Luminous Landscape Forum
The Art of Photography => The Coffee Corner => Topic started by: iconoclast on July 24, 2006, 02:49:02 am
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This is my first post in a LL forum, so perhaps I should introduce myself...
First of all, English is not my native language even though I live in the U.S.A., so bear with me if I don't always make sense. I am involved in photography since 1971. For the last 2 and a half years I am using a digital camera exclusively.
I'd like to ask those of you who are involved in digital photography a question I wasn't able to get an answer to so far, even though I already posted this problem on a couple of prominent forums. It may sound silly and it may be dismissed very easily without even a second thought, but I hope to get to the bottom of this problem. Here it goes:
Previous weekend, we had a perfect sunny day here on Long Island. The sky was blue and almost without a speck of a cloud. As I learned from TV weather report later that weekend, the UV-content advisory was in the purple zone, which was the highest possible. I went with a girl to the beach to take some "summer photos" and I decided to use my polarizer.
I expected no reflections on the water, really deep blue skies and saturated colors. The resulting images were less than spectacular. The reflections on the water were indeed gone. Skies were of various depth of blue, but not much deeper than the ones photographed without polarizer. But that wasn't the biggest of my disappointments. What's worse, overall contrast was lower than normal and my pictures contained visible noise/grain (all pictures were taken as RAW at ISO 100). I use Leica Digilux 2 with a non-interchangeable lens which has an unique filter diameter (69mm). For that reason, there's only one polarizer that fits that lens. And it's been made specifically for this particular lens by one of the best filter manufacturers in the world (Heliopan). So, there's no question of incompatibility or low quality of the filter.
So, I started wondering. In the past, I've heard complaints about results of the D2/polarizer combo. I've never thought much of those complaints until now. Couple of days after I took those pictures, I read a post on another forum. In that post, an owner of Canon 20D wrote of his own experience that was like an exact mirror image of my own disappointment - and it happened exactly at the same time, only he lived in Ireland and I live in NY metro area.
So far, I have been unable to find time for any tests and this weekend weather was not cooperating either. At this point, I am really not sure that I should expect better results from pictures taken in such situation - for the following reasons:- The sensor, especially small one like in my camera, the anti-UV filters built around every camera's sensor and the firmware of the camera may all act alone or in combination producing degraded image quality when dealing with light filtered by a polarizing filter.
- Extremely high UV content in the light may force the camera's firmware to work beyond its normal range of the ability to optimize an image. In that case, I think that even without polarizing filter, I would be able to detect adverse result on the image quality.
- And last but not least, I suspect that the maker of my camera (Leica) predicted potential trouble - otherwise why would they decide to use 69mm filter size (the only lens in existence with this filter size), if not to discourage people from using filters? Leica is well known for their thorough research and sticking to the very highest standards in photography, which makes me think that they didn't want people to use filters with at least this particular model of their camera.
I still intend to do more conclusive testing, but it will take more time and I don't know whether I'll be able to get similar weather conditions...
Did anyone of you ever experienced similar problem or heard of one like that?
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Are you using the automatic settings, or are you shooting in raw?
Keep in mind that a polarizer reduces the amount of light by at least one full stop, and in some cases as much as two stops.
This means that if you leave a camera in fully automated mode, you risk taking pictures at a higher ISO level than the camera can reproduce noiselessly. Using e.g. ISO 400 will create a bit of noise with a Leica Digilux 2, according to various reviews on the net.
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Also, just in case you weren't already aware of it while you were taking the pictures, polarizers do the full extent of their "magic" to the sky only when the sun is at about 90 degrees to the axis between the camera and the subject. At the time, did you check where the sun was? If it was close to behind you or in front of you, that would explain why the sky wasn't much darker than without the polarizer. (If you are already very familiar with using polarizers with film cameras, then apologies for stating what might sound obvious; with new posters here, it's hard to tell what their experience level is...)
And as Jan says, the polarizer cuts down on the light reaching the sensor, which could increase the noise level if the camera is either upping the ISO or reducing the exposure because of it.
Lisa
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Are you using the automatic settings, or are you shooting in raw?
Keep in mind that a polarizer reduces the amount of light by at least one full stop, and in some cases as much as two stops.
This means that if you leave a camera in fully automated mode, you risk taking pictures at a higher ISO level than the camera can reproduce noiselessly. Using e.g. ISO 400 will create a bit of noise with a Leica Digilux 2, according to various reviews on the net.
