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Author Topic: dynamic range  (Read 43293 times)
Ray
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« Reply #40 on: October 05, 2007, 12:33:56 AM »
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Well, I mean in the shadow end of the DR at ISO 100; if the camera exposes the RAW data more conservatively, however, there could be less improvement in the shadows and more in the highlights.  Metering is somewhat arbitrary.
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Since it is unlikely that the photoreceptors or photodiodes on the 40D's sensor have greater capacity than the photoreceptors on the 20D, it would seem unlikely that the 40D could have greater dynamic range in the highlights, no matter what the increase in bit depth. Surely any improvement would be revealed in the shadows, assuming identical exposure adjusted for any variance in ISO accuracy between the two cameras.

Whenever I've tried to compare the DR of 2 cameras (such as the D60 and 20D), I've taken a series of shots with both cameras at 1/3 stop and 1/2 stop intervals, then tried to find as close a match as possible between any 2 images that appear to be equally and fully exposed to the right and which appear to contain equal amounts of highlight detail and/or equal degrees of blown specral highlights.

I've then examined the shadows for noise and detail.
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reissme
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« Reply #41 on: October 05, 2007, 01:09:06 AM »
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For me DR is the most important thing in the whole MFBD discussion. The abrupt way a DSLR (in my case 5D) goes from a defined area to a blown out area is the most disturbing and unaesthetic point in digital photography for my work. As long as a subject is in controlled light, there are little problems, but a reflex or a bit backlight might destroy the whole atmosphere of a photo. My three or four year old back behaves much better (film-like) here and is definitely worth the effort. If Canon and Nikon came along with sensors capturing the same DR I would probably drop the MF thing for a faster and more comfortable working.
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reissme
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« Reply #42 on: October 05, 2007, 02:01:03 AM »
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I recently switched from a Canon 5D (excellent camera) to a P30+ on a Mamiya 645AFDII. 

I was not sure if the DR claims were real or just hype.  According to the specs the DB should have 2-3 stops more dynamic range than the 5D.

After using the back for a few months I am still amazed at the increase in dynamic range.  I have found both shadow and highlights to have more accurate detail.  For example, if I have shadow areas on the 5D that need to be lightened you will see noticeable noise.  With the P30+ there is  less noise and more detail in those shadows. 

Imagine you are shooting landscapes where you would normally use a 2 stop ND grad filter or take two exposures and layer them.  With 2-3 stops extra range, simply forget about it and shoot. 

Combine the DR with no AA filter, better optics, higher resolution and greater bit depth and the difference can be dramatic even for 16x20 prints.

8 bit jpeg: 256 levels
12 bit raw: 4096
14 bit raw: 16384
16 bit raw: 65536

I also switched from a Canon 1DmkII at 12 bits to the 1DmkIII at 14 bits.  The difference with just a 2 bit increase (4X) makes substantial improvements in image quality.  Multiply that difference by 16 when comparing 12 bit to 16 bit images.  More levels means cleaner grads and more realistic detail.  Makes the announced Canon 1Ds MK III look very interesting and will split the advantages of high end DB image quality.
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Hi
I simply prefer to see real side by side pictures of the same high contrast scene that shows the difference between a P30+ on a Mamiya and the 1DmkIII at 14 bits. a true comparison that will show DR in shadows and in highlights, colors and gradation. it can save a lot of discussions about this topic and will show real life shooting instead of camera manufacturer claims or theories . CAN ANY BODY HERE CAN DO THIS SIMPLE COMPARISON AND PRESENT IT HERE???  
 
Menachem reiss     www.reiss.co.il
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Jonathan Wienke
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« Reply #43 on: October 05, 2007, 06:24:40 AM »
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It sounds like you are trying to separate dynamic range and bit depth.  This is possible conceptually, but as I discussed in a previous post, it is not possible in practice, at least with current linear sensors.  Increased bit depth gives both increased smoothness and increased dynamic range.

