a question about all the focus questions

Does a filter(s) on the lens have any impact on the focus of the camera?

Not unless it's dirty, and I'd have to think that it would have to be *very* dirty.

AF works by finding contrast. If a filter is dirty enough to reduce contrast significantly this would affect AF. However, I would think that the camera would be more likely to have problems focusing, but it's not so likely to focus on the wrong distance. Of course, AI Servo will take a shot whether something's in focus or not, so beware of that.

Also, polarizers reduce light by about 2 stops. If light gets too low because of this you'll also have problems focusing.

Again, a filter would have to be very dirty I'd say. So, with caveats, the answer is "No, filters won't affect focus."

I'll simplify Scott's answer:

No.

thank you, I didn't think so but thougt I'd ask.

I hate that word.

A filter shouldn't affect focus, but it sometimes can.

Even a high quality filter can, if sufficiently dark, block enough light to impair a camera's ability to determine focus. I'm not talking about a more or less transparent filter like a UV, mind you. I have in mind something more like a rather dark neutral density filter, a red 25 filter, things like that.

And some filters - even if not dark - can shift focus enough that in extreme cases infinity focus cannot be obtained. I don't claim this is common, but it can happen, it does happen, and I've personally experienced it. I would think that this is more likely to be a problem with a budget filter using mediocre quality glass (which may not even be perfectly square to the lens' optical axis), than with a B+W or Heliopan filter. And more of an issue with a telephoto lens than with a wide angle.
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Michael

I have had focus problems that were a direct result of a UV fikter on a specific lens.. and I have read of others with the same experience.

For me it was One lens,. one filter and specific lighting that would throw foucs off on my 10D.. (it only occured out doors)

At the time the lens was my "best" so I began to think my 10D was at fault. but finally I realized the trouble had started around the time I had installed the new Hoya UV filter.

This filter works fine on any other lens I own.. and the lens works fine with a Canon uv filter.. it was just the specific combo?

Again.. in essence.. this should not be a problem.. but it can happen.

There was an example printed in EOS magazine (I think it was EOS) where someone had problems getting a good focus on a yacht when using a UV filter and perfect without. It was repeatable and may have been lens specific.

I have seen it happen in night conditions where there are bright sources of light (street lighting, car drving lamps, etc.). This is probably due to the reflections caused by the filter/lens combination and it even happens with the very best quality filters.

A linear polarizer will throw the AF system and you should only use circular.

My own view is a hood is far better than a filter for protection, as I prefer not to add another layer of glass in the light path. But that is my view and it usually invites hordes of filter fanatics who think it is some form of evil cult member that works without filters.

KennyG wrote:
My own view is a hood is far better than a filter for protection, as I prefer not to add another layer of glass in the light path. But that is my view and it usually invites hordes of filter fanatics who think it is some form of evil cult member that works without filters.

I'll stand by you on this one Ken. I bought a great (and expensive) Hoya UV because everyone said to buy one, but once I thought about it I just left it in the box. Perhaps I'll use it if I ever shoot in a sandstorm....

I *would* agree as far as the use of a filter for protection and the possibilities of image degradation....

But one look at my UV filters and I am convinced I MUST have them...

I have two that need to be replaced.. one is scratched all to hell.. the other looks.. quite frankly.. like Acid was spilled on it

No idea what caused it.. but there are large pits of discoloration that do not come clean.

In either case.. had this been my lens element

CDS, you are no longer allowed to borrow any of my lenses.

Everything was fine untill those Herons started spitting acid at me!

ok this brings up another question!!!!!!!!! what happens when you shoot through a window?

Depends on the distance. If your subject is just on the other wide of a very dirty window then you could easily lock focus on the window. But if the subject is some distance from the window then you'll easily know if you happen to lock on the window.

If the window is dirty then move closer - you may get a "cloudy" image lacking contrast, but at least you won't get obvious window spots in the image. You can even shoot through a chain-link fence by pushing the camera hood right up against the fence, and if the subject is far enough from the fence then you'll never even know the fence was there. In eithr case, window or fence, a wide-open aperture helps.

Under most circumstances a filter will not affect focus. Under extreme circumstances yes, they can. But, under certain circumstances some cameras will not achieve AF at all. AF is not perfect on any camera and there will be times where MF is the only way to go, filter or no filter.