[a href=\"index.php?act=findpost&pid=71615\"][{POST_SNAPBACK}][/a]
Jani, those particular pictures were all taken in 'auto' mode, but that does not imply that the camera changes the ISO value whenever it sees less light than it is 'comfortable' with. My camera can only change shutter speed or aperture or both, but not ISO speed. And all pictures were taken at ISO set to 100 (all RAW files). There was more than enough light to take correctly exposed shots at ISO 100 even with a polarizer attached. All shots are sharp and are normally exposed. Only the contrast is lower than normal (which I can contribute to using any kind of filter in front of a lens). The thing that I don't understand is the noise (although very fine) which was visible all over the frame, not just in shadows.
Also, just in case you weren't already aware of it while you were taking the pictures, polarizers do the full extent of their "magic" to the sky only when the sun is at about 90 degrees to the axis between the camera and the subject. At the time, did you check where the sun was? If it was close to behind you or in front of you, that would explain why the sky wasn't much darker than without the polarizer. (If you are already very familiar with using polarizers with film cameras, then apologies for stating what might sound obvious; with new posters here, it's hard to tell what their experience level is...)
And as Jan says, the polarizer cuts down on the light reaching the sensor, which could increase the noise level if the camera is either upping the ISO or reducing the exposure because of it.
Lisa
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Lisa, yes, I am aware of that. And no, apologies are not necessary. As I mentioned in my first post, the sky looked a bit darker then without polarizer. I have been shooting for over 30 years with my analog cameras and I always had darker skies with a polarizer (even when shooting in 'auto' mode).
Of course, I was changing my position in relation to the direction of sunrays, but even taking that into account, overall, I should have much darker skies (to a varying degree - depending on the angle).
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can you post a few example pictures. it's better "analysing" it that way.
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I concur with those who have said to post problem images. It's too hard to tell what the issue is without having something to look at. As has been mentioned, polarizers do penalize on between 1 and 2 stops of light, meaning a wider aperture, longer exposure, or higher ISO is required to get equivalent exposure. A polarizer does not by itself increase saturation; all it does is remove certain specular highlights and reflections that can mask the color of foliage (many leaves have a semi-shiny surface) and the color of water. A polarizer's effect on sky depends heavily on atmospheric conditions and the angle between the camera and the sun; when shooting directly into or away from the sun, its effect will be negligible. The increased noise complaint makes me suspect that the camera may have upped the ISO for some reason, or you may have inadvertently underexposed, which will increase noise levels. But again, it's all speculation until we can see the shots.
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I read this discussion while trying to justify the expenditure on this filter.
Is it possible that the poor performance results from "circular" polarization?
The alternative is "linear" polarization.
The difference is how the tiny little lines that cause the polarizing effect
on photons (light particles) are oriented on the filter substrate,
as either concentric circles or parallel lines.
When you look at polarized light through a linear polarizing filter,
then rotate the filter 90-degrees left or right, the contrast
in effect should be most striking. However, with a circular
polarizing filter, the behavior should be different.
(Light reflecting off an electrical insulator is polarized;
conductive reflectors do not polarize photons... see
Unihedron's EM spectrum poster (http://unihedron.com/projects/spectrum) for more.)
You may recall that standard/linear polarizers have a rotating
ring-mount and lever, with which to fine-tune the effect.
I bet the Heliopan 69mm has no such adjustments, due to its
circular polarization.
It's merely a conjecture (hypothesis), but worth considering.
If wrong, thanks anyway for saving me from spending the $$$!
HTH!!!
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I may post a sample... eventually. But right now, I am trying to gather as much input from other photographers as I can.
Just two days ago I had a feedback from a photographer on the Leica Forum, who said he had exactly the same result shooting with the exact same kind of a polarizer (only with a different camera/lens). And his experience was not a random event. Inspired by the discussion, he went out and did test shots with and without the polarizer.
I am not very familiar with the theory of light and polarization effect differences between linear and circular polarizers, but I think there might be some truth to what delnerdo just said. I will investigate that possibility, as well. Thanks!
Oh, BTW! Regarding what Jonathan said about the increased noise - I think that was entirely my fault (I had to increase the overall contrast of a majority of my pictures to get decent prints and therefore, inadvertently, I also increased a perception of noise).
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I read this discussion while trying to justify the expenditure on this filter.
Is it possible that the poor performance results from "circular" polarization?
The alternative is "linear" polarization.
The difference is how the tiny little lines that cause the polarizing effect
on photons (light particles) are oriented on the filter substrate,
as either concentric circles or parallel lines.
No, not quite.
See this FAQ on polarizers (http://www.mat.uc.pt/~rps/photos/faq.polarizers.html) for an explanation.
Yes, circular polarizers can degrade quality more than a pure linear polarizer, but you don't have much of a choice with cameras using beam splitting for autofocus (most, if not all DSLRs, IIRC).
