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Author Topic: Using a polarizer with a digital camera  (Read 41864 times)
Ray
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« Reply #40 on: September 08, 2006, 09:49:21 AM »
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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.
« Last Edit: September 08, 2006, 09:50:45 AM by Ray » Logged
Hank
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« Reply #41 on: September 08, 2006, 10:14:56 AM »
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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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iconoclast
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« Reply #42 on: September 08, 2006, 06:31:31 PM »
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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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Rob
iconoclast
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« Reply #43 on: September 08, 2006, 07:12:55 PM »
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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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Rob
Ray
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« Reply #44 on: September 08, 2006, 08:05:48 PM »
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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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iconoclast
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« Reply #45 on: September 08, 2006, 09:31:23 PM »
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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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Rob
Jonathan Wienke
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« Reply #46 on: September 09, 2006, 05:18:26 AM »
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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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bjanes
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« Reply #47 on: October 05, 2006, 11:35:19 AM »
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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
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