Polarizers are a necessity for mid day shooting.
But there are other occasions when they're important.
I have both the Cokin set up for linear polarizers and a few circular polarizers for each lens.
I use the cir for mid day to get blue in the sky. On one occasion both the cir and linear were appropriate. That was at the Bonneville salt flats where the ground was absolute white and full of glare & the sky was bleached.
If you want a sunset and want to keep some detail in the foreground (rather than silhouettes) then a linear polarizer will help. I have a graduated Cokin that goes as dark as 3 stops (you can adjust to 1 or 2 stop).
But make sure you don't just slap either polarizer on and expect it to work; you have to adjust it, which is easy to do.
This is the best example I have of the value of linear polarizers. I got fog detail in the foreground while reducing the super glare of the sunset to get a definitive orb
My filter is on all the time. It protects my lens and does what a polarizing filter is supposed to do.
I think the most polarizing is the K&N versus stock paper argument.
This is something I have explored for years. Polarizing filters are a great tool for bringing blue skies into rich colors and not bleaching them out by over exposing. They also work great when shooting great bodies of water with glare and shine. Even sunsets. There is a bit of a science since wide angles, zoom lenses and fixed lenses, all react slightly different. You can turn the polirizer to an angle where you are pleased with the image. With a polarizer filter, you are in control. Promaster carries one that's made by Hoya, which is a Tokina company. It's a pretty darn good filter. Cokin, Tiffen, and B+W are also great. I have never had a B+W, but have heard they are the best from other collegues.
I'm sure if I looked I could find an example of a black sky somewhere in the great expanse of the Internet. They're usually shown in the photo mags in "what not to do" scenarios, too.
Just adding some additional thoughts on the discussion above. The black sky effect is particularly acute with super saturated high contrast transparency film coupled with extreme wide angle lenses (<20 mm). Super wide lenses also bring a whole host of issues regarding filter size, vignetting, etc. into play. I'm thinking of Fuji Velvia 50 specifically. I have no idea how the various digital sensors stack up against transparency film in terms of saturation and contrast. I'll see if I can find an example of what I'm talking about. I've probably shot a few over the years, but it's doubtful I kept the transparency (for obvious reasons ).
This isn't Velvia, it's a Canon 1Ds sensor, but it is a high contrast scene using a polarizer. I used this shot as an example of what happens to snow and skies with a polarizer in my photo class. You can see it's difficult to expose the sky, snow and highlight (edge of drift) correctly in one exposure. Had this been shot with Velvia, the sky would have been almost black. You can see the sky isn't really blue at all. The snow isn't purely white because it's reflecting the blue sky. Hope this helps.
Last edited by bricciphoto; 02-20-2008 at 02:35 PM.
If you don't mind, I'll give it a shot. I have a few photos that I think are examples of what you are talking about.
These are all shot with a 40D and a 10-22mm zoom. At the wide end, that's a 16mm equivalent (IOW very wide-angle). All shot with a B+W circular ploarizer.
The polarizer here is adjusted for maximum saturation of the sky. Clearly, it's too dark.
Roughly the same shot, backing off on the polarizer quite a bit. Sky is still unnaturally dark, but it is a more realistic photo.
Same thing: Too dark.
A change in orientation and in the adjustment of the polarizing filter, resulting in a more naturalistic image. Click for EXIF. The exposures in the first two are equivalent; in the second two, identical. The last two photos are underexposed, the LM being fooled a bit by the white rocket. Had I to do it again, I'd add +2/3 EV.
Vignetting (dark corners) is not apparent to me in any of these shots, but might be against a lighter background--modern lenses vignette quite a bit. Processing programs like LightRoom even have settings to deal with it.
Those are good examples Tom. Thanks for helping out!!!
Note the rocket in Tom's last two pics is fighting underexposure while the sky is less saturated (similar to my snow example above). This is very difficult shooting territory even for a pro as it's really pushing extremes into one exposure.
Here, this is Velvia and I'm pretty sure no filtration at the Bonneville Salt Flats. Yucky black sky. There's probably 7 stops difference from the top of the frame to the bottom--maybe more .
(Sorry for beating this nuance to death. )
Here's the same scene with a better exposure. Almost makes me want to go out and shoot some positive film again.
BTW, the photo with the white motorcycle seems to come out better with the polarizer than the black motorcycle.
Here is the similar view show with no filters at all. Compare to black sky above. I confess that I like this better.
One should be careful using a polarizing filter. As Ben said, it can turn your sky black. It can also over-saturate colors, causing then to become too dark, and can alter the colors. And, depending on the quality of the filter, it can reduce the crispness of the resulting image. I noted that my Hoya circular polarizer did reduce slightly the sharpness of the details on the motorcycle, such as the BMW roundel, and tends to make everything slightly greener. Perhaps I need a higher quality filter.
I see they can be pricey -- http://tinyurl.com/3cos3y