Polariser Quality

[Colin500]

Hi,


when I bought my 70-200 a month back, the guy in the shop threw in an old, used polariser they had flying around. Since I've never used one before, I wanted to try it out without spending much — I can always buy a better one later. As it turns out, the quality with the polariser is absolutely terrible, I can't even use it for a "first experience".


I'm posting some samples below; if anybody can tell me whether this is normal for an old/cheap/used/??? polariser? Would an new/expensive one be only half as bad or would there be no noticeable degradation?


Another question I wanted to ask about polarisers: What is the widest lens you can comfortably recommend a polariser for (so that the differences of the effect in different parts of the frame don't become too big)? I'm currently contemplating a CPL for the 70-200 and an ND for the 24-70; darn the different sizes...


Thanks, Colin


70mm no filter


[img]/cfs-file.ashx/__key/CommunityServer.Components.UserFiles/00.00.00.33.43/70.jpg[/img]


70mm with polariser


[img]/cfs-file.ashx/__key/CommunityServer.Components.UserFiles/00.00.00.33.43/70CPL.jpg[/img]


200mm no filter


[img]/cfs-file.ashx/__key/CommunityServer.Components.UserFiles/00.00.00.33.43/200.jpg[/img]


200mm with polariser


[img]/cfs-file.ashx/__key/CommunityServer.Components.UserFiles/00.00.00.33.43/200CPL.jpg[/img]


PS: I'm assuming that it's a circular polariser because AF still works as good as it can, i.e. it doesn't get better with MF. Speaking of focus, these are crops from larger pictures not from where the focus point was, but from where the difference between with and without filter is most visible; the lens can do better than that.

[bob williams]

Colin, The degraded images look more like motion blur to me. A CP filter will darken your image significantly 1-2 stops, If you fail to adjust properly you may end up with slower shutter speeds with the CP filterwhich may result in blurred images. Suggestions: Check your exif data from these images and see if this was the case. Also make sure you are shooting from a tripod, just to remove the human factor, then run the test again and check your results.


I use a Heliopan CP filter and have noticed virtually no image degradation if I do my job properly.


Hope this helps,


Bob

[Colin500]

Colin, The degraded images look more like motion blur to me. A CP filter will darken your image significantly 1-2 stops, If you fail to adjust properly you may end up with slower shutter speeds with the CP filterwhich may result in blurred images.


(...)

It does look a bit like motion blur, but (unfortunately?) I can rule that out — although the shutter speed did change, from 1/400 and 1/640 to 1/125 and 1/200, respectively, that's nothing that the IS can't handle, even at 200mm. And the degrading effect is also clearly visible when looking through the viewfinder, in particular at 200mm.


Thanks, Colin

[Sean Setters]

PS: I'm assuming that it's a circular polariser because AF still works as good as it can, i.e. it doesn't get better with MF. Speaking of focus, these are crops from larger pictures not from where the focus point was, but from where the difference between with and without filter is most visible; the lens can do better than that.
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My first thought was that it was an old linear polarizer. Try this:

Testing Polarizers



You can also easily tell if a polarizer is linear or circular even if it
is not marked.
Hold the polarizer about 2 or 3 inches in front of your eye, and look at
your reflection
in a mirror. Looking from the camera's viewpoint (i.e. with the filter
threads pointing
towards you) you should see the image of your eye in the mirror (as if
looking through a
neutral density filter). Now turn the filter around so that the filter
threads point at
the mirror. If the polarizer is linear you should see the same thing
that you saw before,
but if it is circular it will appear black and you will not see the
reflected image of
your eye. This is because light reflected from your eye passing through
the circular
polarizer exits in the direction of the mirror as circularly polarized
light (let's say
it's left handed circular polarization). When it reflects from the
mirror it reverses its
polarization to the opposite sense - in this case it is reflected as
right handed circular
polarization. The filter only passes left handed circular polarization
when the light
enters from the "camera side" of the filter, so the reflected light is
blocked.
The situation is analogous to the effect of a vertically polarized
linear filter on
horizontally polarized light - little or no transmission of the light
can occur.

Besides that, I must admit that the blurriness does look like motion blur (as if IS wasn't turned on). If IS was indeed on (and I assume it was), you might want to run some tests of the IS with the polarizer off to make sure everything's working correctly.