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Hi, Rob,
I assume you rotated the polarizer while composing with the EVF or back panel monitor to see what orientation of its axis produced the desired effect.
Did you see the desired effect when doing that, but it did not appear in the actual captured image?
Or were you unable to produce the desired effect when composing?
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I read this discussion while trying to justify the expenditure on this filter.
Is it possible that the poor performance results from "circular" polarization?
You may recall that standard/linear polarizers have a rotating
ring-mount and lever, with which to fine-tune the effect.
I bet the Heliopan 69mm has no such adjustments, due to its
circular polarization.
A circular polarizer has an axis of polarization (with respect to its handling of the "incoming" light) - it just does not have an axis of polarization ofits "output" light (which is why it is safe to use on cameras with semireflective mirrors in the metering path, such as many dSLRs).
Thus a circular polarizer will have a rotatable mount just like a linear polaraizer (and must if it is to be able to perform its function inphtpography - selective attenuation of components of the "arriving" light based on the axis of polarization of the light from the image. Just like a linear polarizer, it must be rotated to produce the desired effect, the selective attenuation of polarized components of the light from the image.
You can find a further discussion of this in my tutorial article, "Polarization of Light and Polarizers ", available here:
http://doug.kerr.home.att.net/pumpkin/inde...ationPolarizers (http://doug.kerr.home.att.net/pumpkin/index.htm#PolarizationPolarizers)
Best regards,
Doug
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I assume you rotated the polarizer while composing with the EVF or back panel monitor to see what orientation of its axis produced the desired effect.
Did you see the desired effect when doing that, but it did not appear in the actual captured image?
Or were you unable to produce the desired effect when composing?
Of course, I did change the angle of rotation of the polarizer to get the maximum effect. Also, I changed rotation as I was switching framing from vertical to horizontal and vice versa.
I did see changing effect while rotating the polarizer during composing, but the effect was not as strong as I expected. Same was showing in resulting pictures. Perhaps my expectations were not entirely realistic, since my Digilux 2 gives me very nice, dark skies in sunny conditions even without any polarizer at all.
Doug, thanks for the link. There's a wealth of information many of us can use (I know, I certainly can).
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There is no practical difference between linear and circular polarizers, except:
Circular polarizers reduce light transmission by about 1.5 stops, compared to a little over a stop for a linear polarizer.
Linear polarizers can cause problems with the AF and metering of SLR cameras when the polarizer is oriented in certain directions. Circular polarizers do not have this problem.
The effect of the polarizer is clearly visible in the camera viewfinder; that's how you adjust it to the proper orientation.
ElNerdo is way out in left field; all polarizers have rotation adjustment.
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I've had the same problem Iconoclast had.
Bright blue sky day, used the circular polarizer, and in RAW-Adobe Bridge the sky was "texturized" with niose.
The problem is I used a 20D at ISO 100, (setting: contrast --, sharpness --, saturation -, color tone 0), a Canon EF 70-200 used at 105, f2.8 at f/10 1/400 sec., id est the best conditions to shoot.
Same problems with other lens 20-35 f/2.8 at various aperture and speeds.
Problem solved with Magic Wand to select the sky, Noise Ninja, then Inverted Selection, Unsharp Mask on the building.
This time I was lucky the building border had color tone very different from the surrounding sky, but if I had to shoot a forest on a lake with the blue sky?
Any further explanation about the phenomenon?
Thanks,
Stefano
(the shoot attached was not modified, no sharpening, no Noise Ninja, only reduced to 8 bit/channel, Jpegged to 8, and cropped the to 25%)
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I have the exact same problem on my 350D with the polarizer. it creates noticeable noise at iso100 and at iso200 with clouds in the sky it's simply not worth using a polarizer.
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I've had the same problem Iconoclast had.
Bright blue sky day, used the circular polarizer, and in RAW-Adobe Bridge the sky was "texturized" with niose.
The problem is I used a 20D at ISO 100, (setting: contrast --, sharpness --, saturation -, color tone 0), a Canon EF 70-200 used at 105, f2.8 at f/10 1/400 sec., id est the best conditions to shoot.
Same problems with other lens 20-35 f/2.8 at various aperture and speeds.
Problem solved with Magic Wand to select the sky, Noise Ninja, then Inverted Selection, Unsharp Mask on the building.
This time I was lucky the building border had color tone very different from the surrounding sky, but if I had to shoot a forest on a lake with the blue sky?
Any further explanation about the phenomenon?