Bullcrap. There's also noise to consider, the LEAST significant of which is quantization error. With the vast majority of cameras out there (MFDB, DSLR, and digicams) the shot noise, read noise, dark current noise, and other forms of noise that end up in the RAW come into play before you start having quantization issues. Going from 12 bits to 14 bits is meaningless if the median RAW value is 128 (on a 14-bit scale) when shooting a dark frame, you have only 7 stops between the clipping point and the noise floor.

Dynamic range is simply the number of stops between clipping and the noise floor. Defining the noise floor can be a bit subjective, because some types of noise are more visually acceptable than others, and a simple standard deviation calculation or similar statistical analysis does not account for this. If the most significant factor in the noise floor is quantization error, then increasing bit depth will be beneficial. But if other noise sources are more significant (which in almost always the case) increasing bit depth will NOT increase usable dynamic range. If the black level in the RAW data is below 16, then you should worry about quantization error. Otherwise, you need to deal with other noise sources before increasing bit depth will provide any meaningful benefit.
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John Sheehy
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« Reply #44 on: October 05, 2007, 07:52:26 AM »
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I wonder if its time to do some real world tests.

We can argue 'till the cows come home about wether it's the bit depth that makes the difference (my opinion is that it isn't).
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There is also the issue of whether any perceived benefits are due directly to the actual contents of the extra bits, or just the fact that they may force converters to use more precision.
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John Sheehy
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« Reply #45 on: October 05, 2007, 08:12:10 AM »
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If that higher read-noise at lower ISO speeds is indeed largely due to the noise floor of the A/D converter itself, a better A/D converter could help. However, if Clark is already accusing a 14-bit A/D converter of limiting DR to well under 14-bits (16000:1), I am a bit skeptical that he might be wrongly singling out the ADUs as the main DR bottleneck.

I'm pretty sure he's wrong about that for Canons.  Most other brands of DSLR clearly have a single initial read amplification, and then just use a low-gain amp to drive the ADC for different ISOs.  Nikon D2x for example, and many of the older Nikons, have 15x the blackframe noise in ADUs at ISO 1600, compared to ISO 100.  That small difference in blackframe noise in electrons is answered simply by the ADC noise.  For Canons, however, all the evidence points to the initial read at the photosite just being noisier (in electrons) for the lowest ISOs.  The 1D* cameras and the 5D all have groups of ISOs - x, 1.25x, and 1.6x that all have the same blackframe noise in electrons.  And every other group of three is different from every other group of three.  There are no gaps or spikes in their histograms unique to the 1.25x and 1.6x ISOs, so the scaling is done *before* the ADC, so if the initial photsite reads were all the same, then blackframe noise in electrons would be more close for 160 and 200, 320 and 400, etc.

Also, if you assume that the total blackframe noise is the square root of the sum of photosite read noise squared plus ADC noise squared, no values will satisfy real world blackframe noise at ISOs 1600, and 100, while also satisfying ISO 400, on the cameras I've tested.

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If instead a good part of the noise arises earlier in the trip from electron well to A/D converter, a better ADU would not help as much. And as far as I can tell, Clark's measurements cannot distinguish, since all he has is the output of the A/D converter, not its analog input signals.
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Yes, I think that this is the real problem, IMO.  The 400D and the 40D get read noise down to 1.65 and 1.38 ADU (12-bit) respectively, compared to around 2.0 and higher for previous cameras, so it may be a better ADC in use (the improvements are greatest at the lower ISOs) over previous cameras in the series.  The "pro" cameras have had blackframe noises down around 1.26 for a while now.
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digitaldog
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« Reply #46 on: October 05, 2007, 09:15:01 AM »
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Dynamic range is simply the number of stops between clipping and the noise floor. Defining the noise floor can be a bit subjective, because some types of noise are more visually acceptable than others, and a simple standard deviation calculation or similar statistical analysis does not account for this. If the most significant factor in the noise floor is quantization error, then increasing bit depth will be beneficial. But if other noise sources are more significant (which in almost always the case) increasing bit depth will NOT increase usable dynamic range.

That sync's up exactly with my understanding of all this, certainly with scanners and, unless someone can explain why a linear encoding would be much different, cameras too.

