Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
08-24-2009, 09:57 AM   #1
Senior Member
aCIDfire's Avatar

Join Date: Dec 2008
Location: Czech Republic
Posts: 108
Circular polarizer in macro-photography??

IŽd like to ask if you use CPL in macrophotography (1:1 or more) when using flash (ext. or macro flash). IŽam using 105mm 1:1 macro lens with Metz 58 with diffuser. So IŽd like to know your experiences before buying new CPL. Talking about taking pictures of bugs and parts of flowers outside - on direct sunlight.

08-24-2009, 01:56 PM   #2
Inactive Account




Join Date: Dec 2008
Location: Ames, Iowa, USA
Photos: Albums
Posts: 2,965
I've never used a cpl for macrophotography but think one would remove some specular glare due to reflectionsf rom insect shells
08-24-2009, 02:32 PM   #3
Loyal Site Supporter
pacerr's Avatar

Join Date: Jan 2008
Location: Henry, TN
Photos: Gallery | Albums
Posts: 2,848
PL vs. CPL?

I could well be wrong, and I'd like to see a definitive discussion on this. but my understanding is the CPL effect has to do with phase-differential auto-focus sensors rather than image sensor issues. If this is in fact true, there should be no adverse effects when manually focusing with a PL filter -- which is commonly preferred in macro photography anyway.

If you have a 'linear' polarizer filter you should be able to determine if the effect is what you want -- but I couldn't guess as to how to orient the filter relative to the strobe.

I just did a quick comparison for exposure and AF success using both PL and CPL Hoya filters on a K200D with a Tamron 28-75/2.8 lens and observed NO difference in either parameter on middle distance metallic reflections.

Although I normally manually focus in situations where I use a polarizing filter, I've seen no notable difference in practical results and so far I use either type interchangeably.

-----------------------

Edit: However, this from Wikipedia for example -

" The metering and auto-focus sensors in certain cameras, including virtually all auto-focus SLRs, will not work properly with linear polarizers because the beam-splitters used to split off the light for focusing and metering are polarization-dependent. Circular polarizers include a linear polarizer on the front, which selects one polarization of light while rejecting another, followed by a quarter-wave plate, which converts the selected polarization to circularly polarized light inside the camera, which works with all types of cameras, because mirrors and beam-splitters split circularly polarized light the same way they split unpolarized light.[6]

H2

Last edited by pacerr; 09-14-2009 at 07:02 AM.
08-24-2009, 05:01 PM   #4
Inactive Account




Join Date: Dec 2008
Location: Ames, Iowa, USA
Photos: Albums
Posts: 2,965
QuoteOriginally posted by pacerr Quote
I could well be wrong, and I'd like to see a definitive discussion on this. but my understanding is the CPL effect has to do with phase-differential auto-focus sensors rather than image sensor issues. If this is in fact true, there should be no adverse effects when manually focusing with a PL filter -- which is commonly preferred in macro photography anyway.

If you have a 'linear' polarizer filter you should be able to determine if the effect is what you want -- but I couldn't guess as to how to orient the filter relative to the strobe.

I just did a quick comparison for exposure and AF success using both PL and CPL Hoya filters on a K200D with a Tamron 28-75/2.8 lens and observed NO difference in either parameter on middle distance metallic reflections.

Although I normally manually focus in situations where I use a polarizing filter, I've seen no notable difference in practical results and so far I use either type interchangeably.

H2
In principle reflections from metallic surfaces have NO polarizing effect, hence polarizers are of little use with metals. The polarizing effect is limited to reflections at angles from non-metals.

Here's a graph showing the reflection coefficient as a function of angle for a non-metallic reflector:

Brewster's Angle - HoloWiki - A Holography FAQ

This graph shows the fraction of light reflected from a glass surface (one half is polarized perpendicular and one half parallel to the plane it is traveling in). At zero degrees incidence (straight on, both polarizations are reflected equally, about 4%, for window glass. As the angle increases,the reflection of the parallel component increases smoothly, while the other decreases and is zero at about 56 degrees for window glass. This is called Brewster's angle.

At Brewster's angle, all of the light reflected is polarized parallel to the reflecting surface. The angle of your polarizer can be set to block all this light, thereby removing the reflection. This only works well for light reflected at moderately glancing angles.

So polarizers are of most value when trying to block reflections at moderately grazing angles from non-metallic surfaces.

here's an example showing reduction of glare from a glass-topped table (you can see through the glass on the left, but not the right.

CPL - circular polariser filters - photography-reviews.com - everything you wanted to know about photography

QuoteQuote:
but I couldn't guess as to how to orient the filter relative to the strobe.
The polarizer angle should be perpendicular to the surface causing the reflection. To figure out how to turn the polarizer, stand by a body of water, like a pond, or lake, or puddle and rotate the polarizer until the reflection is a minimum; this is when the polarizer's axis is perpendicular to the water's surface.