Without looking at the original RAW, this is just an educated guess, but since polarizers act as a 1-2-stop ND filter, when shooting at ISO 100 your effective ISO is actually between 200 and 400 because of the light loss from the filter. Increasing exposure time will increase noise; a 2-second exposure will have more noise than a 1-second exposure at the same ISO setting given identical lighting and a 1-stop aperture adjustment to maintain identical overall exposure. In many instances the difference is not very significant, but when noise levels are on the edge of acceptable, sometimes the lengthened exposure times required by the polarizer will increase the noise level to undesirable levels.
Another possibility is that you simply underexposed because you were trying a bit too hard to keep detail in the building wall.
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The noise looks very significant. More than the usual culprits would explain. At first I was thinking it could be that situations that require a polarizer are usually situations with considerable higher ambient temperature. But the resulting noise would not be in this order...
Could possibly be a problem where the Red channel registers lower than expected due to the polarizer, introducing artifacts during debayering because the expected levels for cross channel reproduction are not met.
Does changing White Balance during RAW conversion make a useful difference? (As in something really outrageously low or high?)
Does using the Canon RAW converter make a difference? (Or Leica converter in OP's case).
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Could possibly be a problem where the Red channel registers lower than expected due to the polarizer, introducing artifacts during debayering because the expected levels for cross channel reproduction are not met.
Does changing White Balance during RAW conversion make a useful difference? (As in something really outrageously low or high?)
Does using the Canon RAW converter make a difference? (Or Leica converter in OP's case).
[a href=\"index.php?act=findpost&pid=75322\"][{POST_SNAPBACK}][/a]
In my case, processing a Leica RAW file in ACR does improve some qualities of a picture (mainly contrast), but overall quality is visibly degraded in comparison to shots taken without a polarizer.
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(the shoot attached was not modified, no sharpening, no Noise Ninja, only reduced to 8 bit/channel, Jpegged to 8, and cropped the to 25%)
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Under-exposure combined with a lack of red light in the sky. With the contrast set at -2, the JPEG review image should be barely flashing lighty in little patches in the brightest part of the white paint for optimal exposure. In fact, when you know a deep blue sky is going to be problemic, you might want to take advantage of some RAW converters' ability to render greyscale highlights from just one channel and blow that bright white paint by 2/3 stop.
Judging from the noise, you exposed this image about 2 - 2.5 stops lower than I would have. That means 4 to 6 times the noise in the shadows (the red in the sky is a shadow; the shadow noise is much higher than it would have been, exposed better, at ISO 1600. If you shoot Canon models that came out in 2004 or later, it can not be overstated that under-exposure is *FAR FAR* worse than using a higher ISO with the same f-stop and shutter speed. The only danger of using a higher ISO at a given f-stop and shutter speed is clipping, if you're shooting RAW. I do all my bright sun, non-telephoto work at ISO 200 with my 20D, by default.
Oh yeah, BTW, the 20D is really ISO 115 native, so ISO 100 is not really on my list of ISOs to use. It has 1/4 stop of RAW highlights missing, and Canon stretches the data in the highlights to hide this fact. Why they couldn't have just had ISOs 115 230 460, etc, is beyond me. They cripple cameras to stick to useless standards. Even 125 200 400 ... would have been fine.
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Without looking at the original RAW, this is just an educated guess, but since polarizers act as a 1-2-stop ND filter, when shooting at ISO 100 your effective ISO is actually between 200 and 400 because of the light loss from the filter. Increasing exposure time will increase noise; a 2-second exposure will have more noise than a 1-second exposure at the same ISO setting given identical lighting and a 1-stop aperture adjustment to maintain identical overall exposure. In many instances the difference is not very significant, but when noise levels are on the edge of acceptable, sometimes the lengthened exposure times required by the polarizer will increase the noise level to undesirable levels.
[a href=\"index.php?act=findpost&pid=75303\"][{POST_SNAPBACK}][/a]
These types of shots won't result in any time-related noise. When you do get it, it appears as isolated hot pixels, not readout noise banding artifacts as in the image of the white building against the deep blue sky.
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In my case, processing a Leica RAW file in ACR does improve some qualities of a picture (mainly contrast), but overall quality is visibly degraded in comparison to shots taken without a polarizer.
[a href=\"index.php?act=findpost&pid=75347\"][{POST_SNAPBACK}][/a]
Could you please make it visible to *us*; that's the only way you're going to get any real help. A picture is worth 1000 words; a RAW file is worth 5000 words.
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Could possibly be a problem where the Red channel registers lower than expected due to the polarizer, introducing artifacts during debayering because the expected levels for cross channel reproduction are not met.
[a href=\"index.php?act=findpost&pid=75322\"][{POST_SNAPBACK}][/a]
I don't think there's any kind of threshold effect; just a lack of signal relative to noise in the red channel (and in the green, but to a lesser extent). The red channel gets multiplied by almost 2 to achieve daylight WB - after WB the readout noise in the red channel is higher at ISO 100 than it is in the green channel at ISO 800.