[Jayson]

Funny I was just reading about polarizers yesterday and had some questions. Seeing this thread and wondering if it was a linear or not, I got this from Bryan's review on the 77mm CPL:


"Be sure to get a "Circular" Polarizer Filter if you are shooting with an autofocus camera body such as the Canon EOS bodies. The "Linear" Polarizer Filter interferes with autofocus of these cameras. A Linear Polarizer Filter can be used only on certain manual focus camera bodies with no problems."


So this might not have been user error and the camera would not autofocus correctly if it was the linear polarizer. Also, that last picture I believe would be a statement in motion blur. Hope any of this helps.


Jayson

[Colin500]

My first thought was that it was an old linear polarizer. Try this:


(...)


Besides that, I must admit that the blurriness does look like motion blur (as if IS wasn't turned on). If IS was indeed on (and I assume it was), you might want to run some tests of the IS with the polarizer off to make sure everything's working correctly.

You are right, it isn't a circular polariser! When I hold it between an eye and a mirror, the eye always sees itself! Now matter how I twist and turn the polariser, the only effect that I get is that the reflections from the lateral window disappear (so at least it's a polariser :-/ ).


I am absolutely sure that the IS works fine, it still amazes me every time I use it... Anyhow, it's dark now, but just to be sure I will do another test tomorrow. But the fact that the distortion appears the moment I hold the filter in front of the lens shows that I might as well throw it away.


Thanks, Colin

[neuroanatomist]

Hi Colin,


I think the previous replies have covered the three main possibilities:

1. Motion blur due to light loss (which you may have addressed, but see below)
2. It's a linear polarizer which is affecting your AF system (likely not the case if MF is no better)
3. The old, used, thrown-in polarizer is a crappy piece of glass

I think 3 is a big part of the issue. But, regarding the motion blur, your 70mm no filter example does look noticeably sharper than your 200mm no filter example - since the 70-200mm f/4L IS doesn't give up any sharpness as you move from the center to the border of the frame, the loss of sharpness with increased focal length suggests motion blur. Apologies for asking the obvious, but was IS turned on? Another possibility - were you in P mode? I suspect not, since in both cases the shutter speed changes you indicate should exactly cover the ~1.75 stop of light lost to the polarizer, but if aperture changed as well, the DoF changed and your crops from areas away from the focal point might then be OOF.


On the balance, though, it looks like it's just a bad filter. I use aB+W K&auml;semannCPL, and there's no affect on sharpness.

Another question I wanted to ask about polarisers: What is the widest lens you can comfortably recommend a polariser for (so that the differences of the effect in different parts of the frame don't become too big)?

As wide as you want. It doesn't matter how wide the lens is - it matters what effect you're trying to achieve with the CPL. I suspect you're thinking of the 'make the sky bluer' effect that a CPL has - in that case, you start to see the 'banding' effect of variable polarization when you're using a lens wider than ~18mm (on a 1.6x crop body like yours). You'd be fine with a CPL on your 24-70mm for shots with lots of sky. But, CPLs have many other uses - one great example is cutting the waxy reflections from foliage. I've used my CPL on the wide end of my EF-S 10-22mm lens to good effect for that. Even with an UWA lens like the 10-22mm, when shooting the sky the 'banding effect' can be used to enhance a photo, if done carefully, or the CPL can be rotated to minimize the banding while still providing modest darkening, depending on where the clouds are, etc. The bottom line is that I think it's unfair to categorically state that a CPL should not be used on a UWA lens, it just takes more care, and if used indiscriminately it can be an issue.
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I'm currently contemplating a CPL for the 70-200 and an ND for the 24-70

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I'm not sure I understand this one. CPLs and NDs serve very different purposes. A CPL affects the quality of the light, eliminating reflections which polarize the light; the 1.5-2 stops of light lost with a CPL is a 'side effect'. The sole purpose of an ND filter is to reduce the quantity of light entering the lens. A CPL can do 'double duty' as a relatively weak ND filter, but an ND filter cannot substitute for a CPL.

darn the different sizes...

