CPL's cause blurry image?

USDashCamera

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So I was posting about this on the blackvue section. I realized my 550 had issues looking blurry right around the time I added the CPL magfilter to it. then my new 650 seemed great but then sometimes it was blurry. i started to wonder what caused it... was there some auto focus feature that was focusing on my dash board?

then i thought it must be the CPL, so i take it off and just like that i get a much clearer image again.

did some googling and many photographers have mentioned CPL's especially cheaper ones can cause a blurry image with fast motion.

without the CPL i can pause a car going 60 mph (30 + 30 opposite way) with my 650 and read the plates, but with the CPL its just a blurry mess.

i had never seen anyone mention this before, but it seems to be why my blackvues were blurry.
 
yes it's true adding a CPL may reduce picture quality at times. (trade off) Here is one recent example. Blackvue has low bit rate to start with so double whammy.
 
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Depending on the CPL if you look through it you can see how much darker they're making the picture to start off with - to compensate the camera has to keep the exposure open longer for each frame to get it bright which means that stuff up close (like numberplates) can just be a blur.

It's why I got that Hoya HD filter to try out - it's not as dark as other CPLs so the camera behind it has an easier job and doesn't lose as many stops.

I got good results with the Hoya on the G90 and the LS460W - it was just a big thing to have hanging in the window and it made the already bad G90 mount worse. I've not tried it with the new one though as I have the decimate now. I used the same filter on holiday with my DSLR and the difference was great with no obvious loss of detail on a 24Mp sensor.

I've noticed the latest G90 firmware seems to have issues anyway with this sort of thing in that it's trying to keep the picture bright, again at the expense of close up detail like passing numberplates. I'm thinking of dropping back a firmware version to see if it's a new thing as the older ones were darker but you got the detail.

I don't know what size filter those use but if you could get a Hoya HD filter to fit on it you might find it works a bit better with that Sony sensor in there as long as exposure times don't go up too high.
 
A good vertical polariser filter, which is all that is needed, should reduce the brightness by slightly over 1 stop so motion blur should be doubled - it is removing the horizontally polarised half the light, a circular polariser has an extra stage which will loose a little more light so motion blur for a CPL will be a little more than doubled. A poor quality one may take an extra half stop.

So you have a compromise of loosing readability due to reflections off the glass reducing contrast or loosing readability due to extra motion blur. In good lighting the CPL will win, at low speed the CPL will win, in poor lighting and at high speed no CPL will win. A vertical polariser would be ideal but good ones are not so easy to find.
 
A vertical polariser would be ideal but good ones are not so easy to find.
Have you ever tried such a filter? What are the results compared to, for example VIOFO A129's CPL filter?
 
Have you ever tried such a filter? What are the results compared to, for example VIOFO A129's CPL filter?
I've never tried a side by side comparison, but I don't think there is much difference, and it may depend on which version of the Viofo filter you have since some seem to have better anti-reflection coatings than others.

The images in this thread where all using a non-circular polariser: https://dashcamtalk.com/forum/threads/a129-under-trees-with-some-big-frame-grabs.34305
 
I came across a Hoya 37mm HD CPL on Amazon selling at half the usual price due to an "Open Box" deal. It has a filter factor of 2 which is only a +1 stop exposure penalty. The filter has 8 layers of anti-reflective multi-coating helping to prevent internal ghosting and reflections and protects the lens from dirt and oil, which I have noticed. Using a 28mm to 37mm step up ring I fit it to a Mobius 1 camera with a 6-22mm ƒ/1.6 varifocal lens and the results are truly impressive compared to the cheap generic CPL I had on the lens previously. It was a great find!

There have been three iterations of these remarkable "HD" Hoya CPLs to date, the HD, HD2 and HD3.

The latest HD3 Hoya CPL is an amazing piece of optical glass technology. It offers light transmission of 92% with a filter factor of 1.25x which reduces exposure by merely +0.3 stops! It has 16 layers of a special nano coating, 8 per side. Unfortunately, these only come as small as 49mm and they are quite expensive.

Edit: Apparently a 37mm HD3 CPL filter is listed at B&H photo and elsewhere but so far has never been in stock. Perhaps they can be special ordered?
 
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It offers light transmission of 92%
Does that mean that it only removes 16% of the reflection?

I want a filter that removes 100% of the reflection, so I expect light transmission of 50% at best...
 
Does that mean that it only removes 16% of the reflection?

I want a filter that removes 100% of the reflection, so I expect light transmission of 50% at best...

16% of the reflection?, No, hardly.

Reports are that the Hoya HD3 polarizer works marginally but noticeably better at removing reflections than either the HD or the HD2 which are both superb.
These are among the best CPLs on the market, if not THE best.
 
Hoya make mega filters - I had one with the LS460 abomination I made years ago (quite interesting looking back at those old photos from 5 years ago of the local area!)

https://dashcamtalk.com/forum/threads/hoya-hd-cpl-filter-for-dod-ls460w.6770/

I remember the wife was disappointed when I showed her with the Hoya HD CPL that holiday magazine photos use things like a CPL to get a completely different photo from what you see with your eyes - for example the Hoya CPL from the same spot at the beach from a holiday years ago:

4469544696

Those images are straight out the camera and I love how the CPL makes the sky and details from the sea really pop once the reflections are removed.

I think I'll have to invest in one of those HD3s for sure, I've still not picked up filters for the Mavic 2 Pro I bought last year (unfortunately work is too busy at the min for much playing!)
 
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