Bad Focus With CPL Mounted

[BazzaBoy]

I have noticed that whenever I mount a CPL on my Tamron 28-300mm lens (on a Canon 6D), the focus goes a little haywire. Same thing happens when I mount the UV filter too. While the UV filter is a Hoya, I must admit that the CPL is a cheap ebay purchase.

Before I spend good money on a brand name (e.g. Singh Ray), can someone advise me as to why this happens? Is there a solution with my current combination, or do I have to go in for a brand name filter.

I have read many forums on the web. Many people have the same problem, but no one seems to give a reasonable explanation as to the cause, or offer a good solution.

Any help appreciated.

[J.davis]

I had a CPL focus problem, turned out that when I rotated the filter it moved the lens slightly.
Not a great deal, but enough to move the focus point a bit.
Was a second hand lens though.

[arthurking83]

I have noticed that whenever I mount a CPL on my Tamron 28-300mm lens (on a Canon 6D), the focus goes a little haywire. Same thing happens when I mount the UV filter too. While the UV filter is a Hoya, I must admit that the CPL is a cheap ebay purchase......

What type of lighting conditions?
What type of target(s) are you focusing on?
What focal length are you experiencing issues at(if you haven't positively tested various focal lengths)

Lastly(but almost unlikely given your post about the issues with the UV filter too), are you sure it's a CPL and not a standard(linear) polariser?
The way to check this is to use a mirror.
With a Circular polariser, if you look through one(imagine using it as an eye piece) at a mirror, from one side it can be totally blackened but flip it the other way and it will be translucent, slightly darkened only, but you can still see through it.
If it was a Linear polariser type you don't get the fully darkened flip side .. it's translucent if viewed from either side.

Linear polarisers are known to cause issues with AF systems(not that I've ever experienced that myself tho).

I don't this lens by experience, but the only possible explanation I could think of is that the focus issue could be due to the f/6.3 aperture at some of the longer focal lengths in the zoom range.
(hence the focal lengths question).

If the issue is the same irrespective of the focal length chosen .. I really have no idea other than maybe a hazy filter(s) or some other weird reasoning
If it were at longer focal lengths (where the aperture is starting to close up on you .. say to f/5.6 and smaller) then it could have an impact on focusing.

The best way to assess if the cheap ebay CPL filter is of any worth is to physically test it yourself.
Easy to do and pretty quick.
Just take a photo of a static subject with good detail in it making sure to use a tripod and good light so shutter speed isn't going to cause variable issues. If the lens has VC, turn that off.
One shot without CPL and one shot with CPL on. Compare the images side by side to see if there is TOO MUCH variation between each respective area(s) in the image.
If you don't have software to do live side by side comparisons, the free FSViewer can do that.
It's reasonable to expect a tiny amount of variation between filtered and unfiltered comparison images.
A good filter will simply keep those variations to a minimum.

[ricktas]

Any thing you put between your lens and the subject is going to affect the results. There have been UV filter debates for years on how they quite often soften images, with great examples of how adding extra elements on top of the lens, degrade the image quality. Try stacking 3-4 UV filters and it becomes ridiculously obvious. So whilst one filter may not affect the result that much, it does affect it.

Better quality CPL filters do actually result in less image quality degradation. Whether your particular filter is bad, or not, is only something you can decide. AK ^ also gave you some great reading and ideas in his post.

PS: welcome to AP!

[BazzaBoy]

Thanks arthurking83 and ricktas for your valuable inputs.

I did try out the mirror test. Translucent from one side and completely opaque when flipped over. So, it is a CPL alright. I will try the test and compare images side-by-side.

So, I guess, I am resigned to the fact that I need to go in for a better CPL.

Thanks again.

[arthurking83]

....

So, I guess, I am resigned to the fact that I need to go in for a better CPL.

Thanks again.

So the fact that it's circular means that it doesn't have any effect on the AF system.
So that's a good start point.

The next question is, does it have the same haywire focusing issue at 28mm as it does say at 300mm too?
So what you want to do is try to focus on some point that will completely cover the focus point area at 28mm.
For example, take a medium sized subject with a well defined pattern on it and focus on that first.
Obviously try both without the CPL first, then you want to try with the CPL.
You'll need to be in good light too tho.

As a rough example: flyscreens can work well, or exterior sunshades/blinds .. or whatever .. a paling fence .. etc, etc.
The idea is that you want a regular pattern on the subject that the focus point can clearly see ... BUT!!(very importantly) no messy cluttered background to 'fool' the AF system.

** If there's a chance that the background may distract the AF system, to the point where the AF system may try to focus on the background instead of the subject, then you're not really testing any but the camera's ability to determine subject from background, and the speed at which it reacts.

The issue with this lens is that as you zoom in to longer focal lengths, the variable nature of the aperture and the fact that it gets to smaller than f/5.6 could be part of the issue.
(you haven't defined any focal length specific info on the issue, so we assume that the lens in an overall sense focuses erratically with filters on it).
If it focuses ok-ish at 28mm but then goes erratic at 300mm, then the most obvious answer is the aperture issue.

In that case you may have trouble with any CPL with that camera lens combo.

FWIW: I wouldn't over spend on a CPL.(been there done that)
I have a few CPL's (and single Linear polariser too), but I really don't find that the higher priced .. supposed higher quality models gives you enough extra quality in your images to warrant it's extra expense.
I once paid way too far over the odds for a Super (Duper) Hoya multi-something-or-other uber high end CPL, and it gave me nothing extra in return for the nearly 3x price tag to what I already had.
Some time later I also got another super high quality CPL model, this time from Kenko. This model was their (current at the time) high end Zeta model.
The only reason I bought it was that the sales guy at the photo show had no idea on what he was actually selling me. 82mm Zeta retails for between $250-300 .. and you can't find a medium good model for less than $100. I'd have committed myself if I didn't steal .. err buy it from him
Is it any better in terms of overall quality to some of the cheaper ones I have? not really!

I did have a very low grade Hoya which I ended up throwing out one day as it just fell apart on me once.
IQ was OK. All other aspects were fine too .. focus, colour, transmission, etc. Just that the rotating ring kep losing it's retaining ring, and my hands would get covered in some greasy goop, and I'd have this metallic ring in my hand sometimes whilst trying to simply rotate the filter!
The last time I used it, it was just easier to throw it in a bin than to fluff about trying to get that retaining ring back in again.

I reckon in terms of value for money the Marumi DHG Super would be about as good as you can get. I need a 77mm CPL one day soon and one of those is on my to get list.
I've seen on Marumi's site that they have a new top of the line model filter that allows you to wet yourself on the filter, or even do some line art drawings with texta's and it cleans off very nicely.
If I ever have any more kids in the foreseeable future I may consider one of them instead, but I don't wet myself on my filters, nor do I impersonate Mr Squiggle with them either.