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I don't think there's any kind of threshold effect; just a lack of signal relative to noise in the red channel (and in the green, but to a lesser extent). The red channel gets multiplied by almost 2 to achieve daylight WB - after WB the readout noise in the red channel is higher at ISO 100 than it is in the green channel at ISO 800.
[a href=\"index.php?act=findpost&pid=75355\"][{POST_SNAPBACK}][/a]
That probably explains the relatively large blobs of red signal differences, but does that also explain the overal grainy noise? This image doesn't look like ISO 100 at all, it's that bad. I'm thinking more along the lines of how a polarizer can change the appearance of colors significantly enough so that the default debayering co-efficients per color aren't valid.
Obviously, this is something that should easily be reproducable, but unfortunately the weather here is completely overcast and gray. I'm still going to do some tests though, because it may help identify the real culprit. If for example the UV content is throwing the capture off, it should not be so in conditions lacking UV...
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That probably explains the relatively large blobs of red signal differences, but does that also explain the overal grainy noise? This image doesn't look like ISO 100 at all, it's that bad. I'm thinking more along the lines of how a polarizer can change the appearance of colors significantly enough so that the default debayering co-efficients per color aren't valid.
There's an easy way to test that theory; see what the image looks like without demosaicing. Can you put the RAW file somewhere to download it?
Obviously, this is something that should easily be reproducable, but unfortunately the weather here is completely overcast and gray. I'm still going to do some tests though, because it may help identify the real culprit. If for example the UV content is throwing the capture off, it should not be so in conditions lacking UV...
[a href=\"index.php?act=findpost&pid=75362\"][{POST_SNAPBACK}][/a]
Take RAW images with different combinations filters (CP, UV, CP+UV, none) in manual mode with the same settings on a tripod, exposed so that the one with no filters is exposed to the right. Then we can see what the difference is between them.
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Well, here is a first preliminary test.
Canon DR (300D) Aperture priority @ f11
Artificial daylight @ ~D55
B+W Linear polarizer
Battery happened to be completely charged prior to this test, but earlier tests with depleted battery showed no significant differences.
Following image shows the cccard with 3 exposures:
1. Incorrectly polarized, camera recommended exposure (2s)
2. Incorrectly polarized, recommended exp + 1stop (4s)
3. Correctly polarized, camera recommended exposure (4s)
(http://www.theimagingfactory.com/examples/poltest/poltest1.jpg)
Incorrect polarization = correct pol + 90degr rotation
For incorrect polarization, +1stop results in the tiniest over-saturation, compensated for in ACR by -0.10 exposure correction.
For correct polarization, camera recommended exposure results happened to be perfect ETTR for sRGB.
Clearly, the correctly polarized result is more saturated and has more contrast.
Following image compares the blue patch of the +1stop incorrectly polarized image vs the correctly polarized image @ 200% magnification.
(http://www.theimagingfactory.com/examples/poltest/poltest2.jpg)
Given the significant difference that polarization makes in color rendition, even in this situation, but a complete lack of noise differences, I strongly suspect that I can not reproduce the artifacts with my equipment. It might be of course that a (B+W) linear polarizer has a completely different interaction than a circular polarizer, or something is significantly different in outside conditions.
RAW files:
Correctly Polarized (http://www.theimagingfactory.com/examples/poltest/CRW_6081.CRW)
Incorrectly Polarized (http://www.theimagingfactory.com/examples/poltest/CRW_6085.CRW)
Incorrectly Polarized (+1stop) (http://www.theimagingfactory.com/examples/poltest/CRW_6088.CRW)
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I took the liberty of examining the EXIF data of the noisy shot of the building/sky, and the problem is simply underexposure. The Exposure setting used in ACR was +2.00. With Canon DSLRs, you should expose in-camera so that ACR's exposure setting is between 0 and -0.35 during conversion. If you have to adjust below -0.35, you're going to start having some clipped highlights, and if you have to adjust above 0, you're going to have elevated noise levels. In this instance, the presence of a polarizer is irrelevant; the real problem is operator error.
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In this instance, the presence of a polarizer is irrelevant; the real problem is operator error.
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Indeed! Sometimes the most obvious reason is the correct one. A 1/400th sec at ISO 100 and f10??
In certain bright and contrasty situations, the dynamic range of Canon DSLRs is not great enough. Some of the first shots I took with my new 5D and 24-105 IS lens were of a sunset. I was actually looking for the flare problem in the lens, that everyone was talking about at the time. Exposing for the brightest parts of the image (these were not direct shots into the sun), I discovered the shadows were disturbingly noisy in a way that was impossible to remove, like a fixed irregular pattern like coarse jute or burlap.
