Polarizer vs ND Filter

Both a polarizer and a 0.6 (4x) neutral density filter produce about a 2-stop reduction in received light, thus allowing a longer shutter speed at a given f-stop. Such a long speed is commonly used to blur moving water in photographs of waves and waterfalls -- commonly at sunset.

With the polarizer I have noticed darkening of the sky on one side of some images. I have not encountered this with the non-graduated ND filter. But the ND filter will not eliminate reflections.

Have others encountered this one-sided darkening? How can it be prevented?

Patrick, the darkening of the sky on one side is a very common occurrence when using a polarizer to it's maximum. You might experiment with turning the polarizer while looking through the view finder and adjust the scene with a more even approach overall in polarization.
ND's do not polarize they merely add a neutral amount of darkening to the area in a variety of types and overall strengths.

If you're using a polarizer on a wide angle lens, you may get uneven polarization.

I commonly photograph mountains as well. It appears that I will need to stack polarizer (reduced effect) and ND (more light reduction) filters to produce more even images.

Many expert photographers will advise against shooting at mid-day, but often we don't have that scheduling option (although I agree it is preferable).

You're almost certainly guaranteed uneven polarization when using them with a wide angle lens. I stopped using 'em with mine altogether for that reason. You can 'polarize' a sky in postprocessing if need be.
If you like the effects of a polarizer on land but not the sky, then take two images (one with, one without) and blend those two for the best of both worlds.

The maximum effect of a polarizer is perpendicular to the direction of the sunlight. A wide angle lens covers a wide enough area that in parts of the field of view, the reduced effect from not being at right angles to the direction of sunlight is apparent. This effect is less apparent with longer focal lengths. If you rotate the polarizer to minimize the polarizing effect you can still take advantage of the darkening affect, probably less than two stops. A neutral density filter has the same effect no matter the direction relative to the sun.

The light in the mountains at higher altitudes on a clear day will already be slightly polarized naturally due to the reduced amount of particulates and water vapor. Therefore, it is easy to overuse a polarizer.

If you need to eliminate unwanted reflections in a scene that includes the sky, the suggestions given by the others will work. If you simply need to correct for dynamic range (i.e. to keep the sky from blowing out), an ND-Grad will be best.

I rarely use filters of any kind when shooting color, and then only when their use is absolutely called for. In the mountains, I may use an 81-A or 81-B to warm up a scene that is excessively blue.

--P

"Both a polarizer and a 0.6 (4x) neutral density filter produce about a 2-stop reduction in received light."

I just noticed that the above statement is incorrect if these filters are stacked. If the filters are used separately, the statement is true.

The f-stop reduction for a 0.6 ND is 2 stops (4X filter factor). The f-stop reduction for a polarizer is (approximately) 2 stops (4X filter factor). To obtain the total light reduction in stops, we add the two for a total of 4 f-stops.

--P

I conclude from this:

1. The polarizer can be overused, in which case one side of the image (sky) may be darkened.
2. If a grad-ND filter is used to achieve the same degree of darkening as max use of a polarizer IN BRIGHT LIGHT the sky may become "dead" grey.
3. In stitched panos or wide-angle, edge(es) of the image may be unequally darkened by a polarizer.

Patrick
With regard to your conclusion no. 2:

The whole Idea of neutral density is that it does not change color, only brightness. If you use a two-stop neutral density filter, you should get exactly the same color by increasing the exposure by two stops. If you do this with a graduated ND filter, the sky will be the same and the foreground will be two stops brighter. If the sky is gray, an ND filter will make it a darker gray, if you do not change the exposure settings of the camera. If the sky is blue, the ND filter will make it darker blue, if you do not change the exposure settings. I often use a GND to brighten the foreground rather than to darken the sky.

A polarizer can be used as a neutral density filter, but that is not it's primary intended use. It's reduction of exposure is really kind of a side effect. It is used to reduce glare, e.g. on foliage or water, increase contrast, e.g in clouds, to reduce the effect of haze in the atmosphere, etc. A good polarizer is pretty much a must-have piece of equipment.

I also carry with me an assortment of neutral density filters and graduated neutral density filters. I find them indispensable, though other photographers may prefer to achieve their effect in post processing by HDR or blending. A polarizer, ND filters, and GND filters can be used in conjunction with one another or separately, depending on the situation. If their use does not enhance the photograph, then it was simply not the right choice in that particular situation.

any lens less than 40mm will show the darkening of the sky and the uneven blues, polarizers are best with cloudy skies so that you can pick and choose which part of the sky you want darkened....40mm and longer should give you an even blue sky when the polarizer is used....try it and see

I'm surprised this comment wasn't already made - if you use Live View, you will be able to clearly see the uneven polarization and correct it before you take the shot.