The greatest worry about linear vs circular polarizers with (D)SLRs is not focusing but exposure. Say polarized light inside the camera is reflected by a prism to a light meter; the graph shows that differing amounts of light will reach the meter depending on it's polarization. This leads to unpredictable exposure measurements.

The solution to this problem is to place a so-called quarter wave plate after a linear polarizer (this effectively depolarizes the light that made it through the linear polarizer). This combination of linear polarizer & wave plate is called a circular polarizer.

My experiments with a linear polarizer on a K100D show about a 1/2 stop variation in exposure when the polarizer is rotated looking at an un-polarized light source. I do not see this variation if I use a CPL.

Dave


Last edited by newarts; 08-25-2009 at 05:39 PM. Reason: critical NO inserted.
08-25-2009, 10:09 AM   #5
Loyal Site Supporter
pacerr's Avatar

Join Date: Jan 2008
Location: Henry, TN
Photos: Gallery | Albums
Posts: 2,848
Thanks, Dave, useful info. And I said that wrong didn't I. I should have said a PAINTED metallic surface - a car hood to be exact.

None the less, I find it difficult to see a significant exposure difference in actual practice. A few degrees difference in 'look angle' seems to account for more change than the difference between PL and CPL filters as I use 'em. I don't doubt precise comparisons could define a difference.

Bottom line: after I got over the initial "CPL-BA" I stopped worrying about it and simply used the PL's I had available with whatever exposure adjustments that resulted. That happened very shortly after I saw the price on the larger CPL's.

H2
08-25-2009, 10:32 AM   #6
Veteran Member
MattGunn's Avatar

Join Date: Jun 2009
Location: Wales
Photos: Gallery
Posts: 347
Metalic surfaces definatelt DO affect the polarization of light reflected from then but in a slightly more complicated way than dielectric materials such as glass. Essentially where the reflection goes to zero at the brewster angle for the polarization perpendicular to the surface of a dielectric material, for a metal it goes through a minimum but does not reach zero. A polarizer can therefore reduce reflections / glare from metalic surfaces. The reflection of light polarized light from surfaces can be predicted using Fresnels Equations: Fresnel equations - Wikipedia, the free encyclopedia
For macro photography a polarizer could be used to reduce glare and reflections but more interesting things can also be done. Insect shells can have a strong affect on the polarization of light (Beetle has polarizing twist in its shell - physicsworld.com) and you can get interesting contrast and lighting effects if you have a polarizer of the lens and the light source and try adjusting both.
08-25-2009, 10:39 AM   #7
Veteran Member




Join Date: Feb 2007
Location: London
Posts: 389
QuoteOriginally posted by pacerr Quote
I just did a quick comparison for exposure and AF success using both PL and CPL Hoya filters on a K200D with a Tamron 28-75/2.8 lens and observed NO difference in either parameter on middle distance metallic reflections.
This very much confirms the experience I've had with the K10D and Hoya and a Cokin P linear polarisers.

I've used them both quite a bit, and never noticed any AF or exposure issues. I note Newarts' comment about a half stop difference, so I should make it clear that I haven't done any experiments that can resolve half stop differences. But if that's the only difference, I'd still rather stick with my LPs and save the expense of CPLs.

Last edited by ChrisA; 08-25-2009 at 10:44 AM.
08-25-2009, 05:15 PM   #8
Inactive Account




Join Date: Dec 2008
Location: Ames, Iowa, USA
Photos: Albums
Posts: 2,965
QuoteOriginally posted by MattGunn Quote
Metalic surfaces definatelt DO affect the polarization of light reflected from then but in a slightly more complicated way than dielectric materials such as glass. Essentially where the reflection goes to zero at the brewster angle for the polarization perpendicular to the surface of a dielectric material, for a metal it goes through a minimum but does not reach zero. A polarizer can therefore reduce reflections / glare from metalic surfaces. The reflection of light polarized light from surfaces can be predicted using Fresnels Equations: Fresnel equations - Wikipedia, the free encyclopedia
Matt's correct. I oversimplified.

Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
cpl, flash, k-mount, macro, pentax lens, slr lens
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Looking for Circular Polarizer lurchlarson Pentax Camera and Field Accessories 22 08-01-2010 09:39 AM
Circular Polarizer NecroticSoldier Troubleshooting and Beginner Help 21 04-08-2010 07:50 AM
Which Circular Polarizer should I get? Hannican Pentax Camera and Field Accessories 7 09-25-2008 03:00 PM
circular polarizer hll Pentax SLR Lens Discussion 7 11-27-2007 05:05 PM



All times are GMT -7. The time now is 07:56 PM. | See also: NikonForums.com, part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top