The result of a cheapo CPL...

So I picked up a "multi-coated" CPL (in 62mm) just because it was cheap and I figured if it worked great, if it was crap, I was only out a little bit of $$.

Well it seems that at shorter focal lengths it was okay, but when i got to longer focal lengths it would not focus correctly at all. While looking into why this may be, I ran across this from another thread on this forum:

I'm not sure I understand what this is saying. I have no problems autofocusing when my aperture is 5.6 or narrower. Anyone got any insight (other than the obvious of this being a piece of junk filter in the first place) as to why focusing seems to be so off at longer focal lengths? Thanks.

Your lens autofocuses when the lens is wide open. If your lens is and f/3.5 maximum and you set it to f/11 for the shot, autfocus will happen while the lens is at f/3.5. It only stops down to f/11 when you fully press the shutter. The polarizer limits light whenever its installed. In this case, it limits the light to the equivalent of f/7, even when the lens itself is wide open. Since your camera can't focus with less light than f/5.6 provides, you lose your autofocus. Its the same deal as installing a 2x TC on and f/5.6 lens where you lose autofocus on all but the 1D series.

Ahhh...I see. I did not know that. Good to know. So what happens (with no filters of any kind) if you have a lens where the widest aperture is f/6.3 (like at the long end of some zooms); how does the camera achieve focus if it can't open up to f/5.6 or wider?

So is it still possible to manual focus then?

dan.k78 wrote in post #12528963
So is it still possible to manual focus then?

Yep.

mike_d wrote in post #12528926
Your lens autofocuses when the lens is wide open. If your lens is and f/3.5 maximum and you set it to f/11 for the shot, autfocus will happen while the lens is at f/3.5. It only stops down to f/11 when you fully press the shutter. The polarizer limits light whenever its installed. In this case, it limits the light to the equivalent of f/7, even when the lens itself is wide open. Since your camera can't focus with less light than f/5.6 provides, you lose your autofocus. Its the same deal as installing a 2x TC on and f/5.6 lens where you lose autofocus on all but the 1D series.

That's actually incorrect. The amount of light transmitted isn't the determining factor. The f/5.6 limitation is directly related to the f/number, not the intensity of the light, as it's because of the baseline available to the AF sensors. Essentially,Canon's AF system works like a split-image rangefinder; the lens aperture serving as the baseline. Lenses that are close to f/5.6 "lie" to the AF system about their maximum aperture (though still reporting the actual aperture for exposure purposes). But if their exit pupil is too small, the camera/lens combo will struggle to AF anyhow. It's for this reason that "taping" contacts on a teleconverter will let your camera try to AF, but that you'll often experience the lens "hunting" for focus. You'll experience similar problems trying to manually focus a camera with a wedge-type or microprism focusing screen at small apertures; half the focusing area will often "black out", making focusing nearly impossible.

dan.k78 wrote in post #12528963
Ahhh...I see. I did not know that. Good to know. So what happens (with no filters of any kind) if you have a lens where the widest aperture is f/6.3 (like at the long end of some zooms); how does the camera achieve focus if it can't open up to f/5.6 or wider?

So is it still possible to manual focus then?

The zooms that are only 6.3 on the long end report 5.6 to the camera so it will attempt AF. (at least that is my understanding of it)

cloose wrote in post #12530964
The zooms that are only 6.3 on the long end report 5.6 to the camera so it will attempt AF. (at least that is my understanding of it)

I have noticed that many of the f6.3 lenses don't have the best reputations for AF, though.

Jon wrote in post #12529097
The amount of light transmitted isn't the determining factor.....

Maybe you can explain me.. when using a Manual focus lens with AF Confirmation chip. If I go to lets say F11, the AF sensor never confirms focus.. even if the chip is reporting a F1.4 aperture.. First I said well my adapter is defective, but tried another one with the same results. So I thought the light entering the camera had something to do with the AF sensors. Because if I open the aperture on the lens it will confirm focus. Thanks.

but if you are using an f1.4 lens in the dark, you might be getting less light to the sensor than an f11 lens in the bright sun.

Well this behaviour it's in bright sun. the scenario: Lens 28mm F2.8 M42 full manual, adapter with AF Confirmation chip. The adapter always report F1.4 to the camera (it shows on the top lcd) if I stop to F11(thats my correct exposure) the focus point on the viewfinder never beeps, litup. So i have to open the aperture until i get the AF confirmation and then stop down. So I think that the light entering DOES matter for AF Sensors, even if the camera "think" is in F1.4..

CyberManiaK wrote in post #12533345
Maybe you can explain me.. when using a Manual focus lens with AF Confirmation chip. If I go to lets say F11, the AF sensor never confirms focus.. even if the chip is reporting a F1.4 aperture.. First I said well my adapter is defective, but tried another one with the same results. So I thought the light entering the camera had something to do with the AF sensors. Because if I open the aperture on the lens it will confirm focus. Thanks.

That's because the actual aperture diameter (RF baseline, see above) is the smaller f/11, not the f/1.4 being reported. The camera doesn't even try to AF if the lens' reported aperture is smaller than the camera's cutoff. It will struggle to focus, but may well fail, if the lens is falsely reporting an acceptable aperture. See also my comments about non-reporting or taped teleconverters failing to AF properly and consistently.

CyberManiaK wrote in post #12533462
Well this behaviour it's in bright sun. the scenario: Lens 28mm F2.8 M42 full manual, adapter with AF Confirmation chip. The adapter always report F1.4 to the camera (it shows on the top lcd) if I stop to F11(thats my correct exposure) the focus point on the viewfinder never beeps, litup. So i have to open the aperture until i get the AF confirmation and then stop down. So I think that the light entering DOES matter for AF Sensors, even if the camera "think" is in F1.4..

Why are you even trying to focus with the M42 lens stopped down to f/11?...the DOF zone is so wide that you could miss hitting precise focus on the subject by a wide margin. By refusing to show focus confirmation, the camera is making you focus the right way!

Jon wrote in post #12534018
That's because the actual aperture diameter (RF baseline, see above) is the smaller f/11, not the f/1.4 being reported. The camera doesn't even try to AF if the lens' reported aperture is smaller than the camera's cutoff. It will struggle to focus, but may well fail, if the lens is falsely reporting an acceptable aperture. See also my comments about non-reporting or taped teleconverters failing to AF properly and consistently.

Ohh I see thanks

Wilt wrote in post #12534336
Why are you even trying to focus with the M42 lens stopped down to f/11?...the DOF zone is so wide that you could miss hitting precise focus on the subject by a wide margin. By refusing to show focus confirmation, the camera is making you focus the right way!

I focus Wide open, then stop down. And when i'm going to press the shutter, is where I notice that the AF sensors don't beep. And that lead me to think that maybe I move the camera a little, and is not in focus anymore. You get the point?

CyberManiaK wrote in post #12535536
I focus Wide open, then stop down. And when i'm going to press the shutter, is where I notice that the AF sensors don't beep. And that lead me to think that maybe I move the camera a little, and is not in focus anymore. You get the point?

OK, understand the situation. I certainly wouldn't bother to be concerned about any body movement at f/11 after focusing at f/2.8 unless you walked after focusing !

on the other part of your first question...
Have you tried again in full sunlight just to see? unless the cpl glass is messed up or dirty, there's no way it would stop your lens from focusing. When you say that it wouldn't focus, do you mean that it was hunting back and forth and wouldn't lock or do you mean that it wouldn't try to autofocus? With the aperture issues being discussed above that's a separate issue where the camera won't even try to focus the lens, it won't do anything when you press the focus button