The shots all appear out of focus even though focus was good and depth of field was good, even though the filter was clean. As a guess, I removed the filter and the problem went away.
That probably indicates that you were using a linear polarizer like we used back before the invention of autofocus. AF doesn't work with linear polarizers. When AF came out, polarizer filters were modified with a quarter-wave plate placed behind the actual polarizing material to reorient the polarized light waves in a non-intuitive circular pattern. Thus, we started calling them circular polarizers. This allows AF to continue functioning when using a CPL.
Even though the very cheap AGFA filter you were using is supposed to be a CPL, it appears to have either a defective quarter wave plate placed behind the actual polarizing material or the quarter-wave plate is not oriented as required at an exact 45° angle to the orientation of the main polarizing material. Or maybe it is an out-and-out counterfeit that does not even have a quarter wave plate behind the polarizing material.
Roger Cicala at lensrentals.com did a survey of circular polarizer filters a while back and wrote a blog article about it: My Not Nearly Complete, But Rather Entertaining, Circular Polarizer Filter Article
He compared six different CPLs in 77mm diameter ranging in price from $102 to $200 purchased from a reputable major online seller (listed in alphabetical order).
- $102 - B&W XS-Pro High-Transmission Circular Polarizer MRC-Nano
- $200 - Heliopan Circular Polarizer
- $140 - Marumi EXUS Circular Polarizer Filter (EXUS is an acronym for a form of High-Transmission)
- $150 - Sigma Water Repellent Circular Polarizing Filter
- $103 - Tiffen Ultra Pol Circular Polarizing Filter
- $180 - Zeiss T* Circular Polarizing Filter
He found that all of them were at least 99.9% efficient at polarizing light. He couldn't say any were more efficient than that because 99.9% was the limit of his measuring setup.
He found that all of them were flat enough to not affect IQ more than any of the others. In his words, "They all passed with flying colors."
Where they differed was in how much light they let through when not polarizing light. In other words, he shone already polarized light through them with the filter turned to let as much of it as the filter could allow through. Here are the results from least to most transmissive:
- 55% - Tiffen ($103)
- 58% - Heliopan ($200)
- 66% - Zeiss ($180)
- 68% - Sigma ($150)
- 88% - B&W ($102)(HT)
- 91% - Marumi ($140)(HT)
Roger did point out that some shooters might actually want the ND effect of less transmission when using a polarizer, which is often used in bright sunlight. So it is not always necessarily true that more transmissive translates to better in terms of CPLs.
In terms of spectral response, the two high transmission CPLs (B&W and Marumi) had near identical graphs between 430-700nm, with a flat line from around 500-700nm and a drop off on the blue end of things with significant limiting of UV wavelengths.
The rest had curves similar to each other but different from the two high transmission filters. There was no UV cut or drop off in the blue sections, there was a slight rise through the green wavelengths, then a very modest dip from green to red before a slight rise to infrared.
Neither type had individual differences between color when oriented to block the most light and when turned to block the least.
Roger's first conclusion:
If you are buying a circular polarizing filter because you want some circular polarizing, it doesn’t seem to matter much which one you choose; they all polarize like gangbusters. So I saved you some money today.
He then went on to say:
The second point, one which I’ve been told before I did all this testing, is set the white balance after you put the CP filter on, not before. Because CP filters will have a color cast. Or just shoot in raw and fix it later, which is what we mostly do anyway.
The final major conclusion he drew was what he termed "the painful one":
I didn’t want to test filters; I really didn’t. But people wanted me to. So I chewed up my testing equipment budget to buy laser transmission stuff and an optical spectrometer, spent a few weeks getting everything calibrated and establishing norms, and then a couple of days testing these CP filters. I did this in clear violation of Roger’s Third Law: No Good Deed Goes Unpunished.
After I was done, I told Aaron I had just documented that CP filters had different light transmission percentages and different color casts. And that high transmission filters had one look, and it was different than regular CP filters, which all were really similar. Because I was proud that my investment in time and money had paid off.
Aaron took the filters from me, put them on a piece of paper, took this picture with his cell phone, and said, “Yeah, you’re right.”
Later on Roger followed up with another post in which he tested a couple of cheaper CPLS:
- $35 - Tiffen 77mm Circular Polarizer
- $45 - Hoya 77mm HRT Circular Polarizer UV (ostensibly a High Transmissive filter)
And he measured the following:
- 38% - Tiffen CP with a spectra very similar to the four non-HT filters above.
- 53% - Hoya HRT with a spectra very similar to the two HT filters above.
He observed that the lower transmision seemed to be related to the lack of anti-reflective coatings on the cheaper filters. They were also as flat as his test could measure, just like the first six filters.
His final conclusion:
... There’s not much question that as far as polarizing light, cheap CP filters do it very well. They also, as you would expect from uncoated or partially coated filters, reflect a LOT more light. This is a significant issue on a clear or UV protection filter. I’m honestly not confident if it’s as big of a deal for a polarizing filter.