Polarisers & auto iso

[stamper]

When using my polariser I find that the shutter speed - in aperture priority - changes by as much as 2 to 3 stops when rotating the filter to get the right effect. I sometimes like using auto ISO on my Nikon camera. Yesterday it was set at a minimum of 400/th of a second shutter speed as I was on a moving boat. The question is will the polariser effect be hampered by the auto ISO when the filter is being rotated because the shutter speed was about the 400/th of a second minimum or will the auto ISO behave in the same way as the changing shutter speed? In other words is auto ISO and a polariser compatible? TIA

[Chris Pollock]

You're lucky to have a camera that has a usable auto-ISO mode. The lack of one is my favourite gripe about the 5D II.

I don't quite understand your question. Changing the ISO is just another way of adjusting the exposure; doubling the ISO will have the same effect as halving the shutter speed, except that it will increase the amount of image noise. I don't see why using a polarizer should cause any special problems.

If you're in doubt, you could always take a few test shots and see how they turn out. It's not like we have to worry about wasting film anymore.  

[stamper]

You're lucky to have a camera that has a usable auto-ISO mode. The lack of one is my favourite gripe about the 5D II.

I don't quite understand your question. Changing the ISO is just another way of adjusting the exposure; doubling the ISO will have the same effect as halving the shutter speed, except that it will increase the amount of image noise. I don't see why using a polarizer should cause any special problems.

If you're in doubt, you could always take a few test shots and see how they turn out. It's not like we have to worry about wasting film anymore.  

My point is that when adjusting the filter the image darkens which means that the shutter speed gets slower, as much as 2 to 3 stops. If the exposure didn't change the effect wouldn't work??? My point is does auto ISO stop the exposure changing thus negating the effect of the filter? It is hard to test thus I wanted a answer from someone who is better informed on these matters than I am. BTW I am using the Nikon d300 and the auto ISO is a very useful function which I now use about 90% of the time. A minimum shutter of 400 and a aperture of f/11 with the ISO fluctuating is good in changing light such as taking pictures from a moving ship. As long as the noise doesn't go above 1000 then it is a viable way of taking pictures without getting camera shake or movement blur. If the ISO goes above 1000 then neat image can deal with the noise?

[Mike Boden]

In other words is auto ISO and a polariser compatible?

Yes.

However, I think you're not quite understanding a polarizing filter. Basically, a polarizing filter blocks incoming light that is perpendicular to the axis of the filter. This is used to control light reflections on water or glass, for example. It can also be used to darken the sky. A side effect is a bit of light loss. To compensate, you need to adjust your exposure. Typically when shooting film, an adjustment of the aperture or shutter speed was necessary. Nowadays with digital cameras, it's very easy to adjust the ISO to achieve the same result.