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08-01-2012, 03:26 PM   #1
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three times CPL?

Hi,

i saw on the internet tip how to make own variable ND filter... by two CPLs front cpl has flipped glass... it work... super... but i have any more idea, please tell me what are you thinking about it:

using three CPLs... in line
from the front to back:
1. normal cpl
2. cpl with flipped glass
3. normal cpl
4. lens

my assumption is, that with first CPL i can remove reflections(etc.) and second and third will be as ND...
color shift will be removable with pre-shot with WB cap... or not?

and i know, reflections between filters... accetable victim

what do you know?

08-01-2012, 06:36 PM   #2
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QuoteOriginally posted by bricamry Quote
Hi,

i saw on the internet tip how to make own variable ND filter... by two CPLs front cpl has flipped glass... it work... super... but i have any more idea, please tell me what are you thinking about it:

using three CPLs... in line
from the front to back:
1. normal cpl
2. cpl with flipped glass
3. normal cpl
4. lens


my assumption is, that with first CPL i can remove reflections(etc.) and second and third will be as ND...
color shift will be removable with pre-shot with WB cap... or not?

and i know, reflections between filters... accetable victim

what do you know?
It will work but maybe tint will be hard to deal with. Three glass layers is many from a reflection standpoint.

You can get the equivalent result by using a CPL behind an LPL - no tint and only two layers of glass. LPL can be oriented to remove polarized glare from scene, then CPL used to adjust overall ND factor.

Dave in Iowa
08-01-2012, 10:29 PM   #3
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QuoteOriginally posted by newarts Quote
It will work but maybe tint will be hard to deal with. Three glass layers is many from a reflection standpoint.

You can get the equivalent result by using a CPL behind an LPL - no tint and only two layers of glass. LPL can be oriented to remove polarized glare from scene, then CPL used to adjust overall ND factor.

Dave in Iowa
oh, realy? it is interesting, but bad is, that LPL is bad to find cheap.... but it is very interesting idea thanks.. have you any long explosure picture with this?
08-02-2012, 03:11 AM   #4
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LPL being Linear polarizer, ie. the non-circular- kind which I thought was generally easier to find cheaply? Or is that just in second-hand now?
I think some cheap Chinese ebay type CPL filters actually turn out to be not circular at all... Whether they are any good other than that I'm not sure.
If you aren't worried about the small effect on metering that an LPL may give, you can use two LPLs too.

08-02-2012, 03:36 AM   #5
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QuoteOriginally posted by Moo Quote
LPL being Linear polarizer, ie. the non-circular- kind which I thought was generally easier to find cheaply? Or is that just in second-hand now?
I think some cheap Chinese ebay type CPL filters actually turn out to be not circular at all... Whether they are any good other than that I'm not sure.
If you aren't worried about the small effect on metering that an LPL may give, you can use two LPLs too.
Thx, I know how is LPL =) yop, second hand... good idea! i was thinking about new... the cherapest, how i find is about $35 and it is too much for me (because i even haven't money for lens for $120 and $190 monitor)

Luckily I have one CPL...

BTW I have ND8 filter... it is sh*tty ... i need low by 10EV not 3 incredible crap for $25, what i never used...


then... ypu say, that LPL+CPL is not giving color change and i can remove reflections and bluerize sky? that sounds great FO ND filters
08-02-2012, 03:58 AM   #6
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I actually tried the LPL in front of CPL and it didn't work. It turned out the Panasonic Lumix LPL I had bought was actually CPL. A small investment in a male-male thread connector turned it into a variable ND filter which works quite well. There is a bit of a blue/magenta cast to the images but this can be removed.

To get a polarised effect I don't believe you need a third filter at all though. The ND effect is achieved by polarising the light rather than just restricting all light in the case of a normal ND. So light passing through the first (backwards) filter is polarised. The second polariser is then adjusted between:

- aligned with first polariser - lets through all light
- aligned against first polariser - lets through no light

In both cases the starting light is polarised.
08-02-2012, 04:16 AM   #7
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QuoteOriginally posted by WobblyGoblin Quote
I actually tried the LPL in front of CPL and it didn't work. It turned out the Panasonic Lumix LPL I had bought was actually CPL. A small investment in a male-male thread connector turned it into a variable ND filter which works quite well. There is a bit of a blue/magenta cast to the images but this can be removed.