The part above about the useable data above noise is also the clincher. Who decides at what point we start measuring? In the old days, a company like Agfa and Microtek would have different dynamic range spce's for what was in fact the identical hardware. Let the marketing boys into the mix, the science gets a bit dicey.

Maybe Michael can do a piece on this and put to rest the controversy.

The one guy who I deeply respect to provide the skinny is Mike Collette who basically invented the scanning back (Betterlight). But I haven't run into him or heard from him in a few years, not sure if he's still around. But this guy knows his stuff and can explain it so I can understand it.
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Andrew Rodney
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EricV
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« Reply #47 on: October 05, 2007, 11:54:28 AM »
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Bullcrap. There's also noise to consider, ....[a href=\"index.php?act=findpost&pid=143978\"][{POST_SNAPBACK}][/a]
If you go back and read the previous post I was referring to, you will see that it started:

"Bit depth and dynamic range really are the same thing, provided that
1) System noise is low enough that bits are not wasted digitizing noise
2) The ADC is linear, so each bit represents twice as much signal as the previous bit."

I knew that without the noise disclaimer someone would make a post much like yours (though perhaps without the first sentence), but I guess the point has to be repeated every time.  So, bullcrap to your bullcrap.
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Jonathan Wienke
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« Reply #48 on: October 05, 2007, 12:49:53 PM »
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Even if both conditions are met, bit depth is merely one factor that limits dynamic range, not a measurement of dynamic range itself, just like the displacement of a car engine is a factor that limits its maximum horsepower, but is not useful as an actual horsepower measurement; there's too many other factors that come into play. You still have to specify a minimum number of levels to provide an acceptably low level of shadow posterization (most people would say betweeen 8 and 32, depending on who you you ask) and factor in the fact that white balance scaling is never 1:1:1 between the color channels (there can be over 2 stops of difference between the levels of the red channel and blue channel when shooting with tungsten light) so dynamic range is going to be 3-6 stops less than bit depth, depending on WB of your lighting and how fussy you are about shadow posterization. And then there's the fact that the highest voltage value from the sensor is usually calibrated to produce a slightly-below-maximum outputput value from the ADC, so your maximum numeric output from the ADC is usually 10% or so below the greatest possible mathematical value. For example, Canon DSLRs usually clip about value 3600 out of a possible 4095 maximum. The theoretical best DR you're going to get out of a 16-bit system is about 12 stops, even if sensor and ADC noise is not a factor at all (which is never the case). So you're wrong in theory, and even more wrong in real-world practice.

So bullcrap to your bullcrap on my bullcrap.
« Last Edit: October 05, 2007, 12:58:27 PM by Jonathan Wienke » Logged

BJL
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« Reply #49 on: October 05, 2007, 01:17:27 PM »
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"Bit depth and dynamic range really are the same thing, provided that
1) System noise is low enough that bits are not wasted digitizing noise
...
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Your statement seems true under its hypotheses, but becomes irrelevant if the assumptions are not true.
You are assuming the answer to what is for many of us a critical question:
is other noise, such as pre-amp noise, low enough to be negligible relative to the quantization noise of 12-bit or 14-bit AD conversion?
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Jonathan Wienke
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« Reply #50 on: October 05, 2007, 04:45:59 PM »
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Your statement seems true under its hypotheses, but becomes irrelevant if he assumptions are not true.

It isn't true under any circumstances, regardless of the validity of the qualifying assumptions. Bit depth limits maximum dynamic range, but does not define it. Even in purely theoretical terms, dynamic range cannot be defined by bit depth alone, the issue of acceptable shadow posterization (how many levels are required for acceptably smooth shadow tone gradations?) must be factored in to the calculation, and is going to reduce DR 3-5 stops from bit depth depending on how picky you are about smooth shadow tonality.

And of course, you cannot eliminate real-world considerations such as sensor, pre-amplifier, and ADC noise, the color channel multipliers required for proper white balance (anything other than 1:1:1 is going to decrease DR by making one of the color channels clip sooner than it would otherwise), and the issue of the ADC not outputting 100% of its numerical range.

