Circular vs Linear Polarizer

[Hill Country Hack]

I could not find this topic discussed via the search on the site. I apologise if I'm beating a dead horse.

I recently shot an airshow in full sun with cloudless skies. At the time the sun was above and to the front of me at about 1:00 with striaght above my head being 12:00 and the horizon in front of me being 3:00. There was no option to move as I was on an airbase and we were restricted to the one side. A challange to say the least. I was using a UV filter only and while panning about the sky chasing aircraft shooting in full manual, my exposures varied greatly. I was traversing the lens on both the vertical/horizontal planes and frequently at a diaginal.

While doing my personal de-brief I initially banged my head on the desk for not using my circular polarizer (CP) also. The more I thought about it, the more I came to the realization the CP would only have been of limited benifit. I came to this conclusion because my CP screws into my lens and the end of the lens rotates as it focusses. This would mean if I initially set the polarizer in one position, as I tracked and continually focussed the CP would be rotating back and forth with the lens thus changing the initial setting and I'd still end up with over/under exposures.

I am unsure of the benifit of a linear polarizer for this type of photography. I am not concerned with the loss of f-stops as where I am there is always plenty of bright sunlight during the day.

My other options are to continue in full manual, set my metering to center weighted and continue to adjust the exposure on the fly as best I can. Or shoot center weighted/shutter priority and let the camera set the ISO and apeture.

Would I be better served with a linear polarizer for the mentioned scenario or the other stated options?

Thank you in advance for your thoughts and expertise.

[Doug Pardee]

All polarizing filters rotate to allow you to adjust the polarization effect. The difference between a circular polarizer and an ordinary ("linear") polarizer is that after the polarization is done, the circular polarizer has an extra layer that essentially de-polarizes the light coming out into the lens.

The reason people use a circular polarizer is that autofocus SLRs and DSLRs pretty much universally use a half-silvered mirror, also known as a "beam splitter", that sends some of the incoming image to the focus screen and lets the rest pass through to the autofocus sensors. Beam splitters are sensitive to polarization, and the percentage that gets reflected can vary. In almost all SLRs and DSLRs, the in-camera metering system is looking at the focus screen, so any variance in the brightness can affect the metering.

In practice, though, it's not much of an issue. The difference in reflectivity is maybe 1/3 of a stop, which almost nobody will notice. I've seen many reports of people using "linear" polarizers on DSLRs, and not one has ever had a problem.

But that's not what you needed to know. What you need to know is that a polarizer would only have made things worse. Auto-exposure is your best bet for taking photos in rapidly-varying lighting conditions. For critical photos, you could throw in some exposure bracketing.

Also, if your UV filter is an inexpensive one, it could have been making things worse by introducing flare. You didn't mention if you were using a lens hood, but they'll help to reduce flare.

[brianxlt]

For action Airshow shoots I use a ND filter rather than a PL. The ND will reduce flairs and bright spots on the jets as well as alowe for adjusting settings.

[Doug Sundseth]

The other Doug has many good points. I'd like to add only a couple of things:

1) A UV filter doesn't do anything useful on a current digital camera. UV is filtered out by the camera before the light reaches the sensor anyway. I'm not going to get into the whole "protect the front element" thing except to say that I don't see any value there either.

2) Are you sure the filter ring rotates as you zoom or focus on the lens you're using? (FWIW, it would drive me nuts to have the filter ring rotate during focus.) None of my lenses do. I mention this because I worried about the same thing you're worrying about at one time.

3) The primary use for a linear polarizing filter is to place it on the front of a circular polarizing filter to act as a variable ND filter. Altering the relative angles of polarization alters the total light allowed through. You can't do this with two circ. pol. filters unless you reverse one of them. This can introduce some pretty nasty artifacts, though, so I wouldn't recommend it, especially if you buy anything other than very good filters.

4) The big thing I'd recommend when shooting flying aircraft is to think about whether you need exposure compensation. If you're shooting the shadowed side of the plane, you'll likely need to push up your exposure 1-2 stops (sometimes even more) because the sky will be so bright relative to the plane. Alternatively, manually expose for the side of the airplanes you'll be seeing and let the sky do whatever it will. This only works if you're pretty constantly shooting the lit or unlit side of the planes, though I suppose you could have two different manual exposures and switch rapidly from one to the other.

HTH

[Elmo]

my air show technique is different, I generally shoot ap priority but keep an eye on the shutter speed. I dial in a stop of so of exposure compensation (direction can vary depending on size of aircraft in the frame) to avoid the bald sky and adjust the aircraft in post as necessary. I have also shot some in P mode which works well also. I also use autofocus as you will have enough trouble keeping up with aircraft without fiddling with a bunch of knobs etc.

[Hill Country Hack]

Thank you all for your well thought responses.

Doug S- My filters screw into the front of the lens. The lens does rotate as it focusses. Maybe no more than 1/4 turn either direction. The focus ring does not rotate with zooming in and out.

Brianxlt- It looks like I need to add a ND filter to the bag. Those come at various stops of filtering? If so, what is a good as a general utility ND?

Doug P- Thanks for the information. I was using a hood and I didn't get a single lens flair.

I was using back button focus the entire time at the airshow. I only had to fiddle with one wheel to adjust exposure.

FWIW I get some more practice in a couple of weeks as a B-17G will be in town and I plan on paying it a visit. And yes, it is all unpainted aluminum so it will be SHINEY in the Texas sun. I'll shoot some in Shutter priority and Apeture priority and see what works best. I'll also bracket some shots and shoot at f8 to increase my DOF.

[brianxlt]

You could get the ND2 (.03) or the ND4 (.06) filters for general use ether the screw on or something like the Cokin square filter set. With the ND filter it won't matter if it turns with you focus ring (round or square type) unless you get the graduated type for darkening the sky to ground perspective. For bright sunny day Airshow shooting the ND2 would/should be enough to reduce hot spots, but still give you enough shutter speed 1/500 - 1/750. You would also want to use center focus point servo AF and center weighted metering. I had a SXi so not sure if the XS has these functions or not.

[Hill Country Hack]

Originally Posted by brianxlt

You could get the ND2 (.03) or the ND4 (.06) filters for general use ether the screw on or something like the Cokin square filter set. With the ND filter it won't matter if it turns with you focus ring (round or square type) unless you get the graduated type for darkening the sky to ground perspective. For bright sunny day Airshow shooting the ND2 would/should be enough to reduce hot spots, but still give you enough shutter speed 1/500 - 1/750. You would also want to use center focus point servo AF and center weighted metering. I had a SXi so not sure if the XS has these functions or not.

Thanks again for the input. I had used your suggested settingings except for the center wieghted metering. I'll add that in when I shoot the B-17.

[Doug Pardee]

Originally Posted by Hill Country Hack

I didn't get a single lens flair.

There's always flare. The question is how noticeable it is. Flare that produces the classic sun-flare spots is obviously quite noticeable.


(Photo credit: emilymills on Flickr)

With digital photography, a new variety of flare — the ghost image of highlights, rotated 180 degrees from the original — has turned up. This is triggered by light reflecting off of the shiny front surface of the digital sensor. That light can get re-reflected off of any flat surface in the optical system — possibly the back side of the front element, which is flat in most lenses, but the usual culprit is a filter.


(Photo credit: Dyanna on Flickr)

Usually, though, flare just bounces around inside your lens, scattering light across your image, and thereby reducing the contrast by lightening the darker areas.

Test image without filters:



Test image with inexpensive, uncoated Haze filter:


(Photo credits: Ken & Christine at kenandchristine.com)