreme wide-angle or fisheye lenses can cover an extraordinarily wide field of view–up to 180° in a few cases. So if a lot of your frame is blue sky, there’s a good chance you’ll end up with bands of lighter and darker shades across the sky. A further complication is that you won’t really see the full effect through the viewfinder, and it’s not always easy to see on the camera’s LCD screen with all its reflected light. So you might not notice it until you get your images home onto your computer.
With a wide-angle you can also end up with near black at the top of the frame. That can be a good or band thing depending on what you’re going for. If you look at the sky in the immediate area around the dome in this shot, it’s roughly the same as the shot above. But because this shot is taken at the widest end of at 10-20mm lens and therefore the frame spans the area of the sky 90° from the sun, the blue at the top of frame is much darker. To my eye, the effect is overkill for this particular shot–the gradient is just too strong and sharp in the context of the whole frame–although there are some subjects where it can be used to great effect.
The shot at left shows an extreme case of banding when zooming out to 10mm (15mm in 35mm equivalent). The polarizing effect is most intense in the middle of the frame, which was at around 90° to the sun. As you get both under and over that 90° the effect reduces, resulting in light bands. The shot at left shows that zooming in a bit can eliminate the problem. Where the precise threshold is varies based on things like angle to the sun, what’s in the frame, and what effect you’re aiming for.
In general, it’s not a good idea to use a polarizing filter when shooting for stitched panoramas. Because the polarizing effect isn’t evenly applied across the frame, if you have blue sky in the frame you’ll have a lot trouble avoiding banding across the sky. More effective for darkening sky is a ND Grad filter, but even then you have to be very careful about the filter being perfectly horizontal and your frames aligned.