That should be the least of your concerns. For all your 'light modifying' filters (ND, CPL, etc., but not UV/protection), get them sized to fit your largest lens (77mm in your case), then get step-up rings (e.g. a 67&rarr;77mm adapter for your 70-200mm f/4L IS). See Jayson's recent post,To step up or not to step up, that is the question?, for a discussion on using step up rings.


Hope that helps...


--John

[Colin500]

I think the previous replies have covered the three main possibilities:

1. Motion blur due to light loss (which you may have addressed, but see below)
2. It's a linear polarizer which is affecting your AF system (likely not the case if MF is no better)
3. The old, used, thrown-in polarizer is a crappy piece of glass

I think 3 is a big part of the issue. But, regarding the motion blur, your 70mm no filter example does look noticeably sharper than your 200mm no filter example - since the 70-200mm f/4L IS doesn't give up any sharpness as you move from the center to the border of the frame, the loss of sharpness with increased focal length suggests motion blur. Apologies for asking the obvious, but was IS turned on? Another possibility - were you in P mode? I suspect not, since in both cases the shutter speed changes you indicate should exactly cover the ~1.75 stop of light lost to the polarizer, but if aperture changed as well, the DoF changed and your crops from areas away from the focal point might then be OOF.

Since I got the DSLR, I'm shooting in Av mode 99% of the time; just double-checked, these were all shot at f/4.


I don't understand why the difference in degradation changes with focal length, but I'm pretty sure that the effect has nothing to do with (camera or object) motion blur: When I look through the viewfinder at 200mm, it looks just as horrible as in the samples, when I zoom out to 70mm it looks more decent, but nothing like with the filter removed...

On the balance, though, it looks like it's just a bad filter. I use aB+W K&auml;semannCPL, and there's no affect on sharpness.

If I get the occasion, I'll ask the person that gave it to me whether it was a joke, or a mistake to give me that crappy filter...

Another question I wanted to ask about polarisers: What is the widest lens you can comfortably recommend a polariser for (so that the differences of the effect in different parts of the frame don't become too big)?

As wide as you want. It doesn't matter how wide the lens is - it matters what effect you're trying to achieve with the CPL. I suspect you're thinking of the 'make the sky bluer' effect that a CPL has - in that case, you start to see the 'banding' effect of variable polarization when you're using a lens wider than ~18mm (on a 1.6x crop body like yours). You'd be fine with a CPL on your 24-70mm for shots with lots of sky. But, CPLs have many other uses - one great example is cutting the waxy reflections from foliage. I've used my CPL on the wide end of my EF-S 10-22mm lens to good effect for that. Even with an UWA lens like the 10-22mm, when shooting the sky the 'banding effect' can be used to enhance a photo, if done carefully, or the CPL can be rotated to minimize the banding while still providing modest darkening, depending on where the clouds are, etc. The bottom line is that I think it's unfair to categorically state that a CPL should not be used on a UWA lens, it just takes more care, and if used indiscriminately it can be an issue.

The main effect I was after with the polariser is how it can make reflections seem stronger or nearly disappear. Since I'm new to using any filters, and I had read about these non-uniformity issues with CPLs, I was just wondering at which focal length I can just concentrate on the polarising effect (because the non-uniformity is not so strong as to disturb/require extra care).
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I'm currently contemplating a CPL for the 70-200 and an ND for the 24-70

</div>


I'm not sure I understand this one. CPLs and NDs serve very different purposes. A CPL affects the quality of the light, eliminating reflections which polarize the light; the 1.5-2 stops of light lost with a CPL is a 'side effect'. The sole purpose of an ND filter is to reduce the quantity of light entering the lens. A CPL can do 'double duty' as a relatively weak ND filter, but an ND filter cannot substitute for a CPL.

Sorry, mixing things together: I was planning a "double-strike" into the use of filters, with a CPL on the tele to see what it can do regarding reflections etc., and an ND on the wide-angle to see what I can do with a tripod and a landscape...


Thanks, also for the note on "step-up" rings, I had never heard of them before...


Colin

[Colin500]

Ok, latest tests confirm: This filter is crap!


I just ordered a B&amp;W to see what a real polariser will do...


Thanks for all help and hints, Colin

[neuroanatomist]

Glad to help. Have fun with your new CPL filter!