Circular Polarizer vs Nd Filters?

[SonyA7R]

I use ND Filters to keep my video shutter speed at 1/50. However, I have started to do multiple exposure shots. When it comes to photo's, is there really any difference in stopping down 2 stops than using an ND filter? If the Polarizer feature makes it about 2 stops as well, wouldn't that be about the same? 

 

I would like to get some nice shots with blue skies that aren't multiple exposure though so I am looking at polarizer filters. I am looking into the Hoya Alpha series.

[addicted2light]

Well, I'm assuming you already know that stopping down two stops will extend your depth of field.

 

Using an ND will only "turn down" the luminosity of the scene; as far the camera is concerned it will be the same of shooting at a darker scene.

 

Using a polarizer will achieve more or less the same result, BUT it will also change the tonality and saturation of the sky (darkening it) and eliminate most reflections. To achieve the same 2 stops reduction of the ND filter you will have to be sure to turn the polarizer so that it is at its most effective position. Besides, depending on how the light falls on a scene (I mean literally the orientation of your light source in relation to where you stand) a polarizer will be more or less effective.

 

More, using a polarizer with an extreme wide angle (below 24mm generally) it will yield ugly IMHO results because the sky will often look "striped". This because the polarization of the sky changes with the orientation to the light source, like I said, so using a lens encompassing section of the sky vastly different in this regards thanks to its extreme angle of view will give you these results.

[SonyA7R]

When I say stopping down, I'm talking about on the exposure, not stopping down the aperture. 

[Austerror]

When I say stopping down, I'm talking about on the exposure, not stopping down the aperture. 

Stopping down always = aperture.

 

So what do you mean stop down on the exposure?  Reduce ISO or increase shutter speed?

[Golem]

This is a Sony Alpha forum. The thread title mentions

circular polarizers. For all of you using the mirrorless

[non SLT] cameras, there is no need of circular type

polarizers. There are zero mirrors or prisms located

in the imaging paths of your focus/view and imaging

systems [which really are one and the same system].

 

If there's any mirror or prism involved in magnifying

the mini-micro LCD screen in the EVF, that is not in

the IMAGING path, it's AFTER the imaging path, it's

more or less just part of the eyepiece, similar to any  

contact lenses you might wear while shooting.

   

The above is both common sense, and tested. When

it occurred to me, I tested it on 2 brands of live view

systems, Sony and Canon. I have plenty of both type

of polarizer on hand, since my involvement predates

and postdates the need for the circular type. Results

confirmed that live view photography needs only the

old school linear polarizer, often found languishing in

the "junk bin" of retail shops for a few dollars. 

   

` 

[Golem]

Stopping down always = aperture.

 

So what do you mean stop down on the exposure?  Reduce ISO or increase shutter speed?

  

Right. A "Stop" was a piece of metal with a hole

in it, slid into a lens barrel thru a slot on the side. 

 

Lately, we use a multi-bladed adjustable "stop"

built right into the lens cuz itz more convenient.

But itz still a "stop". It still governs the relative

aperture of the lens [aperture is a fancy word

meaning "hole"].

  

Exposure adjustments equivalent to a full stop

are called an "EV". One EV is the equivalent in

exposure shift to one full stop, or to doubling [or

halving] the shutter time or the ISO rating. IOW

there really are no 2 stop filters. Correctly, they

would be a 2 EV filter, or a filter with a 4X factor.

  

This is not P&S turf. Might as well get it straight ! 

 

` 

[weinhous]

From Wikipedia at http://en.wikipedia.org/wiki/Polarizer#Circular_polarizers

 

Quoting: "Linear polarizing filters were the first types to be used in photography and can still be used for non-reflex and older SLR cameras. However, cameras with through-the-lens metering and autofocusing systems - that is, all modern SLR and DSLR - rely on optical elements that pass linearly polarized light. If light entering the camera is already linearly polarized, it can upset the exposure or autofocus systems. Circular polarizing filters cut out linearly polarized light and so can be used to darken skies or remove reflections, but the circular polarized light it passes does not impair through-the-lens systems."

 

That quote came from Ang, Tom (2008).Fundamentals of Modern Photography. Octopus Publishing Group Limited. p168. ISBN 978-1-84533-2310.

 

So, it would seem that the effect of using a linear polarizer will depend on the through-the-lens-metering technology and upon the autofocus technology in use in any particular camera body. Will very likely vary from camera-body to different camera-body.

 

Certainly there is no harm in trying a linear polarizer and monitoring the resulting exposure and autofocus.

[Golem]

What wiki says is obsolete,or at least misleading. It's 2008 info

and last I checked we're up to 2015, and thaz in digital years,

not dog years :-) 

  

The problem is that it speaks of "cameras with through-the-lens

metering and autofocusing systems - that is, all modern SLR and

DSLR - " and states that these cameras "rely on optical elements

that pass linearly polarized light." 

  

The way it's written, the "modern SLR and DSLR" are presented

merely as a handy, familiar example of the types of cameras in

question, namely "cameras with through-the-lens metering and

autofocusing systems". 

  

SLR type cameras are NOT an example of type of cameras that

need circular polarizers. They are THE type of camera that needs

them. In today's market, nearly all other "cameras with through-

the-lens metering and autofocusing systems" are live-view types,

commonly called "mirrorless" cameras. Well Bunky, no mirrors,

no need for circular polarizers.

   

It's not mirrors per se that require circ pols. A Nikon F or similar

dinosaur never needed them, even with their TTL metering. It's

the more complex designs that involve SEMI-SILVERED mirrors

that need circ pols. A polarizer plays tricks with semi-reflecting

surfaces. Thaz their value when the semi-reflecting surfaces are

in the subject matter. But when the semi-reflecting surfaces are

parts of the inner hardware of the camera, it's not good to play

tricks with them.

  

And yes, polarizers only work on semi-reflecting surfaces. If your

subject matter includes a mirror, or chromed metal, etc etc there

is no effect on those by any polarizer. A polarizer will not "shut

down" a mirror lens [aka "cat" lens].

  

Now if you really wanna know how a circular polarizer can still

reduce reflections in the subject but not screw up your cameras

that have semi-mirrors in their innards, go look that up, but NOT

in wikipaedia. There's plenty of cool diagrams etc out there so I'm

not gonna try it via text-only right here .... you're welcome !  Let's

just scare off the timid folks by summarily saying it's something to

do with Phasing, and Wave Retardation. For those not scared off  

yet, here's one place you can find a diagram: 

  

     http://www.apioptics.com/circular-polarizers.html  

  

   

`  

[weinhous]

Posted 2008 background information aside, the points that I was trying to make were that…

1. if the dexel transducers are in any way linear-polarization-angle sensitive, a linear polarizer may confuse them
2. we don't know the polarization sensitivity of the dexels tasked with exposure determination and/or autofocus implementation in all mirrorless camera sensors at the microscopic level
3. there may be some mirrorless camera sensors that intentionally use polarization in their dexels (or even beam-splitters).

So, one can't assume that linear polarizers will absolutely work for all mirrorless camera sensors. They will probably work for many, but the only way to tell will be to put them on a camera and pay attention to the measured exposure and to the quality of the autofocus and judge whether they make sense and are working.