Nikon D40x focus problem

[Cremaster]

I have a Nikon D40x that works great except when I am using a circular polarizer. I have noticed that it doesn't focus correctly with the polarizer on. I notice this especially with the 55-200 VR lens. In addition to the polarizer, I also have a UV filter on both lenses. Any ideas what might be causing this?

 

[ghostman]

Are you sure it's a circular polarizer and not a linear polarizer?

If it is a circular polarizer, I'd imagine it's because you're not getting enough light for the AF to work. What's the wide-open aperture on the focal length you're using?

 

[OulOat]

That lens has a wide open aperture of f4-5.6, which is around the minimum aperture the AF system needs to work properly. The polarizer cuts the light by a stop or two (the purpose of the polarizer), so your AF system won't work properly. There is nothing wrong with your camera; you will have to upgrade your lens to something faster if you want to use AF with a polarizer.

 

[Cremaster]

Thanks ghostman and OulOat, I wasn't aware of the amount of light that a polarizer cuts.

 

[soydios]

Originally posted by: OulOat
That lens has a wide open aperture of f4-5.6, which is around the minimum aperture the AF system needs to work properly. The polarizer cuts the light by a stop or two (the purpose of the polarizer), so your AF system won't work properly. There is nothing wrong with your camera; you will have to upgrade your lens to something faster if you want to use AF with a polarizer.

Bright sunlight still provides plenty of light for the autofocus system to work, even with a polarizing filter.

What filter exactly do you have, OP? If it's a linear polarizer then it will cause problems with the phase-detect autofocus system. If it's a circular polarizer then something else is up.

 

[magomago]

Originally posted by: soydios

Originally posted by: OulOat
That lens has a wide open aperture of f4-5.6, which is around the minimum aperture the AF system needs to work properly. The polarizer cuts the light by a stop or two (the purpose of the polarizer), so your AF system won't work properly. There is nothing wrong with your camera; you will have to upgrade your lens to something faster if you want to use AF with a polarizer.

Bright sunlight still provides plenty of light for the autofocus system to work, even with a polarizing filter.

What filter exactly do you have, OP? If it's a linear polarizer then it will cause problems with the phase-detect autofocus system. If it's a circular polarizer then something else is up.

That was my initial thought as well.

Also, the Circular Polarizer shouldn't be cutting out THAT much. How many stops do you typically lose? I'm sure it depends on the quality...so then is there a general range? I remember reading that it was typically 1/3rd of a stop...but I don't know if that is right.

edit:

doing some searching reveals widely quoted 1.5-2 stops of lost light.

 

[Deadtrees]

I hope you're not using both of UV and PL filter together.

 

[OulOat]

Originally posted by: magomago

Originally posted by: soydios

Originally posted by: OulOat
That lens has a wide open aperture of f4-5.6, which is around the minimum aperture the AF system needs to work properly. The polarizer cuts the light by a stop or two (the purpose of the polarizer), so your AF system won't work properly. There is nothing wrong with your camera; you will have to upgrade your lens to something faster if you want to use AF with a polarizer.

Bright sunlight still provides plenty of light for the autofocus system to work, even with a polarizing filter.

What filter exactly do you have, OP? If it's a linear polarizer then it will cause problems with the phase-detect autofocus system. If it's a circular polarizer then something else is up.

That was my initial thought as well.

Also, the Circular Polarizer shouldn't be cutting out THAT much. How many stops do you typically lose? I'm sure it depends on the quality...so then is there a general range? I remember reading that it was typically 1/3rd of a stop...but I don't know if that is right.

edit:

doing some searching reveals widely quoted 1.5-2 stops of lost light.

The amount of light cut depends on the angle of the polarizer to the light source, ie. directly facing the light source cuts much less light than 90 degrees from the light source. But no matter what, using a polarizer on his lens is pushing it.

 

[Deadtrees]

Originally posted by: soydios

Originally posted by: OulOat
That lens has a wide open aperture of f4-5.6, which is around the minimum aperture the AF system needs to work properly. The polarizer cuts the light by a stop or two (the purpose of the polarizer), so your AF system won't work properly. There is nothing wrong with your camera; you will have to upgrade your lens to something faster if you want to use AF with a polarizer.

Bright sunlight still provides plenty of light for the autofocus system to work, even with a polarizing filter.

