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Author Topic: Why is there only 14 bits in Canon/Nikon ?  (Read 8003 times)
John Sheehy
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« Reply #20 on: February 28, 2008, 04:56:32 PM »
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just curious

why would full well very by ISO?

No.  "Full well" is a feature of the sensor's photosites.  It is how many electrons one has captured when it is full.  A camera may or may not use the full range of the sensor at its lowest ISO.

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And does read noise very much by ISO?
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Read noise as quoted in electrons may or may not decrease at higher ISOs, depending on the particular design.  Most digital cameras have almost the same read noise in electrons at all ISOs, varying sometimes slightly by the contribution of the analog-to-digital converter and second-stage amplifier noises.  Canon DSLRs and recent Nikon DSLRs have read noises that decrease rapidly, as measured in electrons, from the lowest ISO to about 4x to 8x that ISO, and decreasing more slowly, if at all, after 1600 or 3200.

Read noise as quoted in a percentage of maximum signal always increases with increasing ISO.  With the cameras that have almost the same read noise in electrons at all ISOs, the read noise relative to maximum signal at any ISO is proportional to the ISO.  For the Canon DSLRs and recent Nikons DSLRs, this read noise increases slowly at the low ISOs, and starts to almost double with a doubling of ISO between the higher ISOs.
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DiaAzul
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« Reply #21 on: February 28, 2008, 05:10:34 PM »
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What are the odds that Canon and Nikon, working on totally different technological/supplier communities, come up precisely at the same time with 14 bits sensors, nearly within days of each others?


Cheers,
Bernard
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Probably the same as Mclaren coming out with an F1 car using surprisingly similar technology to Ferrari within the same season  

But then, Nasa didn't go to the moon either

Looks like we need a photo  conspiracy blog. Manufacturers...they're just out to get you!
« Last Edit: February 28, 2008, 05:11:19 PM by DiaAzul » Logged

David Plummer    http://photo.tanzo.org/
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« Reply #22 on: February 29, 2008, 12:46:17 AM »
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I'm not sure what it means but if I do a RAW conversion of a 5D file and a P30 file the P30 fills the histogram (16 bits) the 5D does not (12 bits)

1. The appearance of the histogram does not depend the very least on the bit depth.

2. I displayed both images with ACR; after WB on a grey square, resetting brightness and contrast to 0 and increasing the "exposure" of the P30 image by one EV, both histograms are filled.

Note, that ACR makes an automatic -1 EV adjustment to P30 and P45 images @ ISO 100, +1 @ ISO 400 and +2 @ ISO 800 (I don't know how this is @ ISO 200). I don't know the reason, but if you want to see the shot with the original exposure, you need to compensate for the automatic adjustment (it is not indicated anywhere by ACR!).

3. Several squares show clipping in both images.

Now, what about the raw data?

The first two attached images show the raw histograms.  

The 5D image shows the perfect exposure. The exposure is maximized, but nothing clipped (apart a few stray pixels). Note the saturation level: 3692.

The green in the P30 images is significantly clipped: the white square (the bottom left) is completely clipped, *but nothing else*. The saturation level is 65535.

So, ACR's showing several squares as clipped is plainly incorrect - but why?

Let's see, what makes the ACR histogram sop different from the raw histogram?

*Note*: the following does not include

a. demosaicing,

b. color transformation from the camera's color space in sRGB or aRGB.The missing steps would cause other differences as well.

The first step is white balancing, on the middle grey square. This yields the following coefficients:

5D:  red = 2.04, blue = 1.35

P30:  red = 2.48, blue = 1.29

Now, let's see the histograms after WB application and mapping ("gamma encoding").

The third image is of the 5D. It looks good, except that there is some red clipping (the rightmost bump comes from the ominous white square of the checker). This red ended originally somewhere at 1830; after WB application it became 3733, and that is over the current white point (which started out with the saturation level).

The solution is either decreasing the brightness, or increasing the white point. I chose the latter one; see the fifth image, showing the histogram with white point 3942 - and there is no clipping any more.

The same could be achieved in ACR by either reducing the "exposure", or, much better, increasing the "recovery" (which does not recover anything in this case, because there is nothing lost).

Now, to the P30 shot. The fourth image shows the histogram after WB application and mapping. The bump of the white square vanished completely, i.e. it is cipped; thisis due to the WB application. Again the red dictates the upper limit: it ends around 29000, that with the WB coefficient 2.48 yields 72000. After increasing the white point to 72000 the red and blue clipping vanishes (see the sixth attached image). However, the green remains clipped: that is truy lost.

Finally, one note to the P30: it has a (small) weekness in comparison of the 5D. The difference in response between the red and the green is much higher with the P30. This results in a somewhat reduced dynamic range *in this lighting*.
« Last Edit: February 29, 2008, 12:50:45 AM by Panopeeper » Logged

Gabor
marcmccalmont
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« Reply #23 on: February 29, 2008, 02:47:15 AM »
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I have noticed sometimes the P 30 clips the green channel even if the displayed histogram or blinkies don't show it, A quirk of the P30? I guess I have to be more conservative when I ettr.
Marc
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Marc McCalmont
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« Reply #24 on: February 29, 2008, 07:10:08 AM »
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1. The appearance of the histogram does not depend the very least on the bit depth.

2. I displayed both images with ACR; after WB on a grey square, resetting brightness and contrast to 0 and increasing the "exposure" of the P30 image by one EV, both histograms are filled.

Note, that ACR makes an automatic -1 EV adjustment to P30 and P45 images @ ISO 100, +1 @ ISO 400 and +2 @ ISO 800 (I don't know how this is @ ISO 200). I don't know the reason, but if you want to see the shot with the original exposure, you need to compensate for the automatic adjustment (it is not indicated anywhere by ACR!).