To get a polarised effect I don't believe you need a third filter at all though. The ND effect is achieved by polarising the light rather than just restricting all light in the case of a normal ND. So light passing through the first (backwards) filter is polarised. The second polariser is then adjusted between:

- aligned with first polariser - lets through all light
- aligned against first polariser - lets through no light

In both cases the starting light is polarised.
oh, it seems as my first solution... em, have you any sample photos with it? please? )

And no light thing... i tested my IR 720nm filter and i used ISO 200, f22 and 4:30 min and result was slightly underexcponed... it was afternoon, no clouds, after mid-day heat... that decrease of ligth i gonna have )

08-02-2012, 09:53 AM   #8
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Stacking CPL's will probably work but why? Good CPL's aren't cheap and 3 will probably degrade the image. Buy an ND filter. If you're on a really tight budget, go to your local welding supply and get good quality helmet lens. I've seen a few good shot with this method. Under $10 plus some duct tape.
08-02-2012, 10:08 AM   #9
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QuoteOriginally posted by reeftool Quote
Stacking CPL's will probably work but why? Good CPL's aren't cheap and 3 will probably degrade the image. Buy an ND filter. If you're on a really tight budget, go to your local welding supply and get good quality helmet lens. I've seen a few good shot with this method. Under $10 plus some duct tape.
yea i saw it on net... i will use twice CPL.... but:

1. for DIY I am extremly inept (i fu*ked up one lens (old tele-zoom) and second never assemble(A50/2)...)
2. cheapest variable nd is over $50, but two CPLs are $20... (yes, mother of cheapnes)
3. yes i thought about ND1000 but i dont know, how i will meter explosure wit it...
08-02-2012, 10:56 AM   #10
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There's some confusing info here. I've done a lot of experimental work in this area and am sure of what I say.

A linear polarizer lets light of one polarization vector pass, so light passing thru it is polarized in one direction.
A cpl is a linear polarizer followed by a quarter-wave plate which effectively de-polarizes linearly polarized light.

Consider a reversed CPL followed by a normal CPL. Light first strikes the quarter-wave plate so becomes effectively de-polarized; it then passes an LPL where it becomes completely polarized, this polarized light can be either blocked or passe depending on the orientation of the LPL it passes before becoming effetively de-polarized by the final quarter-wave plate. this setup's first step is to depolarize incident light so won't affect the sky etc. Various tints will be added to the scene depending on relative orientations of the filters

If the first polarizer in the stack is an LPL, it will serve the function of modifying any incident polarized light according to how you rotate it (get rid of reflections off glass, etc. The second polarizer will then act as an ND filter as it is rotated with respect to the first polarizer. The second filter can be either a CPL or LPL. A CPL is desired for some cameras for consistency in exposure control, etc. Little or no tinting is added with this set-up.

A set-up involving 3 CPLs can emulate the LPL+CPL at the cost of strong, variable tints, more reflective surfaces, poor performance, and cost.

LPL's are currently being manufactured and are not costly - get multi-coated versions.

if a CPL is not mandated by your camera's optical system, the best setup will be LPL - LPL. both multicoated, both high transmission for polarized light.

Dave
08-02-2012, 11:18 AM   #11
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QuoteOriginally posted by newarts Quote
A set-up involving 3 CPLs can emulate the LPL+CPL at the cost of strong, variable tints, more reflective surfaces, poor performance, and cost.
Oh, thank you, sir. I will try it in store, to see it with own eyes If i get LPL and it wil realy works, that will be perfect... But I'll buy without MC... because i cant have that high expense...
08-02-2012, 11:50 AM   #12
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certainly the cheapest high stop filter is the welders lens, and the fit Cokin P holders if you don't ant to use tape
they throw a colour cast though so a test shot to correct WB with a white card is advised
typically they measure out at 10-13 stop for about $5
08-02-2012, 12:20 PM   #13
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QuoteOriginally posted by reeftool Quote
Stacking CPL's will probably work but why? Good CPL's aren't cheap and 3 will probably degrade the image. Buy an ND filter. If you're on a really tight budget, go to your local welding supply and get good quality helmet lens. I've seen a few good shot with this method. Under $10 plus some duct tape.
I've read this suggestion before and tried it. Color cast was bad and IQ even worse - waste of $10 and time spent, IMHO
08-02-2012, 12:25 PM   #14
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QuoteOriginally posted by philbaum Quote
I've read this suggestion before and tried it. Color cast was bad and IQ even worse - waste of $10 and time spent, IMHO
stacked cpls will have a lot of the same issue if you are doing it on the cheap. good CPLs are expensive, in which case may as well just get a good 10 stop ND
08-02-2012, 01:57 PM   #15
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Here's an test showing severe tinting that can occur when a reversed CPL is used in front of a polarizer stack used as a variable ND filter.

The left hand image -where a cpl is used in the rear of a stack, shows no significant tinting. The other images show a couple of the tints that may appear when a reversed CPL is used as the front element.


Dave in Iowa

In my experience, IQ remains good even with inexpensive PL's, at least for normal & wide focal lengths.
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