But the OP's shot in this thread is just plain underexposed.
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We seem to be homing in on the essence of this problem. Everything points to the underexposure with polarizers.
Since several of us had a similar problem with different circular polarizers and digital cameras of different makes and models, is it safe to say that to get proper exposure in those circumstances, we should override the in-camera meter by +1.5 to +2 EVs? Any other conclusions?
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We seem to be homing in on the essence of this problem. Everything points to the underexposure with polarizers.
Since several of us had a similar problem with different circular polarizers and digital cameras of different makes and models, is it safe to say that to get proper exposure in those circumstances, we should override the in-camera meter by +1.5 to +2 EVs? Any other conclusions?
One should always review the histogram after shooting to verify proper exposure, regardless of whether a polarizer is used or not. I'm skeptical of the notion that a polarizer consistently causes underexposure; I've never experienced such a problem. But I use circular polarizers; linear polarizers can cause problems with metering and autofocus and should never be used on SLR cameras, film or digital. But if one uses a circular polarizer, there's no reason to always dial in +2 EC.
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For correct polarization, camera recommended exposure results happened to be perfect ETTR for sRGB.
Actually. CRW_6081 is actually more than a stop away from any RAW clipping. The brightest RAW pixel is about 1900, out of 3967.
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Actually. CRW_6081 is actually more than a stop away from any RAW clipping. The brightest RAW pixel is about 1900, out of 3967.
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Yes, my apologies, I suppose "perfect ETTR" is a misnomer in this case. ACR in ProPhotoRGB suggests there is still another 1.3 stops to go. In sensor space it may be even more.
I only shot upto +1 stops for both cases, and the brightest incorrectly polarized image matched the normal exposure of the correctly polarized image. The results were already conclusive in that there is absolutely no reason to believe that I can replicate the problem...
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Yes, my apologies, I suppose "perfect ETTR" is a misnomer in this case. ACR in ProPhotoRGB suggests there is still another 1.3 stops to go. In sensor space it may be even more. [a href=\"index.php?act=findpost&pid=75720\"][{POST_SNAPBACK}][/a]
ACR is not mathematically accurate with the exposure slider. There is no real exposure bias control in ACR. There should be one that biases the RAW data in a completely linear fashion, and that one should be called "Exposure". ACR's "Exposure" is a combination of exposure, and built-in curves.
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ACR is not mathematically accurate with the exposure slider. There is no real exposure bias control in ACR. There should be one that biases the RAW data in a completely linear fashion, and that one should be called "Exposure". ACR's "Exposure" is a combination of exposure, and built-in curves.
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Well, ACR internals are supposed to be linear space, so the exposure slider may well be mathematically accurate. The histogram however, only shows colorspace converted data.
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Well, ACR internals are supposed to be linear space, so the exposure slider may well be mathematically accurate. The histogram however, only shows colorspace converted data.
[a href=\"index.php?act=findpost&pid=75724\"][{POST_SNAPBACK}][/a]
Shoot a greystep card at -4, -2, 0, +2, and +4 EC, and then do the opposite with them in ACR. Very different renditions.
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Perhaps Iconoclast is trying too hard to be an iconoclast here. His results make no sense to me. I just tried a Hoya circular polarizer on my Sigma 15-30, at 30mm with my 5D. It's a bright, sunny day. With camera in aperture priority mode and meter in pattern mode (evaluative), pointing the camera at a scene consisting of half sky and half river and fields, I got the following results at f10; with polariser, 1/40th to 1/80th depending upon the circular adjustment of the filter; without polariser, camera pointed at the same scene, 1/200th sec.
I see no reason why the camera's metering system would not read a correct exposure with the addition of a circular polariser.
I once met a guy on my travels who was using a 10D with a polariser permanently fixed to his zoom lens. He'd filled up all his flash cards and had nowhere to store the images, so I downloaded them on my laptop and burned them to CD for him. During the process, I noticed that a lot of his images were not sharp. His shutter speeds were too slow. I explained to him that the polariser was causing this and that it wasn't a good idea to have the polariser permanently fixed to his lens.
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Shoot a greystep card at -4, -2, 0, +2, and +4 EC, and then do the opposite with them in ACR. Very different renditions.
[a href=\"index.php?act=findpost&pid=75741\"][{POST_SNAPBACK}][/a]
Yes, but that is to be expected because that is related to the tonal curve (which will be different for any RAW converter).
The clipping point however should react completely predictable. If it takes a +1.3 exposure correction in ACR to clip, then a +1.3 exposure bias in camera capture should exactly move the data to the point of clipping.
