poor focus when using circular polarised filter on EF 100-400 IS II at 400mm zoom

[Sunbird24]

I recently purchased a K&F concept Nano-K 77mm CPL filter to used with my EF 100-400 IS II at 400mm zoom attached to my EOS 6DII body and the first test shots were with it at 400mm but all were out of focus. I then did some online searching but came up with mixed results and answers so did some more testing using different focal lengths and some with an EOS70D body. In all cases at 400mm the focus is out but as I reduced the zoom it appears to improve the focus. Some say the autofocus does not work with F/8 and above and the CPL filters lose between 1 and 2 stops effectively making the lens somewhere between F/8 and F/11. Up until now I have only used UV filters for protection and since a lot of my subjects are at a distance and moving my standard settings are 1/1600th @ F/11 in good lighting and 1/800th @ F/8 and I reduced these by one stop or more for the testing. For the past several years I have never had focussing problems caused by the equipment.

Is this a limitation of this particular lens?

Might it be improved by a software upgrade?

Does this happen with other brands of CPL filters?

Am I pushing the limits a stop too far?

I am debating with myself whether to return the filter or to retain it for some use later.

[Tronhard]

Hi and welcome to the forum:

To me, logic suggests that if the lens worked well without the filter then the lens is not the issue - a flawed piece of glass in the system will undermine any lens.  My suspicions therefore fall on the filter. 

Could it be that it has a fault in the manufacture?   I have used polarizing filters on this and other lenses without any issue: they were Hoya, Marumi and B+W brands.   It seems to me that the logical thing to do is test the lens with a different filter and see if that improves.

[Sunbird24]

Thanks Trevor for the welcome and input. I updated the firmware on both lens and camera and it appears to be a lot better but its getting dark so using a much higher ISO. I will check again tomorrow morning when conditions should be the same as this morning.

[Waddizzle]

I ran into the same issue with the lens and camera.  That lens does not like CPL filters when used with camera that use Phase Detect AF, which is what is used in the 6D Mark II.  My advice is to forget about using the CPL filter with any lens when shooting with the AF 6D2.

If you wish to use it, then use it on a tripod for shooting scenarios like landscapes.  Auto Focus without the filter.  Swith the lens to MF, install and adjust the CPL filter, and then take the shot.

[jrhoffman75 · Accepted solution]

Welcome. You have performed thorough testing. I believe you have identified the problem; it is the light loss associated with the CPL filter. As you reduce the zoom length you increase the aperture. When you reach a certain point the light becomes enough to allow AF. The 100-400 zoom is a great lens, but it is aperture limited. It barely tolerates the Canon 1.4X teleconverter at 400mm (which puts it at f/8 equivalent) on my 1D  X Mark III. I don’t know that other brands of filter would have any better light transmission.