interesting article

[BJL]

I don't understand your comment that you are not referring to a 'correct metered exposure' scenario
[a href=\"index.php?act=findpost&pid=63225\"][{POST_SNAPBACK}][/a]

Of course we want "correct" exposure, but my original phrase was "on meter", not "correct".
Perhaps all I am saying is that in some high ISO speed situations where shutter speed and aperture are constrained so that only ISO speed can be varied, the "correct" choice of ISO is not always the one that the meter confirms. It can instead be a higher one which the meter will call overexposed (when on meter exposure wastes highlight headroom), or a lower one which the meter will call underexposed (when "on meter" exposure causes highlight clipping in pre-amplification or A/D conversion.)

Which is similar to advice about choosing a suitably high speed film, with the digital version in some cases effectively using ISO 800 film but rated at EI 400 (to hold shadows better), or ISO 400 film rated at EI 800 (to hold a strong highlights).

[Ray]

I was considering the case you previously raised with your ISO 1600 vs "ISO 100 four stops underexposed": when light is scarce so that even with maximum possible sensor exposure there is no chance of any photosites getting "full" and using base ISO would involve underexposure. Then the best procedure seems to be to raise the ISO (pre-amplification before A/D conversion) so that the A/D convertor sees a "full strength" signal, with the brightest highlights producing near maximum levels in the RAW digital output. Probably often "on meter", but maybe an even higher ISO and so a bit "over meter" when the highlights do no go far above the metered mid-tones and so the mid-tones (and shadows) can be pushed further right on the histogram.

BJL,
I still can't follow the point you are making, unless it's the obvious one that a higher ISO is useful when the shutter speed at a lower ISO is too long. The first 2 shots I took at dusk were at 100th and f8 using an IS lens. The 4 stops of underexposure at ISO 100 could have been corrected by using 1/6th sec at the same aperture. This would have resulted in some of the photosites (relating to the sky) getting full. Surely the situations, where light is so scarce that maximum possible sensor exposure will not fill any of the photosites, are very rare. Some possible examples might be night photography with no artificial lighting or moon, or taking a shot with the lens cap on.

However, 1/6th of a sec is too long for a hand-held shot, even with the benefit of IS and a short focal length. F8 was also giving me too shallow a DoF. If those shots were intended to be more than experiments, I would have used a tripod, MLU, 1/3rd sec exposure at f11 and ISO 100. If I didn't have a tripod handy, I would probably have taken a shot at 1/25th, f11 and ISO 800 and another at 1/50th, f11 and ISO 1600, just in case the first was not acceptably sharp.

All these exposures would result in a small amount of highlight-warning flashing in the sky (observed in the shot at 100th, f11 and ISO 1600) indicative of a full exposure to the right. It was necessary to use -1 EC during conversion to recover the small degree of blown highlight. If the sky had been overcast and grey, I could probably have given an extra 1/3 to 1/2 stop of 'overexposure'.

I still can't see any advantage to using a higher ISO other than the opportunity to use a faster shutter speed and/or smaller aperture, but perhaps a little more experimentation is required   .

[BJL]

BJL,
I still can't follow the point you are making, unless it's the obvious one that a higher ISO is useful when the shutter speed at a lower ISO is too long. .
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A possible key: "pre-amplifier" or "read" noise and "quantization" noise in A/D convertors are factors too, not only photon ("shot") noise and sensor dark current noise.

It should be clear that I am not talking about using higher ISO to get a higher shutter speed: I am talking about the very case that you first raised, of using a higher ISO with the same shutter speed and same f-stop, perhaps to the point that the light meter says that you are overexposing, and the straight output looks too bright, but highlights are not blown-out and you can restore desired luminosity with PP like minus compensation.

The point of this is that it takes the same signal (and same photon and dark current noise) from each photosite and pre-amplifies it more before A/D conversion, which reduces the final effect of noise that enters "midstream": quantization noise in the A/D converter and read noise in the analogue steps just before A/D. This benefit is because that equal amount of midstream noise is added to a signal that is already stronger, due to having already been amplified more. Since this stronger signal produces higher levels in A/D output, it needs lower subsequent amplification in the digital domain to get a given luminosity level in the final displayed image. And thus the mid-stream noise gets amplified less in the digital domain, and comes out weaker in the final output and the final displayed image.