Problem of course is that bright colors may commonly clip sooner than neutral highlights. In addition, the detail tab settings will SIGNIFICANTLY change the histogram as wel. So you may see clipping in the ProPhoto histogram and still have RAW data latitude. Only a RAW data histogram (possibly rendered in "stops" for perceptual uniformity) would show the true ETTR latitude.
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Yes, but that is to be expected because that is related to the tonal curve (which will be different for any RAW converter).
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Obviously, clipping points will be different, but unclipped, rendered tones will be different, too, which proves beyond a doubt that there is no simple linear adjustment to the data with the "exposure" slider. ACR does not allow you to expose all the way to the right, and get a reasonable render with negative exposure compensation. You need another converter for that.
Like everything Adobe, ACR has some great features, and some big, gaping holes.
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iconoclast
May I go back to your very first post in this thread where you write that you went down to the beach with a girl to shoot some summer pictures. Now, I hope that the girl was only along for the ride - the use of a polarizing filter on flesh is not a pretty effect at all, certainly on colour film, where the effect is that of turning tan into brickwork.
If you take the time to look carefully at some of the better model photographers who do this kind of work, you will not see many of their pictures where the highlights have been polarized off the skin! On the contrary: the industry makes great use of the application of makeup and sun-oil products to create, purposely, a noticeable shine. There are, of course, as with everything else, exceptional cases, as in some fashion work where the whole deal is about creating saturated colour in fabric or even location. This can be harsh as hell, but if the effect is satisfying to the commissioning editor...
You can, of course, use it (the filter) with success if you keep the figure small in the frame and make the location the principal feature, particularly if you can manage to get the beautiful fluffy white clouds that some places seem to create - then, the polarizer will do its work well and be worth its weight in gold.
As with so much else: use with caution!
Best wishes - Rob C
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iconoclast
May I go back to your very first post in this thread where you write that you went down to the beach with a girl to shoot some summer pictures. Now, I hope that the girl was only along for the ride - the use of a polarizing filter on flesh is not a pretty effect at all, certainly on colour film, where the effect is that of turning tan into brickwork.
If you take the time to look carefully at some of the better model photographers who do this kind of work, you will not see many of their pictures where the highlights have been polarized off the skin! On the contrary: the industry makes great use of the application of makeup and sun-oil products to create, purposely, a noticeable shine. There are, of course, as with everything else, exceptional cases, as in some fashion work where the whole deal is about creating saturated colour in fabric or even location. This can be harsh as hell, but if the effect is satisfying to the commissioning editor...
You can, of course, use it (the filter) with success if you keep the figure small in the frame and make the location the principal feature, particularly if you can manage to get the beautiful fluffy white clouds that some places seem to create - then, the polarizer will do its work well and be worth its weight in gold.
As with so much else: use with caution!
Best wishes - Rob C
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Thank you, Bob C.
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FWIW, I use polas (B/W schneider and Hoya) and don't see anything resembling the problems you folks are discussing. To avoid brand wars, I'll only say that I'm not using Canon.
Is there any possibility that all this is something specific to Canon's sensor technology? Serious question, and flames are deflections.
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Is there any possibility that all this is something specific to Canon's sensor technology? Serious question, and flames are deflections.
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No, I don't believe so and I have 3 Canon DSLRs, the D60, 20D and 5D. I think what has probably happened here is that iconoclast has used manual mode, setting the same exposure that he got without the polariser. The EXIF data shows an exposure of 1/400th at f10. That would be an underexposure even without a polariser, at ISO 100.
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That makes a lot more sense Ray. The whole discussion is a head scratcher verging tempest in a teapot.
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FWIW, I use polas (B/W schneider and Hoya) and don't see anything resembling the problems you folks are discussing. To avoid brand wars, I'll only say that I'm not using Canon.
Is there any possibility that all this is something specific to Canon's sensor technology? Serious question, and flames are deflections.
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No, it's not Canon-specific. I use Leica Digilux 2.
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No, I don't believe so and I have 3 Canon DSLRs, the D60, 20D and 5D. I think what has probably happened here is that iconoclast has used manual mode, setting the same exposure that he got without the polariser. The EXIF data shows an exposure of 1/400th at f10. That would be an underexposure even without a polariser, at ISO 100.
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I've used 'program' mode. And my EXIF data looks like that:
- exposure: normal program
- shutter speed: 1/250; 1/400; (two different groups of photos)
- aperture value: f/5.6
- max aperture value: f/2.0
- ISO speed rating: 100
Indeed, those photos taken with faster shutter speed (1/400) look more degraded than others.
