Polarizing filter...

[aroy]

Stupid question...

Could some cheap filter make my photos soft, and unsharp?

YES. Bad filters can have all sorts of effects. It is after all a glass/laminate and if the medium is defective it will affect the image. Normal UV filters from Hoya cost around $10-20, so there is no use going for cheaper filters. Polarizing filters cost a bit more, but they are still below $75 for most sizes. As long as the filters are of good quality and kept clean they have minimal impact on the image.

Now there is an interesting class of filters called "Softars". They were used in the film era to soften up portraits, but are rarely used in digital age, as software can do that for you (and more).

 

[Horoscope Fish]

OK, here is the ZIP file with 3 folders:

18-105 Bad
18-105 Good
35mm

https://drive.google.com/file/d/0Bz565Iyy7PBXcDNkT2d1X2l6Sk0/edit?usp=sharing

This "Bad" folder contains 4 photos, two photos with, and two without polarizer, at 18mm and 50mm, all with f8. I named it "Bad" because I did not screwed the filter to the end, so it was loose. The folder named "Good" contains the same, but this time I screwed the filter to the end hard, so it was on it's place. The third folder is named "35mm", and it is made with 35mm f1.8 G with some cheap Baner polarizer, just two files "with", and "Without" filter.

I would like to hear your opinions, how much my cheap filters are affecting IQ.

What I notice in looking at the shots in both "Good" and "Bad" folders is that shots taken with your 35mm appear sharp, both with and without the filter attached. The rest of your images have what look like focus issues issues to me. Below are two shots, both taken at 18mm, and both appear out of focus to me. I don't mean "soft", I mean out of focus. I don't think this is a filter issue, I have to think this is a technique issue.



....


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[alfaholic]

I see that 35mm looks sharper, but I also see that both lenses looks sharper with filter attached. They look sharper to my eyes, or maybe there is less light reflection, I do not know.


As for those two shots, each one is made with a tripod, f8 aperture, so the only thing that can cause this is maybe because I forgot to turn off VR.
I will make another two shots without VR.

 

[alfaholic]

Here you have 4 photos, two with, and two without polarizer, again 18 and 50 milimeters, f8, all with VR off:

https://drive.google.com/file/d/0Bz565Iyy7PBXS1NKbXIwbGhzcHM/edit?usp=sharing

Again, the ones with polarizer looks a bit sharper to my eyes..

 

[Horoscope Fish]

I see that 35mm looks sharper, but I also see that both lenses looks sharper with filter attached. They look sharper to my eyes, or maybe there is less light reflection, I do not know.

I agree your polarizer shots look better. I also think there's a lot more reflective light than what we're aware of and it's removal does nothing but enhance a shot as far as I can tell. It's why I have a CPL for every lens I own and rarely shoot without one.

As for those two shots, each one is made with a tripod, f8 aperture, so the only thing that can cause this is maybe because I forgot to turn off VR.
I will make another two shots without VR.

I don't know what the issue could be but there's a very significant lack of sharpness there. Are you using Auto-focus or are you focusing manually? I would definitely make sure VR is turned off and maybe use a wireless release, or the self-timer to delay the shot a few seconds, in case you're vibrating things by pressing the shutter button. I say this because what I saw just doesn't look like a DOF issue to me; it looks more like front/back focus to my eye.

If anyone else has an opinion I'd sure like to hear it!

....

 

[alfaholic]

All photos are made with a tripod, camera set to self timer, auto-focus, f8 aperture, so there is no vibration, and I think there can not be so much back/front focus while using f8, or I am missing something.

Look at this photo:

https://drive.google.com/file/d/0Bz565Iyy7PBXeGFzNWdiOFduczQ/edit?usp=sharing

It is 50% of the actual size, f8, I focused 2 or 3 meters from my position, so everything should be in focus to infinity.
If you look bottom of the photo, I cropped it so that is the point I focused, and if you look at the rest of the image where you have things 5, 10, 100, and 1000 meters away, you will notice that nothing looks sharp, so if it is back/front focusing issue, something must be in focus, but at f8 and 18mm it is impossible to have everything out of focus, even if lens is back/front focusing. Please correct me if I am wrong...

So, you think the last photos are still not sharp enough, the ones I made with VR turned off?

 

[Jules]

Well looking at these I tend to agree with Horoscope Fish here.
The polariser shots look sharper because they have cut out the reflections, thus making them appear sharper, you'll need to get the last link in the chain checked out here which is your D7000 if you didn't already when you got your lenses calibrated, with the 18mm F8 shot, if nothing is in focus you could be looking at a misaligned sensor/mirror, it only takes a fraction to throw these things out, as you say at 18mm and F8 something should be sharp!
You used Auto focus, single point? Hopefully not group on the coin shots, basic I know but I have to ask to get a clear indication of what was going on, I don't think it's the filters but something else, if it's not the AF it's likely the mirror needs calibrated but it's a cart and horse thing, ones no good without the other, so to speak.
You need to eliminate the D7000 as the faulty element, can you borrow a body and try your lenses on that?
Pretend to be buying and shoot a card full on your lenses in a shop to compare maybe?
If your lenses are within spec and it seems they are, it has to be the body, leaving VR on rarely causes any severe problems on a tripod in my experience. I forget all the time and sell Landscapes two feet wide taken that way...

 

[alfaholic]

Thank you.

The body was checked few months a go, at the same time my lenses are calibrated. D7000 works perfectly with my 35mm f1.8 G now.

I made some more tests, and everything is OK, the problem with my beach photos can be camera shake as well. Polarizer lowered my shutter speed down so at 1/200 it is possible to shake the camera a little bit.

As for Polarizing filter, it is possible that I did not screwed it well, which turned out to be the problem while testing. After I screwed the filter very hard, and had trouble to take it off few months a go, I just turn one or two circles, so I can take it off easily.

Of course, VR can cause the same problems with exposures longer than one second, so it must be turned off to eliminate softness in ling exposures. That happened to my coin photos in "Bad" folder.

Thank you all for your help.

 

[STM]

Any filter adds two glass surfaces to a lens. A circular polarizer puts FOUR surfaces in front of your lens. If the two surfaces of each piece of glass are not perfectly plano-parallel then it can cause distortion. If the two polarizers are not perfectly parallel that too can cause distortion. The bottom line is DON'T GO CHEAP.

 

[Blade Canyon]

A circular polarizer puts FOUR surfaces in front of your lens. If the two surfaces of each piece of glass are not perfectly plano-parallel then it can cause distortion. If the two polarizers are not perfectly parallel that too can cause distortion.

A circular polarizer is just one piece of glass. You're probably thinking of a variable neutral density filter, which is two circular polarizers stacked together.

 

[Eyelight]

Been studying on this question and doing a little reading.

1st thought is, need to make sure the filter in use is a CP and not a linear polarizer. Linear will confuse the AF.

If it is a linear filter, there is a note on Wikipedia's page that mentions a linear polarizer can defeat the action of the anti-aliasing filter. Never heard this before, but it kind of makes sense and it would mean some pics with the filter could be sharper.

As fas as focusing, if the focal point is close to the camera (3 meters), a shift toward the camera of 1 meter has dramatic effect on DOF and 2 meters will wipe out most of the background focus. So, if the filter is confusing the AF, this could explain why nothing is in focus.

A tripod set in sand is better than handheld, but not as steady as one set on solid earth.