On the other hand, noise already present on the sensor comes out at the same level either way. So if you think only in terms of photon and dark current noise, there would seem to be no difference.


Aside: Dalsa spec sheets show that increasing read speed (on their Full Frame CCD's at least) can about double the total dark noise, = sensor dark current noise+read noise. So read noise can be quite significant, at least if processing has to be done fast. Also, excessively slow read-out  can increase total dark noise: apparently letting the signal to sit around too long in the electron wells allows additional dark current noise to accumulate.

BJL,
I still can't see any advantage to using a higher ISO other than the opportunity to use a faster shutter speed and/or smaller aperture, but perhaps a little more experimentation is required[a href=\"index.php?act=findpost&pid=63324\"][{POST_SNAPBACK}][/a]

You already did an extreme version of this experiment and confirmed what I am saying: ISO 100 underexposed by four stops versus ISO 1600 "on meter"!

[Ray]

You already did an extreme version of this experiment and confirmed what I am saying: ISO 100 underexposed by four stops versus ISO 1600 "on meter"!
[a href=\"index.php?act=findpost&pid=63400\"][{POST_SNAPBACK}][/a]

BJL,
That's true. I've already confirmed that adjustable ISO serves a real and worthwhile purpose (apart from producing a brighter review on the camera's LCD screen). But the experiment has not confirmed the point you are making, which I now understand to be as follows.

In the case where, say, one stop of highlight headroom is available at a certain exposure at, say, ISO 100, there would be essentially 2 ways of exposing fully to the right to use that headroom. One is to halve the shutter speed using the same ISO setting of 100. The other is to keep the same exposure but jump up to ISO 200.

I have no doubt that the latter option is preferrable to using the same exposure at ISO 100 (which would essentially be 1 stop underexposure for ETTR purposes). But I do have doubts that the latter option would produce better tonality than the first option as described (ie ISO 100 and half the shutter speed).

In other words, if there's a contest between double the photon count and double the preamplification gain, then I'd expect that double the photon count will produce better tonality however marginal it may be. It's this factor I haven't yet tested, but you've put me in a position where I now feel that I need to test it.

If I'm wrong, I'll eat my hat   .

[BJL]

One is to halve the shutter speed using the same ISO setting of 100. The other is to keep the same exposure but jump up to ISO 200.
... if there's a contest between double the photon count ...

Ray, you still do not get what I am saying, even after I used italics in my previous post! There is no contest involving double the photon count. I am comparing different ISO speed setting options that involve exactly the same photon count at each photosite due to using exactly the same shutter speed and aperture, but different amounts of pre-amplification so that those same photon counts are converted to different voltages being input to the A/D converter, giving different digital levels going into the RAW output. My goal is "exposing the A/D convertor to the right", giving it the highest input voltage that it can handle so that the brightest parts of the image are represented in the RAW file by near-maximum digital levels.

And this is specifically about situations calling for higher than base ISO speed in order to get "on meter" exposure, so it is best to stay away from examples involving ISO 100!

[bjanes]

Of course we want "correct" exposure, but my original phrase was "on meter", not "correct".

Which is similar to advice about choosing a suitably high speed film, with the digital version in some cases effectively using ISO 800 film but rated at EI 400 (to hold shadows better), or ISO 400 film rated at EI 800 (to hold a strong highlights).
[a href=\"index.php?act=findpost&pid=63282\"][{POST_SNAPBACK}][/a]

The above works well for film with exposure determined from middle gray. However, with ETTR in digital, one should place the highlight just short of clipping (data value 4095 in the raw file with a 12 bit AD converter). The easiest way to do this is probably to take a spot reading from the highlight and expose according to a previously determined offset from the nominal meter reading that places the highlights properly. This is often about 2 1/3 stops over the indicated exposure.

