Help me with a weird problem

Hi all. I am going to describe to you a weird problem that I encountered today and wonder if you can give me an explanation. It goes like this:
I have a Pentax A* 300mm which I was messing about with today. I decided to see what sort of results I'll get with a CPL filter, so I stick one on. Suddenly everything is out of focus. I thought it could be a bad filter but it works fine on other lenses. I then thought it might be due to the
'UV' filter that is on the lens. I removed that but it still is no good.
So who has any ideas. Could it be because of the coating on the lens? I'll await your response.

what camera are you using ? It happens only with the CPL filter or even without the filter the lens doesn't focus well ?

There are two types of polarizer: circular and linear. Linear polarizer are known to throw AF off with some lens/camera combo. For AF cameras, you should always use a circular polarizer. You should verify to make sure your polarizer is really a circular one. Beside that, I have no idea as to what could cause your problem.

If I understood well is a manual focus lens isn'it ?

Thanks
It is the K5 that I use. Its only with the CPL, otherwise the focus is perfect.
I have bought it as a CPL and it says so on the box. Can you tell me how to make sure it is what it says?

That's right Cintelli, it is manual focus.

Since your lens is manual focus, any polarizer will work with it, unless you use the AF hexagon to get focus confirmation. If it says on the packaging that it's a CPL, then I'd assume it is.

Very strange Naim... If it was an AF lens , sometimes It happens that reducing the contrast the AF system doesn't work properly and shots even when it is not perfectly focused. But focusing manually you might not have this problem , unless you are using a 1,7 af adapter or quick-focus shift option.
Set manual focus on the camera and use AF-lock . Use the depth of field techique to focus a wide area. If it is still blurry there must be something on the CPL and try to use a linear Polirizer on the non-Af lenses and keep the CPL for AF.

Ciao

Alberto

Do you mean the picture looks soft? If that's the case then it may be due to a low quality polarizer.

A picture would speak a thousand words here, I feel!

I haven't noticed any problems with using a LPL on AF lenses btw - but I admit I have not checked whether there's a problem with critical focus at large apertures etc.

The only thing I can think is that the reduction in light means you need a slower shutter speed, so causing more camera shake. This would not be quite so obvious on shorter focal length lenses.

The filter would have to be truly horrendous to make your images look so soft that you thought the focus was out, I think... and it would affect all lenses equally.

No guys when I say 'out of focus' I mean out focus, like totally blurred. When I first looked in the viewfinder I thought I had changed the diopter by accident. I haven't taken photos with it but I promise you the viewfinder is well out of focus. I'll take a photo tomorrow and then upload it but in the evening because I have to take my son to rugby festival that will last all day! O joy!

OK guys here are the shots.
This one is with the filter
[IMG]http://i376.photobucket.com/albums/oo209/Naim-Khan/NAIM9240.jpg[/IMG]
and the shot without the filter
[IMG]http://i376.photobucket.com/albums/oo209/Naim-Khan/NAIM9241.jpg[/IMG]

I hope you can appreciate the difference
Cheers

Naim

That looks like camera shake to me. Was the SR on? Maybe it's just too slow a shutter speed.

Regards

Definitely user error ie. camera shake. 1/60th second is way to slow for a lens of this focal length, you were lucky with the second one!

Probably the 2 stops or so lost through the filter causing
camera shake.

Even the 2nd shot should be sharper it's a top lens.

Edit: Just noticed they are both 1/60th and the iSO was changed. You need at least 1/300th with a 300mm lens?
It still looks like camera shake.