Polarizing filters

Posted: Mon Oct 11, 2004 12:54 am

There is often times questions from users on what a polarizing filter does & why is it often used.

Here's an example. An average shot of a small stream in open shade with leaves, pine needles etc. Generally speaking it's a good area to shoot some photographs right? Well, with no polarizing filter we see in this first shot that the reflection of the clear blue sky above is quite overpowering, washing out the majority of the stream itself.

Date/Time 09-Oct-2004 16:30:34
Make Minolta
Model DiMAGE 7
Flash Used No
Focal Length 55 mm
Exposure Time 1.41 sec
Aperture f/9.5
ISO Equivalent 100
Metering Mode matrix (5)

Now, with the polarizing filter installed & simply rotating it, we can cut that reflection to a bare minimum. Rotating back & forth will increase/decrease the amount of reflection, so it's a good idea to play with it until you get the desired effect.

Date/Time 09-Oct-2004 16:30:46
Make Minolta
Model DiMAGE 7
Flash Used No
Focal Length 55 mm
Exposure Time 1.41 sec
Aperture f/9.5
ISO Equivalent 100
Metering Mode matrix (5)

Posted: Tue Jan 25, 2005 8:22 am

Thanks Shaun,

as a total filter-ignorant i was wondering: i've always heard that a Polarizer is used to make the sky more blue and to enhance colors in general, is that true?
and are polarizers and ND filters the same thing?
and if no what is the ND filter?
and what about their effect on exposure?
and...

i could go on forever but i'll stop here.

peace,
Trombone

Joined: 08 Jun 2004 Posts: 121 Location: San Francisco

Trombone,

Yes, a polarizer filter will make the sky appear more blue. Light is polarized. You can think of it as vertical and horizontal light. The polarizer filter allows you to selectively block one of the two by rotating the lens. A simple fun experiment is to go to your local department store, pick up two pair of polarized sun glasses. Hold the first up to the light and note the light coming through. Now hold the second pair up in front of the first with the lenses parallel with the first pair. Note the light still comes through. Now rotate the second pair of sunglasses so that they are 90 degrees to the first and note that the light no longer passes though. This is because one pair is blocking the horizontal light and the other the vertical light. When you take a picture with the sky in it, there is a considerable amount of reflected light. You are basically cutting that in half and thus darkening the sky.

No, an ND filter is not the same as a polarizing filter. If you were to do the same trick above with non-polarized glasses, it wouldn't work. ND filters just cut down the amount of light coming in to the lens allowing you to have a longer exposure time. They are great for those smooth (blurred) waterfall shots you see so often. A split ND gradient has one half of the lens darkened so you can darken sunsets to keep them from blowing out the foreground.

Both polarizers and ND filters will require that you also consider the exposure. Polarizers block about 1/2 the light so you can figure about double your exposure time. ND filters come in a variety of stops and will effect the exposure time accordingly.

If you shoot outdoors, a polarizer is a must. ND filters are great for slowing the shutter and bluring any moving subjects.

Cheers,

Posted: Mon Jan 31, 2005 2:47 am

Darv,

Simply: Thank you.

Cheers!
Trombone

Joined: 30 Jul 2004 Posts: 20 Location: La Mirada, CA

I have Canon glass, and I'm wondering if the brand name of the filter makes a big difference??? I'm interested in getting a circular polarizer for my Canon 70-300mm f2.8 IS - I have the option of getting Canon or generic brand for less than half the price. Is it worth it to buy a generic brand for less, or will the Canon filter be worth paying double for?

Joined: 08 Jun 2004 Posts: 121 Location: San Francisco

A "big" difference? Probably not but that's a pretty good lens you have there so skimping on the filters doesn't make any sense to me. I don't know if that means you would have to buy a Canon filter though. You didn't give the name of the other brand so it's hard to say how they compare. There are other brands out there that I would say are at least as good as the Canon filters (some might even argue even better) and probably not as expensive. For what it's worth, I have been buying Tiffen filters based in part on recomendations from other photographers and I have been pleased with the results but then I am currently shooting digital only.

In retrospect to my own exprience, the one thing I would do differently is to buy an oversized filters and use a stepup ring. This would have given me the abilty to use the one filter on all lenses I plan to purchase in the future.

Hopefully some others will chime in on what their experiences are in this area because I'd love to know more myself.

Cheers,

Joined: 30 Jul 2004 Posts: 20 Location: La Mirada, CA

Hi Darv,
thanks for the response. I'm not sure what brand the filter is, at B&H it just says "General Brand General Brand 77mm Circular Polarizer Glass Filter" - here is the item: http://www.bhphotovideo.com/bnh/controller/home?O=details_accessories&A=details&Q=&sku=241621&is=REG

I also have the Canon 28-70mm f2.8L lens, with the Canon Circ Polarizer - I just can't justify buying another one for $125. Sure it works great, but if I could save $54 on a filter, you betcha I'll go for it, as long as the "general brand" can perform equal to or near the Canon equivalent. I'm just curious if anyone out there has had any experience with both a generic brand and the Canon filters.

Posted: Mon Mar 14, 2005 11:36 am

Eventually I'll have both a ND and Polerizing filter. Which should I buy first? Which will get the most based on me taking typical landscape type shots?

Joined: 08 Jun 2004 Posts: 121 Location: San Francisco

You will want to go with the polarizer filter first and get the ND second. The polarizer filter will do some of what the ND (increase exposure time) as well as help to reduce glare and reflections.

Cheers,
Darv

Joined: 23 Jan 2005 Posts: 34

I noticed at the top of this topic Shaun mentioned something about rotating the filter back and forth. I am going to purchase a polarizing filter in a few days, but my thinking was that with a Circular polarizing filter, why would you need to rotate it to change the amount of reflection? I guess I am still just a bit confused

Posted: Tue Mar 22, 2005 9:30 pm

http://dpfwiw.com/polarizer.htm#proof

read this, it should help you understand.

Posted: Wed Apr 20, 2005 11:55 am

Thanks for the clear description. I was always curious about polarizing filters.

Joined: 26 Mar 2005 Posts: 240 Location: Southwest Florida

I have a simga tele lens 70-300mm, when I put my polarizer filter on and am zoomed in at 300 my camera can't auto focus? Anyone else wih this problem?

Posted: Sat Apr 30, 2005 8:40 am

Sarah5344 wrote:

I have a simga tele lens 70-300mm, when I put my polarizer filter on and am zoomed in at 300 my camera can't auto focus? Anyone else wih this problem?

This is actually a common problem. What’s happening is when you zoom out to 300mm your lens drops down to f5.6. At 5.6 there is very little light entering the camera in which the AF system uses to focus. Most if not all AF cameras will not focus beyond 5.6 so when you add the polarizer to the equation you’re reducing the light level even more. In bright sunlight you should not have this problem but as the light levels dip this becomes an issue.

Joined: 23 Jan 2005 Posts: 34

I have run into this problem too. thanks lots for the explanation