For the best signal to noise, one should use the lowest ISO that is feasable for the shooting conditions. It is hardly surprising that in dim light a high ISO may be needed. The amplifier gain is then increased to compensate for the reduced exposure, so that the highlights will be at the desired location in the histogram. If  one uses an inappropriately low ISO for the exposure, underflow of the AD converter may occur, shadow values will be lost and there will be a reduced number of tones at higer signal levels. In such cases, one has not really exposed to the right.

With JPEG, some cameras may use different noise reduction for high ISO, but this is not well documented with raw. Ron Parr (who publishes a digital FAQ at Duke Univesity), has stated that he thinks that, at high ISO, Canon merely reduces the sharpening and he has seen no evidence that additional NR is given. In the case of Nikon, the Nikon guru Thom Hogan has stated that Nikon applies no NR to the raw image.

[Ray]

Ray, you still do not get what I am saying, even after I used italics in my previous post! There is no contest involving double the photon count. [a href=\"index.php?act=findpost&pid=63486\"][{POST_SNAPBACK}][/a]

BJL,
Why is there no contest involving double the photon count? There should be. As bjanes has written (and I agree with him on this point), the best tonality will be achieved by exposing fully to the right using the lowest ISO setting which permits an adequate shutter speed at the desired aperture. If I can double the photon count at a lower ISO without blowing wanted highlight detail, why should I not take that opportunity? What possible advantage is there in using a higher ISO than is needed?

Now, it makes some sense if you are advocating this technique (of achieving ETTR through increased preamplification rather than increased photon count) as a personal style of shooting because (let's say) you have a tendency to underestimate the shutter speed required for a sharp image. This would be analagous to someone setting all clocks forward by a few minutes because he/she has a tendency to be late. (Not that that really works because one tends to be constantly aware that the clock is really 10 minutes fast   ).

[jani]

The above works well for film with exposure determined from middle gray. However, with ETTR in digital, one should place the highlight just short of clipping (data value 4095 in the raw file with a 12 bit AD converter). The easiest way to do this is probably to take a spot reading from the highlight

... which may involve purchasing another camera.

So it's not quite as easy as you 1-series zealots think.

(Yeah, yeah, I know the 30D has a "spot" metering mode, that the 5D really has it, and Nikonians are probably laughing right now, but I still think it was a funny point to raise.)

[Chris_T]

The above works well for film with exposure determined from middle gray. However, with ETTR in digital, one should place the highlight just short of clipping (data value 4095 in the raw file with a 12 bit AD converter). The easiest way to do this is probably to take a spot reading from the highlight and expose according to a previously determined offset from the nominal meter reading that places the highlights properly. This is often about 2 1/3 stops over the indicated exposure.
[a href=\"index.php?act=findpost&pid=63508\"][{POST_SNAPBACK}][/a]

How you describe "ETTR in digital" is exactly how I set the exposure for a slide in a film camera using manual mode and spot metering. If this is indeed the case, it would be a lot simpler for the ETTR writers to simply refer to this technique which many film camera users are already familiar with. Of course to use this technique, either the film or digital camera must support manual exposure, and preferrably has spot metering.

[Chris_T]

Here is an explanation of sensor ISO as it applies to digital sensors:

http://www.normankoren.com/digital_cameras.html#ISOspeed
[a href=\"index.php?act=findpost&pid=63003\"][{POST_SNAPBACK}][/a]

Thanks. Koren's site has a wealth of info.

[bjanes]

How you describe "ETTR in digital" is exactly how I set the exposure for a slide in a film camera using manual mode and spot metering. If this is indeed the case, it would be a lot simpler for the ETTR writers to simply refer to this technique which many film camera users are already familiar with. Of course to use this technique, either the film or digital camera must support manual exposure, and preferrably has spot metering.
[a href=\"index.php?act=findpost&pid=63543\"][{POST_SNAPBACK}][/a]

Chris,

I love your answer, which is in accord with the maxim of expose for the highlights with transparency film. It may be nothing new to most of us, but it is apparently for the photo expert Jeff Schewe who maintains that exposure for digital is entirely different than with film.  