3. Several squares show clipping in both images.

Now, what about the raw data?

The first two attached images show the raw histograms. 

The 5D image shows the perfect exposure. The exposure is maximized, but nothing clipped (apart a few stray pixels). Note the saturation level: 3692.

The green in the P30 images is significantly clipped: the white square (the bottom left) is completely clipped, *but nothing else*. The saturation level is 65535.

So, ACR's showing several squares as clipped is plainly incorrect - but why?

Let's see, what makes the ACR histogram sop different from the raw histogram?

*Note*: the following does not include

a. demosaicing,

b. color transformation from the camera's color space in sRGB or aRGB.The missing steps would cause other differences as well.

The first step is white balancing, on the middle grey square. This yields the following coefficients:

5D:  red = 2.04, blue = 1.35

P30:  red = 2.48, blue = 1.29

Now, let's see the histograms after WB application and mapping ("gamma encoding").

The third image is of the 5D. It looks good, except that there is some red clipping (the rightmost bump comes from the ominous white square of the checker). This red ended originally somewhere at 1830; after WB application it became 3733, and that is over the current white point (which started out with the saturation level).

The solution is either decreasing the brightness, or increasing the white point. I chose the latter one; see the fifth image, showing the histogram with white point 3942 - and there is no clipping any more.

The same could be achieved in ACR by either reducing the "exposure", or, much better, increasing the "recovery" (which does not recover anything in this case, because there is nothing lost).

Now, to the P30 shot. The fourth image shows the histogram after WB application and mapping. The bump of the white square vanished completely, i.e. it is cipped; thisis due to the WB application. Again the red dictates the upper limit: it ends around 29000, that with the WB coefficient 2.48 yields 72000. After increasing the white point to 72000 the red and blue clipping vanishes (see the sixth attached image). However, the green remains clipped: that is truy lost.

Finally, one note to the P30: it has a (small) weekness in comparison of the 5D. The difference in response between the red and the green is much higher with the P30. This results in a somewhat reduced dynamic range *in this lighting*.
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This is an excellent demonstration that can be supplemented by Guillermo Luijk's discussion of [a href=\"http://www.guillermoluijk.com/tutorial/dcraw/index_en.htm]DCRaw[/url], even though Panopeeper's RawAnalyze tool can largely supplant DCRaw for this type of analysis.

Particularly important are the -H options in DCRaw, which control clipping of highlights in white balancing. It the white balancing multipliers are greater than 1 (as is usually the case) and integer math is used, one can have clipping from overflow. If the multipliers are less than 1, then there will be no clipping but one may have color shifts.

-H 0 forces at least one multiplier be equal to 1, and the rest will be greater or equal to 1
 -H [1-9] forces at least one multiplier be equal to 1, and the rest will be less or equal to 1

Also important is the saturation command, -S; the following is a quote from from Guillermo's essay:


"However the saturation level -S is closely related to the behaviour of the RAW developer in the highlights so it is very interesting to be able to set it as DCRAW like any other RAW developer will use a standard saturation values table for each camera model, but this value could not be adequate for our particular unit:

    * If our camera saturates the RGB channels in a level lower to that considered by DCRAW we could experiencience magenta cast issues with effects similar to those found in Fig. 6 because of channel missalignment.
    * On the other side if our camera would saturate at levels higher than the standard value used we would be unnecessarily losing highlights information in the development process.

That is why it is a good idea to know exactly the saturation level of our camera's RGB channels and apply it to optimise the development. We shall take an example from a Canon 40D RAW file with highlight areas shot at ISO100:"

The saturation table may also be off with ACR, and this may explain its behavior with the P30.

Bill
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ejmartin
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« Reply #25 on: February 29, 2008, 06:20:00 PM »
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I have noticed sometimes the P 30 clips the green channel even if the displayed histogram or blinkies don't show it, A quirk of the P30? I guess I have to be more conservative when I ettr.
Marc
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IIRC there are no separate gains for different ISO with this camera, rather simply a metadata tag that tells the raw converter what factor of two to multiply the raw data by before conversion (1/2 for ISO 100, 1 for ISO 200, 2 for ISO 400, etc).  This resonates with panopeeper's comment about EV compensation that is applied in ACR.  If the native ISO of the camera were 200, then ISO 100 is an overexposure by +1EV which is then subsequently pulled during raw conversion, which would explain the lack of headroom in the dynamic range.  Consequently it would be better to shoot at ISO 200.
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emil
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« Reply #26 on: February 29, 2008, 08:37:10 PM »
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The saturation table may also be off with ACR, and this may explain its behavior with the P30
ACR's understanding of the P30 saturation is correct (it can not be missed easily, it's 65535).

However, ACR's saturation setting is off for several other cameras.

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IIRC there are no separate gains for different ISO with this camera, rather simply a metadata tag that tells the raw converter what factor of two to multiply the raw data by before conversion (1/2 for ISO 100, 1 for ISO 200, 2 for ISO 400, etc).
That would be a too easy explanation. Although I don't have suitable images with the P30, but I do some with the P45. The situation is the same: an auto adjustment will be applied by ACR. However, those images demonstrate clearly, that the ISO setting is effective on the raw data. I would not vouch on the analog origin of that data (the histograms are incredible), but it is not obviously "uoresed". And, anyway, if the pixel values are stretched, as some cameras are doing it, then there is no reason to adjust the brightness, in fact that must not be done.

On the other hand, it is possible that some earlier Phase One camera acted that way, and Adobe has not noticed yet, that the ISOs are now true, or at least they look like true.
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Gabor
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