What filter exactly do you have, OP? If it's a linear polarizer then it will cause problems with the phase-detect autofocus system. If it's a circular polarizer then something else is up.

In terms of AF EV working range, that seems to make sense but when I think about an other factor, I doubt it. If you read camera descriptions, each states how this camera's AF is for lenses faster for certain /f value. I couldn't find that information for D40x, but I assume it's probably f/5.6 as most of other cameras. As Op is using f/4 - 5.6 lens, PL filter ceratinly plays a factor there.

Well, though my arguments sounds logical to me, I'm not so sure about this. Does anyone have clear information on this issue?

 

[Cremaster]

After further review, I discovered it was NOT a circular polarizer. My bad. Thanks to all who weighed in on this.

 

[soydios]

Originally posted by: Deadtrees

Originally posted by: soydios

Originally posted by: OulOat
That lens has a wide open aperture of f4-5.6, which is around the minimum aperture the AF system needs to work properly. The polarizer cuts the light by a stop or two (the purpose of the polarizer), so your AF system won't work properly. There is nothing wrong with your camera; you will have to upgrade your lens to something faster if you want to use AF with a polarizer.

Bright sunlight still provides plenty of light for the autofocus system to work, even with a polarizing filter.

What filter exactly do you have, OP? If it's a linear polarizer then it will cause problems with the phase-detect autofocus system. If it's a circular polarizer then something else is up.

In terms of AF EV working range, that seems to make sense but when I think about an other factor, I doubt it. If you read camera descriptions, each states how this camera's AF is for lenses faster for certain /f value. I couldn't find that information for D40x, but I assume it's probably f/5.6 as most of other cameras. As Op is using f/4 - 5.6 lens, PL filter ceratinly plays a factor there.

Well, though my arguments sounds logical to me, I'm not so sure about this. Does anyone have clear information on this issue?

It's purely a software feature lock. If you're using a 2x teleconverter with an f/5.6 lens, thus ending up with wide-open f/11, but you're in very bright sunlight, there's still plenty of light for the autofocus system to work. Third-party companies do this with their f/6.3 lenses: IIRC it reports f/5.6 as its maximum aperture at the telephoto end instead.

 

[Deadtrees]

Originally posted by: soydios

Originally posted by: Deadtrees

Originally posted by: soydios

Originally posted by: OulOat
That lens has a wide open aperture of f4-5.6, which is around the minimum aperture the AF system needs to work properly. The polarizer cuts the light by a stop or two (the purpose of the polarizer), so your AF system won't work properly. There is nothing wrong with your camera; you will have to upgrade your lens to something faster if you want to use AF with a polarizer.

Bright sunlight still provides plenty of light for the autofocus system to work, even with a polarizing filter.

What filter exactly do you have, OP? If it's a linear polarizer then it will cause problems with the phase-detect autofocus system. If it's a circular polarizer then something else is up.

In terms of AF EV working range, that seems to make sense but when I think about an other factor, I doubt it. If you read camera descriptions, each states how this camera's AF is for lenses faster for certain /f value. I couldn't find that information for D40x, but I assume it's probably f/5.6 as most of other cameras. As Op is using f/4 - 5.6 lens, PL filter ceratinly plays a factor there.

Well, though my arguments sounds logical to me, I'm not so sure about this. Does anyone have clear information on this issue?

It's purely a software feature lock. If you're using a 2x teleconverter with an f/5.6 lens, thus ending up with wide-open f/11, but you're in very bright sunlight, there's still plenty of light for the autofocus system to work. Third-party companies do this with their f/6.3 lenses: IIRC it reports f/5.6 as its maximum aperture at the telephoto end instead.

One thing I'm sure is that it's not purely a software feature lock but a hardware design limitation. It being purely software lock just doesn't make sense if you think about af module system and sensitivity.

EDIT: http://photo.net/canon-eos-digital-camera-forum/00RSsu

 

[soydios]

Originally posted by: Deadtrees
One thing I'm sure is that it's not purely a software feature lock but a hardware design limitation. It being purely software lock just doesn't make sense if you think about af module system and sensitivity.

EDIT: http://photo.net/canon-eos-digital-camera-forum/00RSsu

Ah, I see now. That link has some fairly good explanations. I take back what I said earlier about the software lock, my fault.