To properly measure DR, one needs a standardized method of measuring all noise factors, weighting the negative visual impact of each, and defining an "unacceptable" threshold value for the combined result. It's easy to define the clip point, but we're still measuring the location of the noise floor with a rubber ruler. To do this appropriately, I think that the following factors need to be considered:

Chroma error: The distance between a recorded *a*b coordinate and its actual location in L*a*b space.

Luminance error: The difference between the recorded L value and its actual value in L*a*b space.

Frequency: High-frequency noise is less visually intrusive than low-frequency noise; fine grain is not as annoying as big blobs.

The fun part, of course, is defining the weighting for all these factors and devising a formula to calculate a noise measurement that accurately defines how intrusive any given set of noise factors is on a given image, and then defining a threshold noise value that could be agreed on as the noise floor. Then there's the issue of defining the correct methodology for capturing the image(s) that can be analyzed to accurately calculate all of this stuff...
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Eric Myrvaagnes
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« Reply #51 on: October 05, 2007, 08:48:11 PM »
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The fun part, of course, is defining the weighting for all these factors and devising a formula to calculate a noise measurement that accurately defines how intrusive any given set of noise factors is on a given image, and then defining a threshold noise value that could be agreed on as the noise floor. Then there's the issue of defining the correct methodology for capturing the image(s) that can be analyzed to accurately calculate all of this stuff...
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Maybe Dan M will come up with an action to do this, more or less, except for needing a bit of adjustment when it doesn't work ...  
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TechTalk
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« Reply #52 on: October 05, 2007, 09:34:08 PM »
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That sync's up exactly with my understanding of all this, certainly with scanners and, unless someone can explain why a linear encoding would be much different, cameras too.

The part above about the useable data above noise is also the clincher. Who decides at what point we start measuring? In the old days, a company like Agfa and Microtek would have different dynamic range spce's for what was in fact the identical hardware. Let the marketing boys into the mix, the science gets a bit dicey.
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Absolutely right Andrew.

There is a scientific measure for dynamic range. It is measured at the sensor output and it is called signal to noise ratio and expressed in decibels (dB). The specification should include the sensor temperature as raising or lowering the sensor temperature changes the signal to noise ratio and therefore the dynamic range.
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Ray
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« Reply #53 on: October 06, 2007, 03:00:25 AM »
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Whilst it's understandable that getting all manufacturers of imaging devices to adhere to an international standard for measuring dynamic range is probably unrealistic in view of the fact that a certain amount of bullshit is required to sell the product, there's no reason why disinterested individuals and reviewers cannot do their own comparisons and publish the results.

Well, I suppose there is at least one reason and that's the lack of the cameras to test. I don't suppose anyone would like to lend me his P45+, would he?  

From the perspective of the buyer, it's more meaningful and useful to know that camera (A) has a higher DR than camera B and to be able to view the difference in terms of greater or less shadow noise/detail in images with equal highlights.

As long as the methodology is the same for each comparison (and the target and lighting the same, of course), the results should be valid and probably more meaningful to the non-technically minded than a decibel figure.
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TechTalk
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« Reply #54 on: October 06, 2007, 01:05:53 PM »
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Whilst it's understandable that getting all manufacturers of imaging devices to adhere to an international standard for measuring dynamic range is probably unrealistic in view of the fact that a certain amount of bullshit is required to sell the product, there's no reason why disinterested individuals and reviewers cannot do their own comparisons and publish the results.

Well, I suppose there is at least one reason and that's the lack of the cameras to test. I don't suppose anyone would like to lend me his P45+, would he?   

From the perspective of the buyer, it's more meaningful and useful to know that camera (A) has a higher DR than camera B and to be able to view the difference in terms of greater or less shadow noise/detail in images with equal highlights.

As long as the methodology is the same for each comparison (and the target and lighting the same, of course), the results should be valid and probably more meaningful to the non-technically minded than a decibel figure.
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It would also be useful to compare under different conditions. Two cameras that give similar reults under one set of conditions can give noticeably different results under other conditions. Comparing after long sequence captures and different ambient temperature conditions will affect the result. The dynamic range is not fixed.

