strange filter problem

I recently went on an outing that I knew would involve multiple shots near water and I also knew that reflections would be a major issue. I was shooting with a relative's 100-400 L, and I didn't have a 77mm circular polarizer around. I went to my local camera store and picked one up. They were all out of Hoyas and I really wanted to have one with me, so I got the Sunpak that they had in that size. I'm shooting and sure enought, glare off the water is a problem. I put the CP on and the camera says it achieves focus lock, but the image is out of focus in the viewfinder . I then tried to manually focus and could not get a sharp image either in the viewfinder, or after the shot on the LCD. Take the filter off, everything is fine : . I came home, tried the look-through-the-filter-in-the-mirror trick, and sure enough it is a CP (not linear). I don't have another 77mm lens here (my 24-70 should be on tomorrow's UPS truck), so the best I can do is hold the filter in front of a 72mm lens, and I have no focus issues. Has anyone ever heard of anything like this before? My next step, assuming I have no trouble with other 77mm lenses, is to try a different CP on the 100-400. Is it possible that there is some issue with the lens, or is there something with longer focal lengths and CP that I don't know about?

It could easily be down to the light loss through the polariser, noting that a polariser loses on average 1.5 stops. Assuming the Sunpak is poor and gives you 2 stops light loss, then the lens's aperture reduces to f8-11. Your autofocus doesn't works at f8, let alone at f11, according to Canon. In practise you may find that autofocus may work at f8 providing there is sufficient light and colour contrasts in your scene.

You might want to try one of the top end polarisers such as the Lee Filters, Hoya Pro1 or the B+W HMC, trying to minimise the light loss.

Were you shooting into the sun? Did you have the polarizer stacked on a UV filter? One problem with lower-end non-coated filters is that they are more subject to flare. I imagine that if the light is bouncing around between two stacked filters (or even between lens and single non-coated filter,) it could introduce focus problems - even manual focus - especially if you are shooting into the sun. The specular highlights from the water could also do that.

I happen to use Sunpak CPs and love them - have never had any similar problems, but that doesn't mean it can't happen.

Same exact thing happened to me with a generic CP that I bought at a local shop (maybe it was the same one). Any thing above 200 MM would show the problem. 200 MM or less no problem. I returned it and ordered a Hoya from B&H. Should have it on Tuesday. I wrote about my issue here:

https://photography-on-the.net/forum/showthre​ad.php?t=53429

I understand that, but it was impossible to acheive accurate focus even manually. The problem with the autofocus was simply that when it did lock, it was significantly out of focus, but this was the best that could be acheived with any manual adjustemnt from that point.

Not directly into the sun. There were specular highlights off the water at times, but the same things happens when testing the combo indoors or outside in cloudy weather. I also tried with and without a UV filter thinking maybe that was the problem. It may very well be that the sunpak is not coated and causes some strange light reflections off the surface of the coated L lens... We'll try when my 24-70 gets here today, and then we'll go the camera store and see where we end up.

Read my post above. It will work on you 24-70 but you don't want to keep that filter. It will not work at 200 MM and above if it is anything like mine.

OviV, that is very similar to what I have found. I have some blurriness even at 100mm though. I too have taken the same picture using the same settings with the camera mounted on a tripod and the CP definietly makes the difference. I'll be taking mine back today, so we'll see how I fare with another brand. Glad to see I'm not alone though

Any filter can mess up autofocus.. it is not likely but it does happen...

...as for a cheap polarizer.. it seems this is a recipe for disaster... also are you sure yours was "circular" (and by that we don't mean "round")

It seems to me that this question has answered itself from the beginning...

You purchased a product that you were unsure of the quality of,.. tried it and it failed miserably..

Now you know.. it's a bad filter

Return if you can.

The 100-400L has reported an aperture to the camera, depending on its focal length, and then you have gone and lost at least one stop. The 100-400 isn't the best lens of the bunch for low-light work and in anything less than really bright sunlight a CPL will kill the AF.

You will find it won't effect the AF with a 24-70 or 70-200 2.8 where losing a stop isn't a big deal. I have had exactly the problem you describe, even with the best B+W CPL that you can get on the 100-400L. It would sort of work on very bright days, but was too unpredictable, so I never bother with a CPL on that lens.

Kenny,

I don't believe that the extra stop is what causes the problem at least in my case. I was unable to get "focus" even manually. I believe it was focused but the CP was creating the weird effect. It was visible through the viewfinder and looked like a double vision effect.

Ovi

This is exactly my point. I completely understand the loss of 1 stop and the effect that this has on the AF, but it is also not possible to acheive any better focus manually.

CyberDyneSystems, as I said in my original post - I verified that it was, infact, a CP and not a linear with the test that is outlined here.

UPDATE: So I took the lens and the filter to the camera store today along with a few example shots. The guy who owns it is a friend of mine and he had never really seen anything like this. Two things we found were interesting to note:

1. Another sunpak CP on my 100-400 produced tack sharp results using both AF and manual.

2. The problem filter on a new 300mm 2.8L that was in the store produced the same out-of-focus images.

His best guess is that there was some problem with the QC of the filter- either the glass itself or the coatings. He took the lot number and the order date and will send the sample shots and the filter back to Sunpak. Either way, the new one works well...

I agree with him.
I had a similar problem with my 75-300 with a cheap UV filter. The camera would achieve focus lock but the recorded image was way out. Took the filter off and the image was sharp.
Solution don't buy cheap filters.

"The problem filter on a new 300mm 2.8L that was in the store produced the same out-of-focus images."

How did you fit a 77mm diameter filter onto a 300/2.8L - now that is a neat trick........

You know, I thought about that as I was typing b/c if I remember correctly the 300 2.8 uses a 52 or 58mm filter and I think that I remember hearing that you needed to use a square filter or holder for that lens. I was not outside while he was testing the different lenses and filters (I was inside looking at a few enlargement that were ready) so I can only assume that he held the filter in front of the lens and had the same results. Next time I'm there I'll ask for sure.