bkijek Loc: Oak Lawn, Illinois

Hello -

I read an article today that, in passing, said that the use of polarizing, ND and other filters throws off the auto-focus feature in cameras. Is this true? In what way? I don’t quite understand.

rmalarz Loc: Tempe, Arizona

Geez, that's news to me, and both my digitals and one film camera that has AF. I'd better not let them read, or even know about, the article.
--Bob

bkijek Loc: Oak Lawn, Illinois

Thanks, I didn’t quite believe I was hearing correctly (actually, I heard the statement on a YouTube video, not an article!).

Steve Perry Loc: Sylvania, Ohio

The old linear polarizers could mess with AF, but those are few and far between now. The Circular Polarizer (type, not shape), are fine with AF and are what everyone uses. The only time I could see it affecting AF is if you were close to the EV limit of the AF system and then put one on. If the two stop reduction of light put it below the camera's rated EV, then it may not focus - although, with a modern camera this would also mean it was nighttime LOL!

SonyA580 Loc: FL in the winter & MN in the summer

Steve beat me to it!

Some of the older non-circular polarizers will throw off the autofocus of today's cameras but the new circular ones should not have any effect except to increase aperture to compensate for less light coming through the lens.

jeweler53

Autofocus SLRs and DSLRs almost universally use a half-silvered mirror called a "beam splitter" to send part of the light coming in from the lens up to the focus screen while the rest of the light continues on to the autofocus sensors.

Beam splitters tend to reflect a different percentage of the light depending on the light's polarization. A standard "linear" polarizer has polarized light coming out the back into the camera, so as you rotate it, the reflection percentage varies.

The built-in metering in almost all SLRs and DSLRs reads off of the focus screen. The meter is calibrated to expect a certain percentage of light to be reflected from the beam splitter. If a different percentage is reflected, the built-in metering will be off. If you're using auto-exposure based on that built-in metering, your exposure will be off.

A circular polarizer is a regular polarizer with a "depolarizing" layer on the back, so that the light going on to the mirror (and sensor) isn't polarized. This makes for predictable metering and auto-exposure.

That's the theory.

In practice, none of this matters. The variation in reflectivity from the aluminized beam-splitters used in SLRs and DSLRs is on the order of 1/3 stop. Few photographers who are using the built-in meter will notice a variation of 1/3 stop in metering.

So, if you want to do it the right way, use a circular polarizer. If you only care about results, the old-fashioned "linear" polarizer will work fine.

Steve DeMott Loc: St. Louis, Missouri (Oakville area)

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Steve Perry Loc: Sylvania, Ohio

never mind :)

bkijek Loc: Oak Lawn, Illinois

Thanks for the input, everybody. One more subject this “newbie” understands thanks to the UHHers.

MT Shooter Loc: Montana

Any polarizer or ND filter has the potential to affect the AF depending on the lens and camera used. Many entry level and older cameras cannot AF past F6.3. Polarizers are effectively costing you 1 1/2 to 2 stops of light. ND filters cost you from 2 to 16 stops of light depending on the strength of the filter. If your combination of camera, lens, and filter puts you maximum aperture at F8 or slower your AF could be struggling or even completely incapable of finding focus.
This problem is nothing new and is older than AF itself, one just needs to understand the issue and deal with it accordingly.

rmalarz Loc: Tempe, Arizona

Care to share which video?
--Bob

ballsafire Loc: Lafayette, Louisiana

I just think you are "barking" up the wrong tree…just forget about it and carry on.

selmslie Loc: Fernandina Beach, FL, USA

The video probably misstated or you misunderstood what is involved.

As others have mentioned, linear polarizers can mess up your meter and autofocus. That's why you need a circular polarizer.

ND filters reduce the amount of light reaching the AF sensor. If you start out with a lens that does not open very wide (like some of the popular zooms) autofocus in low light becomes difficult or slow. A polarizer usually reduces light by 1.3 stops or more so it can also affect focusing on a lens with a small maximum aperture.

Lenses with wide maximum apertures (f/2 or f/1.4) won't have a problem with autofocus unless you use a very dark ND filter.

bkijek Loc: Oak Lawn, Illinois

Thanks.

jerryc41 Loc: Catskill Mts of NY

Not that I've experienced.