Polariser plus SLT - blotchy sky problem

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David Kilpatrick
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Polariser plus SLT - blotchy sky problem

Unread postby David Kilpatrick » Mon Jul 16, 2012 10:31 pm

I do not use polarisers often as I find they generally disagree with digital sensors and their cover glass/filters, if you know anything about polarised light you'll understand my concerns - the silicon medium itself has a surface which polarises incident light, and the cover glass/AA filter also polarises light or acts as a polarising filter.

But I do carry one, a very good high-end £100+ Hoya low density type, and when in the USA I used it on the Alpha 77 for a few shots in different locations where that very dramatic effect of deep blue sky would have worked well.

The pictures are barely usable - JPEGs would have been hopeless - as the polariser really screws up A77 WB and where it has a strong effect on an even blue sky, the result becomes blotchy with visible artefacts, a bit like looking through a car windscreen with a polariser.

I conclude that the semi-reflecting pellicle of the SLT, like all such pellicles, is interacting badly with polarised light even from a circular polariser although it should not do this.

Anyone else notice this problem with SLTs?

David

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Re: Polariser plus SLT - blotchy sky problem

Unread postby dewarp » Tue Jul 17, 2012 1:07 am

I cannot say that I've noticed anything like this. But then, I never use a polariser on a clear sky (can usually be easily adjusted in PhotoShop) but only when I want to improve the separation between clouds and hazy sky. I'll look a bit closer tonight.

regards - Peter

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Re: Polariser plus SLT - blotchy sky problem

Unread postby mikeriach » Tue Jul 17, 2012 12:22 pm

Not tried it on the A77 and anyway I've not seen any blue sky for a long time!
Will experiment as soon as conditions allow.

Mike
Only an A77II and Tamron 16-300 left sadly. Sony cannot supply what I want so rightly or wrongly I've branched out to someone who does and I'm very happy.

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Re: Polariser plus SLT - blotchy sky problem

Unread postby johnstra » Tue Jul 17, 2012 12:51 pm

This is a bit disturbing as I often use a polariser. They are not just for blue skies. They intensify the colours in foliage etc in the right conditions. I think I am using the same Hoya as David. It is the circular polarising type of course. My A77 is away for repair but I think that I have used this filter with it and have not noticed a problem. However, when I think how bad the weather has been since I bought the A77 I could be wrong but I do have some pics with blue skies that look as if I used it. If only it's use could be indicated in the metadata! I will try it as soon as I get the A77 back.
Bob Johnston

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Re: Polariser plus SLT - blotchy sky problem

Unread postby dewarp » Tue Jul 17, 2012 1:23 pm

I had a look at my images and cannot see the effect mentioned. Attached is one where there is an expanse of blue sky. Taken with a circular polariser, as confirmed by the vignetting of the corners. Raw file converted to jpg in Lightroom and shrunk further using Irfanview.

regards - Peter

1205200901.jpg
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Re: Polariser plus SLT - blotchy sky problem

Unread postby Dr. Harout » Tue Jul 17, 2012 9:27 pm

I will try 3 different polarizers: a cheap one used on cokin adapter, a Hoya ultra something 62mm and a Zeiss 72mm T* bought specially for my 24/2
I'll report any abnormalities or "normalities"...
A99 + a7rII + Sony, Zeiss, Minolta, Rokinon and M42 lenses

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Re: Polariser plus SLT - blotchy sky problem

Unread postby David Kilpatrick » Tue Jul 17, 2012 10:00 pm

Peter - I'm seeing some uneven patches in the sky on your shot. Mine lighter than this are OK, anything a bit darker and using say the Landscape colour setting or a Vivid conversion shows worse sky quality. I note with interest that it looks much worse on my iMac 27 than on my MacBook Pro, as if the calibration of the 27 inch screen is more critical about these deep blue tones and emphasises tone breaks.

Maybe I need a 10-bit colour screen...

David

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Re: Polariser plus SLT - blotchy sky problem

Unread postby dewarp » Wed Jul 18, 2012 1:00 am

Hi David

OK, I suppose it may be due to my working screen being an early 19 inch Philips LCD and not a modern professional job. However, I also have not noticed the effect on prints from my Epson 3880, which tends to show up things that I cannot see on my monitor. However, it is rare that I print an image with a clear blue sky, and even rarer that it would have been taken with a polarizer.

In theory it should be possible to record/plot the RGB values for a line across the screen and see if there are step changes. Can anyone out there do this easily?

Regards – Peter

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Re: Polariser plus SLT - blotchy sky problem

Unread postby Greg Beetham » Wed Jul 18, 2012 1:57 am

Peter the darker blue area of sky right of centre does show as having low contrast ragged edge or a blotchy edge on my monitor; I think it looks more like a processing/firmware/software/size-reduction associated snafu rather than a light control (mechanically induced) problem, (it’s not something that is glaringly obvious, but it is there).
Greg

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Re: Polariser plus SLT - blotchy sky problem

Unread postby dewarp » Wed Jul 18, 2012 3:54 am

I am happy to provide the ARW file to anyone who wants to do some pixel peeping.

regards - Peter

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Re: Polariser plus SLT - blotchy sky problem

Unread postby David Kilpatrick » Wed Jul 18, 2012 3:36 pm

I've processed a few more files now and I think it's a combination of the polariser, SLT, the behaviour of the A77 sensor, and difficulty of getting this depth of blue to render without tone breaks on any normal monitor.

One thing is certain, the polarised images are very difficult to process well and shots taken without a polariser on two other cameras often look much better :-(

David

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Re: Polariser plus SLT - blotchy sky problem

Unread postby Greg Beetham » Wed Jul 18, 2012 3:53 pm

David it’s not a quantisation type effect then?
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Re: Polariser plus SLT - blotchy sky problem

Unread postby David Kilpatrick » Wed Jul 18, 2012 6:57 pm

There are some artefacts which don't just look like a 6-bit screen (iMac!) view of the colour. Without access to a high end colour system I can't judge what might be caused by LUT quantisation, and what might be from the camera.

David

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Re: Polariser plus SLT - blotchy sky problem

Unread postby Lonnie Utah » Wed Jul 18, 2012 7:05 pm

My best guess is the blue patchiness in the image above is due to the focal length of the lens, and the angle of the sun in the sky. Given the shadows in the photo, the darkest patches are almost 90 deg from the sun, and that's where you'll see the strongest effect of a CPL in the sky.

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Re: Polariser plus SLT - blotchy sky problem

Unread postby Greg Beetham » Thu Jul 19, 2012 12:07 am

Sorry Peter for debauching your nice image, I added an adjustment layer with a clipping mask based on hue to see if it would highlight the loss of smooth transitions in the sky, (for some reason), one would almost think that the rough edges would be visible in the viewfinder if the camera was on a tripod (using digital zoom?) and the problem is related to the camera itself and not the processing afterwards.
Greg

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