Circular Polfilter advice please

AchimT

AchimT

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I've bought a B+W AUC CPL filter, Kaesemann with MRC coating. I've tried to use it outdoors (sunlit meadows, trees, white cows, reflecting roofs, some red metal in the scenery, or medium-distant trees with reflecting leaves, but I wonder whether I am using it properly.

I know I should be more or less lateral to the sun, and I can see the effect of the filter, but I don't fully understand how it works and what I am supposed to do to get the max.

The situation is this: depending on the position I rotate it into, it takes different amounts of light away (I'd say two to three stops), and in the position which takes the most light away, the images come out with a tad more colour perhaps, and maybe a somewhat warmer (more golden?) hue, whereas in the position 90° back from this (takes one f-stop of light less away), the images look a little colder, so probably a more bluish hue.

Neither of the two positions really improves the image, as compared to taking the same pic without the filter. When the sun could touch the lens opening, I often shadow the lens, unless I want glare effects. Does this diminish the effect of the filter?

I've been told I'd be getting a much bluer sky for example, but I only get a marginally bluer sky. Nothing worth writing home about. And the grass may look a tad more juicy, but it's minimal and I could achieve that in post processing easily.

I wonder, what am I doing wrong. Can anybody help me with advice?

Thanks in advance for all comments
 
It's difficult to say what is happening here. The effects that you mention are what you would expect, apart from the light loss, which is likely to be nearer 1.5 than 2 or 3 stops.

Are you sure you're using it with the sun at right-angles to the angle of vision? That is when it is most effective.
 
Thanks for your reply, Jeremy.

In the 'bluish' position, I get for example f7.1. In the 'golden' position, I get f8. With the filter off, I get f9 or f10. Maybe the f10 was due to change of position.

I'm trying to use the filter at 'rightish' angles as much as possible.

I wonder, which is the 'right' position for optimal use of the filter? The 'blueish' position or the 'golden' position? Or should they be used for different purposes, and if yes, for which purposes? And if for example the golden position is 0°, and the blueish position is 90°, will then 180° and 270° deliver yet again different results, or will 180° be identical to 0°, and 270° identical to 90°?

Apologies for asking so much, maybe at the right time of day and with the right scenery and angles, all these questions would answer themselves, but right now, I feel lost and would like to find a starting point - I can't even decide which position to use for what? The 'golden' position will give me the results the filter has been recommended for (richer colours, less reflections etc.), but will this always be the best position? Or is sometimes 90° or 180° the better choice? And if so, then when?

Apologies again for all the questions, and thanks for your help!
 
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All you need to do is go out into the landscape, and point your lens at 90 degrees to the sun. Look through the viewfinder and rotate the polariser. You should see the image change as you do so.

I find that a polariser doesn't give much of a result - or at least not a very pleasant one - when the sun is very low in the sky. I always reckon about 2 or 3 hours before sunset/after sunrise is optimal, in my experience. But really it shouldn't be necessary to analyse things that carefully.
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The blue-ish v warm-ish aspect of the filter that you have noted, while definitely there, is relatively unimportant. You should notice a far greater effect when you rotate it.

Otherwise I don't know what else to suggest.
 
Just try it out held in front of your eye rather than through the camera to get a better idea of what it is doing.


Steve.
 
Polarisers are all about angles, and rotation. But they don't do anything unless there is polarised light in the scene. Blue skies are polarised (overcast are not) and reflections are also polarised (all except off metal).

For blue skies and the 90 degrees to the sun thing, make a 'gun' with forefinger and thumb at right angles. Point at the sun and rotate your hand around the finger. Your thumb will point to an arc in the sky which is the zone of max polarisation, ie where the darkest blue will be.

From this, it follows that if you're shooting a sunset, the zone of polarisation will be above and behind you, ie no benefit. It also follows that if the sun is directly above, then the zone will run a full 360 degrees around the horizon. All other positions will be somewhere inbetween. Rotate the filter for best effect - it's easy to see through the viewfinder.

Exposures will vary according to how much polarised light the filter is cutting out, though even on an overcast day you'll usually lose 1.7-ish stops, depending on the filter.

The other angle to know is Brewster's Angle for reflections, between 30 and 40 degrees to the surface. At this angle, and with correct filter rotation, reflections will disappear almost completely.

Try this with a dark shiny car, with plenty of reflections. Stand 3/4 to the front, hold the filter to your eye (make sure it's the right way round!*) and rotate. At some point, reflections on the bonnet and roof will go, then turn the filter another 90 degrees and reflections along the side will go, and those on the bonnet return. Turn it another 90 degrees and the bonnet reflections will go again - the effect at zero degrees and 180 degrees rotation is the same. Now, with reflections reduced along the side, walk back and forth towards the car and note how the reduced reflection moves with you - at Brewster's Angle.

*It's possible that your filter has been fitted in the mount back to front. Check it by looking at your computer screen - when the filter is the correct way, the screen will go black as you rotate. When it's the wrong way, there will be hardly any change on rotation.
 
f/7.1 to f/10 is ~ 1stop. If your camera is set up for 1/3 stop adjustments then every three aperture adjustments =1 stop. You'll also see two small tick marks between the large ones on the meter. The large tick marks are 1 stop apart.
 
Thanks a lot for all this valuable advice. I had indeed tried it in the later afternoon, I think it was around 4pm.

I had also tried to look through the filter without the camera, and saw a difference according to the angle of rotation. But I had expected a stronger effect both with or without the camera. I have to try the filter around mid-day.

Thanks also for the info regarding the f-stop calculation. I had not been aware of this. I thought one step in the aperture is called one stop. So indeed my CPL filter only costs me 1 stop maximum.

I'll also do the screen test to see if the filter maybe has been mounted the wrong way around.

I probably just have to learn when using this filter improves the image, and when it doesn't.

Thanks again for all this valuable advice!
 
As above. Polarisation is one of the things you can't do in post processing, so it's worth persevering with it. Some people swear by these filters, others are less enthusiastic. I'm somewhere in the middle. I have one, but use it fairly selectively. It's just another tool, to help you achieve certain results, and you'll soon find what works for you.
 
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