To convert dB to bits, 6dB = 1 bit.
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Ray
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« Reply #55 on: October 07, 2007, 12:30:49 AM »
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It would also be useful to compare under different conditions. Two cameras that give similar reults under one set of conditions can give noticeably different results under other conditions. Comparing after long sequence captures and different ambient temperature conditions will affect the result. The dynamic range is not fixed.

To convert dB to bits, 6dB = 1 bit.
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Dynamic range in decibels is usually the figure that manufacturers quote for their sensor. It seems to be more of a theoretical figure relating to photosite size and bit depth. The dynamic range of the camera incorporating the sensor can be quite different since other design factors come into play.

Comparing DR under different conditions could provide additional useful information. How is DR affected by extremes of ambient temperature or long sequences of capture after the camera has heated up? But first I'll settle for a basic comparison at normal temperatures and at the various ISO options.

I don't think I've ever seen a DR comparison, pixel for pixel, between a 35mm DSLR and a high resolution MFDB. It could be safely assumed that a P45 at ISO 50 would have more DR than a Canon 1Ds2 at ISO 100, but how much of that is due to the greater pixel count of the P45 and how quickly is that advantage dissipated as ISO is increased?

There's a lot of testing which could be done which, as far as I know, isn't being done, at least outside of the laboratories and in a format readily appreciatedby the layman.
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TechTalk
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« Reply #56 on: October 07, 2007, 12:58:43 AM »
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Dynamic range in decibels is usually the figure that manufacturers quote for their sensor. It seems to be more of a theoretical figure relating to photosite size and bit depth. The dynamic range of the camera incorporating the sensor can be quite different since other design factors come into play.

Comparing DR under different conditions could provide additional useful information. How is DR affected by extremes of ambient temperature or long sequences of capture after the camera has heated up? But first I'll settle for a basic comparison at normal temperatures and at the various ISO options.
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The dynamic range quoted by sensor manufacturers is not theoretical. It is measured on a test platform. You are right that incorporating a different set of electronics for powering, cooling and reading the sensor (among other factors) inside a back or camera will affect the signal to noise ratio.

As for temperature effect, as an example the Kodak 39MP sensor specs at 71.4dB @ a sensor temperature (NOT ambient) of 40° C. (this may seem like a high temperature, but if you were stuck in a black metal box with an electrical charge being applied to you every so often, you'd get warm too!). The dark current doubles (or halves) with every 6.3° C change in temperature.
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Ray
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« Reply #57 on: October 07, 2007, 03:10:57 AM »
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The dynamic range quoted by sensor manufacturers is not theoretical. It is measured on a test platform.

Yes, of course. I meant theoretical in the sense of an ideal rarely, if ever, reached in practice once the sensor is incorporated into the camera.

Do digital backs have thermometers that give you a read-out of the temperature of the sensor? I don't think it's particularly useful in itself knowing the DR of the sensor is 71.4db at 40 degrees.
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Mike W
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« Reply #58 on: October 07, 2007, 10:19:17 AM »
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Jezus. H. Christ!

This topic really took on a life of it's own. :-)
And to think only two people originally posted.

What a fickle audience ;-)

Anyway, thanks to all for the usefull comments and links. I'll be sure to study all of it soon.
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BJL
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« Reply #59 on: October 07, 2007, 06:13:54 PM »
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Do digital backs have thermometers that give you a read-out of the temperature of the sensor? I don't think it's particularly useful in itself knowing the DR of the sensor is 71.4db at 40 degrees.
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Since 40C is quote hot (except perhaps by the standards of Queenslanders like Ray), it seems to be used to give a cautious, pessimistic value by using the upper end of normal expected operating range. CCD's do not generate a great amount of heat AFAIK, especially the Full Frame type of CCD's in MF backs which are only active for a brief time during each exposure, not for the far longer periods involved in producing video output for viewfinders.

Bear in mind that Kodak publishes its sensors spec's for use by expert customers who buy the sensors directly, expecting them to be read by the technical staff at companies and scientific labs which incorporate the sensors into other devices such as MF backs. So there is very little room for fudge and spin in the way the spec's are presented.
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