Some people keep the CPL filter on their lens all the time as an additional camera lens protector (the way others use a UV filter). If this is you, just make sure that you have the CPL rotated correctly when you're not wanting the effect.
A polarizing filter is not something you want to leave on your lenses at all times though since it reduces light transmission and it can potentially make the sky look unevenly gradient when using wide-angle lenses.
It's generally not a good idea to use a circular polarizing filter with a wide-angle lens. This is because CPL filters work best when they're at a 90-degree angle away from a light source, such as the sun, and a wide-angle lens usually covers more than 90 degrees.
It can remove glare from still water, but is less effective on ripples. It can darken and intensify the color of the sky at 90 degrees from the sun on a clear day. At night, it might have the same effect for moonlit scenes.
When you're photographing at night, you want to get as much light as possible into your lens. A polarizer is going to reduce the amount of light and force you to use a longer shutter speed or a higher ISO setting. So if you're shooting in the dark, do yourself a favor and remove the polarizer.
The short answer is Yes…you can use a polarizing filter for portraits. In fact there are many situations where a polarizing filter can greatly enhance your portraits. There are some things you have to keep in mind, however.
Most photographers, especially those shooting outdoors, use both filters individually for various purposes, although they can also be used together simultaneously.
Can You Use a CPL Filter and an ND Filter Together? Both ND and CPL filters work well together, but often need to be used separately when combating specific issues. For example, water reflections and dull skies are two issues an ND filter can't address.
A CPL Filter can dramatically improve contrast and clarity in daylight, especially on sunny days. Most noticeably, a dash cam CPL Filter can minimize glare and reflections in your windshield.
Quick Tip #1: Use It On Overcast Days—The polarizer helps saturate a blue sky depending on the angle to the sun. If it's overcast, there is no blue, but it can add a touch of snap to darker clouds. Use it to eliminate flat gray sky reflections onto shiny surfaces to eradicate glare that robs color saturation.
You should always rotate the CPL filter 360 degrees and check the level of contrast, saturation, and reflection it is boosting or reducing, and then only decide the degree at which you want to use the filter.
They work differently. Basically, a polarizer is used for blocking light reflected off a surface, while an ND just makes the whole scene darker. Polarizing filters can enhance the color of the image while ND filters just block the light entering the camera. It does not change the overall color of the image.
Let me start with this statement: every landscape photographer should own a circular polarizer. If you're tired of unwanted glare and reflections, struggle to make wet surfaces look good, or can't seem to get the contrast and colors to pop in your image, a polarizing filter is exactly what you need.
Since landscape photographers are often most concerned about depth of field, keeping your aperture constant and changing your shutter speed or ISO will likely be necessary after affixing a polarizer to your lens. A polarizer's effect on your scene changes as you rotate it.
A CPL filter – which stands for circular polarizer/linear – is a glass attachment that can reduce the glare from reflected surfaces. How does it help? Polarized light comes from light that has been reflected off of something.
A 3-stop or 0.9 density ND is ideal for waterfalls in bright sunlight, slowing the exposure to a second or so, depending on the f-stop and ISO used. A strong 10-stop or 3.0 ND filter can blur clouds over several minutes, even on a bright sunny day.
A UV filter won't protect your lens from much more than dust and scratches. If you're shooting at the beach or in the desert, putting one on is a good idea, but otherwise, you're probably fine without one. UV filters have a small effect on the quality of your images. Most of the time, it won't make a difference.
Conclusion. My test confirmed that UV filters definitely have a negative impact on image quality; however, I was surprised that the UV filters only caused very minor changes to the image. The sharpness was not affected at all and the UV filter only made a tiny change to the color and lens flare.
It's a valid question and the short answer is yes, you can stack lens filters.
The polarizer is used mostly at 90º from the sun, so you don't have to worry too much about reflections/flair. Re: can I use polarizer filter together with UV filter? Yes but beware, it may cause some vignetting at the corner.
Are Polarizing Filters Worth It? Polarizer filters provide a means of cutting glare, improving contrast, and eliminating reflections in your photos. Since these unique advantages cannot be found with any other lens filter, polarizers are worth it for any type of photography.
The filter needs to fit the diameter of your camera's lens therefore check your camera lens first. The diameter size is indicated on the top in millimeters (Ex: 16mm, 35mm, 50mm, 55mm, 65mm, 77mm, 82mm, 100mm, 300mm, etc.). In theory, one polarizing filter of the correct size should fit all.