Nalu Loc: Southern Arizona

The polarizer should not be the determining factor, just the extender. Depending on what body you have, you should be able to AF, but probably only with a single point.

leftj Loc: Texas

Well that didn't answer the question.

AzPicLady Loc: Behind the camera!

I have never used an extender, but I nearly always use a polarizer. It does not affect autofocus. I have heard that some extenders do and some don't, so you would need to check yours to see about that. I have 3 Canons, and they autofocus at any f-stop.

oldtigger Loc: Roanoke Virginia-USA

Mount a fixed aperture F16 lens on your canons and i seriously doubt any of them will auto focus.

jeep_daddy Loc: Prescott AZ

You should be fine. The camera doesn't know the filter is there.

oldtigger Loc: Roanoke Virginia-USA

Most people buy filters because they want to change what the camera records.
If the filter did not change what the camera sees, it would be pointless to buy the filter.

amfoto1 Loc: San Jose, Calif. USA

The camera will TRY to autofocus....

It might struggle and focus slower or "hunt" a lot due to the 1 to 2 stops light "lost" to the filter (depending upon how the filter is adjusted). Just how much it struggles, hunts or slows depends a lot on ambient light conditions, subject contrast and other external factors... but at least it will try to AF.

You didn't mention the specific camera and that can make a difference. Canon 7DII is restricted to center AF point only, with any f/8 lens & teleconverter combo. 5D Mark III can AF an f/8 combo with a small group of points at the center (4 or 5, if I recall correctly). 80D is limited to some 27 of it's AF points (sometimes less, depending upon the lens). Only 5DIV and 1DX Mark II can AF an f/8 combo at all AF points (61 of them, in both cases). I don't know the exact f/8 capabilities of 5Ds-series, original 1DX and earlier 1Ds/1D-series, but they all have some limitations I'm sure. Most or all other Canon models are "f/5.6 limited" (such as an f/4 lens with 1.4X or an f2.8 lens with 2X).

Canon cameras are designed and programmed to disable autofocus whenever they detect a lens/TC combo that exceeds their focus system rating. So, for example, with any of the "f/8 capable" models you can use a 1.4X on that lens. But the camera would automatically disable AF if you attached a 2X TC to the same lens, making for an effective f/11 combo. The camera isn't making this decision by evaluating the actual light available through the lens... it just decides whether to allow AF to try or to turn it off based upon the lens/TC combo recognized.

In fact, it's possible to tape up a couple of the contacts on the teleconverter so that the camera doesn't "see" it and will try to autofocus, even when the combo exceeds the camera's rated capabilities. Result will vary wildly, depending upon all the ambient conditions and just how dark the lens/TC combo is.... viewfinder will be darker and EXIF focal length data will be incorrect... but under good conditions it may be able to autofocus. If interested in trying this trick, Google for more info. I'm sure I've seen web sites that illustrate which electronic contacts to tape over.

On another note, I rarely use a C-Pol on telephoto lenses. Funny thing, when I bought my 100-400mm II lens the retailer threw in a free 77mm C-Pol.... which I don't recall ever using on the 100-400mm but have used quite a bit on other lenses... mostly normal to wide angles. I also have a drop-in C-Pol for use with a couple super telephotos, that hasn't seen very much use over the past 15 years (probably could have saved the money spent on it!).

Also, several manufacturers are now offering "HT" or "high transmissive" C-Pols. These aren't as dark and don't "cost" as much light as the standard type of C-Pol, but seem to give about the same polarization results. Might be worth considering (I know the top of the line B+W "Kaesemann" is now HT only... they've phased out their standard strength).

oldtigger Loc: Roanoke Virginia-USA

My nikon D810 forces a reduction in the number of AF sensor sites when it detects a TC on the lens.

jeep_daddy Loc: Prescott AZ

Really? You can't figure out what I meant by my remark? I'll tell you "the camera doesn't know it's there because there's no electronics involved for the camera to know that it's a stop or half a stop less light." So in reality, the camera doesn't know it's there. Yes, the end result is that the filter does it's magic on the image, but the computer in the camera is not going to disable the auto focus because of a CP filter. If you were shooting in low light, you might experience slow or inaccurate focus by your camera, but you won't have a CP filter on the lens in low light. Even if I install a 3 stop ND filter my camera will still autofocus.

oldtigger Loc: Roanoke Virginia-USA

A filter cannot add anything to the light coming from the scene, it can only subtract.
Therefore if it is indeed a filter, and making a change, then there will be less light for the AF sensors to work with.

mwsilvers Loc: Central New Jersey

Reading both your post and jeep-daddy's above yours, I think you're both agreeing on the same thing but just phrasing it differently.

oldtigger Loc: Roanoke Virginia-USA

jeepdadddy says"
"Even if I install a 3 stop ND filter my camera will still autofocus."
I say:
"it might not"

How can you feel those two statements are even remotely similar?

mwsilvers Loc: Central New Jersey

Because the way I read the posts, you both seem to be saying that the AF will be affected if the available light is low enough. And further that CPLs or NDs can reduce available light significantly enough to affect the camera's ability to AF in a way similar to very low natural light with no filter mounted. Additionally, based on the OP's original question, I believe you are also both in agreement that a filter which lowers available light by a few stops will not alter the maximum aperture of the lens, which was the whole point of the thread. If you guys actually sat down and talked, I bet you would both be in complete agreement on this topic. But I think you're misinterpreting his meaning based on the way he phrased his response. If you meant something different then this, I misinterpreted your posts.

jeep_daddy Loc: Prescott AZ

You got it mwsilvers. I don't know why he can't see it. Yes, with a 3 stop ND filter I will not have accurate focus etc., but it will still attempt to focus and I've even had it be accurate on a bright day. But when it comes to a filter, it doesn't alter the maximum aperture of the lens. Teleconverters do affect or alter maximum aperture and when the cameras computer detects this, it simply disables the AF of the camera.

I put a 100-400 Canon zoom on my body and it has a max aperture of f/5.6@400mm. When I install the 1.4x teleconverter it's max aperture becomes f/8 and still autofocus'. But if I put my 2x teleconverter on then the max aperture becomes f/11 and autofocus is completely disabled. No amount of filter on the front of the lens, as long as the camera detects f/8 or greater aperture, will completely disable the AF. It might be slow or might not be accurate, but it will still attempt AF.

oldtigger Loc: Roanoke Virginia-USA

Your two statements do not agree with each other:
"Even if I install a 3 stop ND filter my camera will still autofocus."
...."Yes, with a 3 stop ND filter I will not have accurate focus etc.,..."
There are times when neither one of them is even true.

Place a piece of cardboard to cover 2/3 of the objective lens on your f/5.6@400mm.
How well does the auto focus work now?
The response, speed, and accuracy of your AF is dependent on the level of light
reaching the AF sensors, not the aperture setting.
I respect your abilities as a photographer and have no issue with what you are trying to say,
only with the way you express it.