polariser problem

I have just purchased a curcular hoya polariser and I noticed when using with my 16-45mm lens that there is a dark band in the centre of my blue skies. I have never seen this before when I used to use a polariser with fim, any ideas?

Unless you were using a longer focal length (i.e. smaller viewing angle therefore less sky visible), or if you were at a different angle to the sun (most polarization effect seen when at 90 degrees to sun), the effect should not change with digital cameras.

What you describe would not be an unexpected behavior with a polarizer used on a wide angle lens.

Another possibility would be that your new polarizer is of higher quality and is giving a greater effect than your old one.

There are two types of Polarising filters, one Linear, and I can not think of the other Have you the right one: Post a photo for us.

There is Circular and Linear polarizer. The type of polarizer should not have much effect on the end result.

If you think about it, since a polarizer has the most effect at 90 degrees from the light source, the effect will decrease as you change your viewing angle away from that point.

If you center your camera with the sky such that it is 90 degrees from the sun, the center will be getting the most effect of a polarizer. As the angle changes (i.e. as the view extends towards the outer edges of the frame), the effect of the polarizer decreases. Thus the darker band in the middle.

This effect will be greater with wider angle lenses as the edges of the frame extend further away from the 90 degree mark.

Rule of thumbs,
Don't use a polariser with focal lengths beneath 30-35 MM.(maybe 28, sometimes you get away with it, but certainly not shorter.)
Otherwise uneven polarisation will occur.

Martin.

So I am right in thinking in using the polariser with a focal length of say 18mm could have caused this problem and I really should only use above 35mm?

Yes. I'm not sure of the best focal lengths but other factors likely affect this such as time of day, amount of clouds, etc...

It is probably best to remove the filter if the effect becomes undesirable. Also important to remove if you're making panoramas from multiple images that include large swaths of sky, regardless of lens focal length. (since the issue comes from the difference of viewing angles from the location of the light source - the sun).

As explained by amoringello in his previous answer, the ideal polarisation effect is achieved when you are pointing your camera at a 90 degrees angle towards the sun.
Every lens beneath 35 mm has a wider angle of view as 90 degrees. So using a pol.filter will result in an uneven polarisation effect in the sky. Somtimes you won't notice it when using a 28 mm lens. But that has to do with other conditions.
From 24 mm and downwards, there is no escape. Even in post processing you will have a hell of a time needed to correct it.

HIH,
Martin.

Whilst I sort of agree with the comments here I also need to add that I frequently use a polariser with both my Sigma 17-70 & Sigma 10-20, even the Sigma 10-20 at widest produces images without any uneven polarisation.... a polariser does need to be used carefully at the wider angles but I find it invaluable in some situations and would certainly not discount using one even at wide angles, just be careful !!

Simon

I am lead to believe that if you use a polariser at wide angles you need to reduce the amount of polarising by only partially rotating the filter?

Polarisers at wide angles don't work too well as the amount of polarisation varies over the frame. This looks especially strange in blue skies.