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I've used 'program' mode. And my EXIF data looks like that:
- exposure: normal program
- shutter speed: 1/250; 1/400; (two different groups of photos)
- aperture value: f/5.6
- max aperture value: f/2.0
- ISO speed rating: 100
Indeed, those photos taken with faster shutter speed (1/400) look more degraded than others.
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Sorry! I got my wires crossed here. I see you haven't posted an image. The image I'm referring to is the one posted by Stefano, taken with a 20D, who claims he's noticed the same problem. Checking the EXIF data again of his image, I see that he had his camera in aperture priority mode with an exposure bias of minus 1.67 stops. During conversion in ACR, he applied +2 EC which tends to indicate the shot was underexposed by more than the exposure bias of 1.67 stops. For a full exposure to the right, one should be applying a negative EC in ACR of around -0.35 EC, as Johnathan has said, and sometimes even -1 EC.
I'd say that Stefano's shot was underexposed by around 2.5 stops.
We can't really comment on iconoclast's problem until he posts an image with the EXIF data.
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We can't really comment on iconoclast's problem until he posts an image with the EXIF data.
I am having a problem with posting a sample photo. Almost all pictures in the series in question are nudes and I do not have an authorization to make them public. Those shots where the girl is clothed, do not illustrate the problem very well. Anyway...
The whole discussion so far tended to prove that the underexposure was the culprit. I couldn't imagine how that could have happened to my photos, but it seemed logical to me. So I went back to the EXIF data. Normally, I read only the 'Camera Data 1' in Photoshop's File Info. This time, I went to the actual EXIF data in the 'Advanced' panel and... surprise... I found out that the exposure has been biased 33/100 or 1/3 f/stop. This is still not a really large underexposure, but it might affect a general level of noise. Especially so, that my camera generally is very noisy (that's why I only use ISO 100). So, turns out that those shots where the shutter speed was 1/250 were not biased and looked better and those with the shutter speed 1/400 were biased and looked worse.
Don't ask me why the exposure has been biased. I don't know. My model had the camera in her hands for about 2 minutes. Perhaps she pressed some buttons she shouldn't press... Yes, I should have known better! Sorry for the "tempest in a teapot", as Hank put it. Even though the problem now looks minor and totally created by my inadvertent mistake, I feel a bit embarassed.
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Is there any possibility that all this is something specific to Canon's sensor technology? Serious question, and flames are deflections.
At this point, I'd say it's user error, and not related to any camera brand or technology. FWIW, I have a 1Ds and 1D-MkII and have not had exposure problems with polarizers.
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Obviously, clipping points will be different, but unclipped, rendered tones will be different, too, which proves beyond a doubt that there is no simple linear adjustment to the data with the "exposure" slider. ACR does not allow you to expose all the way to the right, and get a reasonable render with negative exposure compensation. You need another converter for that.
Like everything Adobe, ACR has some great features, and some big, gaping holes.
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Well, I did some experiments with overexposures of a MacBeth Color Checker exposed under daylight at 2, 3, 4, and 8 seconds with a Nikon D200. The 2 second exposure gave proper results with ACR at default settings. The white patch RGB ProPhotoRGB reading of the 2 s exposre was 247, 248, 246. I converted the NEFs with ACR into ProPhotoRGB with the exposure adjusted to give a reading of 248 from the remaining white patches, except for the 8 second exposure (+2 EV) where I used an exposure adjustment of -2. With this degree of overexposure, it is not possible to bring the white square below 255.
I also converted the NEFs with Iris 5.33 and recorded the 12 bit raw values.
The results are shown in the table below. W1 is the white patch on the left and W2 is the next patch to the right. McR, McG, and McB are the red, green, and blue patches respectively. The RGB values and saturation (from HSB readout of PSCS2) are shown in the table below. The image is in sRGB for web display (converted from the ProPhotoRGB)
[attachment=1017:attachment]
[attachment=1018:attachment]
With +0.6 EV overexposure, W1 begins to blow in the green channel, and with 1 EV of overexposure, the green channel is blown in both W1 and W2 as shown by the raw readouts. However, even at +2EV none of the RGB patches were blown in any of the RGB channels and the color values and saturation of all these patches were virtually the same up to - 1 EV of exposure compensation, and changed only slightly even with -2 EV.
Therefore, I conclude that exposure compensation in ACR is linear in real world shooting situations with up to one stop of highlight recovery. Above this, the conversion is nonlinear, since one can not bring the white patch down under 255. According to Bruce Fraser in his ACR with PSCS2, one should expect no more than 0.5 to 1 stop of recovery with ACR. I conclude that ACR highlight recovery produces good results with ETTR under these conditions, and one can shoot fully to the right within these limits.
Bill