The problem really boils down to one of convention...digital sensors are NOT like film. They do not react the way chrome nor neg film reacts to light. Therefore you really can't use old film exposure techniques...the aim is to get as much data captured as far up the scale as you can while still preserving usable textural highlight detail. This is NOT "over exposing" it's "proper exposing"-for a digital sensor.
[a href=\"index.php?act=findpost&pid=62533\"][{POST_SNAPBACK}][/a]

[bjanes]

Ray, you still do not get what I am saying, even after I used italics in my previous post! There is no contest involving double the photon count. I am comparing different ISO speed setting options that involve exactly the same photon count at each photosite due to using exactly the same shutter speed and aperture, but different amounts of pre-amplification so that those same photon counts are converted to different voltages being input to the A/D converter, giving different digital levels going into the RAW output. My goal is "exposing the A/D convertor to the right", giving it the highest input voltage that it can handle so that the brightest parts of the image are represented in the RAW file by near-maximum digital levels.

And this is specifically about situations calling for higher than base ISO speed in order to get "on meter" exposure, so it is best to stay away from examples involving ISO 100!
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I did an analysis of the following exposure situations according to the methods used by Roger Clark for the Canon EOS 1D Mark II. Interested readers should refer to Roger's web site for details.

[a href=\"http://www.clarkvision.com/imagedetail/evaluation-1d2/index.html]http://www.clarkvision.com/imagedetail/eva...-1d2/index.html[/url]

1. Scene exposed for ISO 100 with camera set to ISO 100

2. Scene exposed for ISO 1600 with camera set to ISO 1600

3. Scene exposed for ISO 1600 with camera set to ISO 100

The results are shown in the table below. [attachment=506:attachment]

At ISO 100 the sensor wells contain 53,000 electrons at highlight exposure to the right where the raw file data number is 4071 (as calculated from the gain of 13.02 electrons/data number). If one exposes for ISO 1600, since the sensor is linear, it will have about 3313 electrons and the raw file data number will be  4090 (calculated from the gain of 0.81 electrons/DN). Read noise is 16.6 electrons at ISO 100 and 3.90 electrons at ISO 1600.

Under these shooting conditions, almost all of the noise is accounted for by shot (electron sampling or Poisson noise) and read noise. Dark current is not significant for these exposure times. The table shows the noise calculations for a 10 zone scene. The shot noise predominates at high signal values and the read noise for low values. The total calculated noise is also shown along with the signal to noise ratio (SN) for each zone and the data numbers for a 12 bit linear file and for gamma 2.2.

For the ISO 1600 exposure with the camera set to ISO 100, AD underflow will occur in the shadows assuming 12 bit integer math, and the model predicts that the shadows will be clipped. To apply +4 stop exposure compensation to the ISO 100 exposure, it is necessary to multiply the raw data number by 16.

Now if we look at Ray's test shots we do see shadow clipping in the ISO 1600 exposure with the camera set at ISO 100 and considerably higher shadow noise due to the higher read noise. Shot noise is the same since the number of photons reaching the sensor is the same in both cases. The model goes a long way in explaining the observed results without invoking any additional NR.

[BJL]

BJL,
Why is there no contest involving double the photon count?
[a href=\"index.php?act=findpost&pid=63516\"][{POST_SNAPBACK}][/a]

Ray (and bjanes)

I mean that the alternatives that I am discussing do not involve one with double the photon count of the other. I am discussing different ways of processing the same photon counts, by applying different degrees of pre-amplification (different ISO settings) to them.

This is different from the ETTR scenario, which involves adjusting the "sensor exposure level" by changing the shutter speed (or f-stop) to vary the amount of light gathered by each photo-site.

[BJL]

For the best signal to noise, one should use the lowest ISO that is feasable for the shooting conditions.
[a href=\"index.php?act=findpost&pid=63508\"][{POST_SNAPBACK}][/a]

More specifically, one should use the lowest shutter speed that is feasible (assuming fixed f-stop), because that maximizes the among of light received by each photo-site, and so maximizes the S/N ratio of the signal coming out of the photo-sites (reflecting combined dark current and photo noise).

The amplifier gain is then increased to compensate for the reduced exposure, so that the highlights will be at the desired location in the histogram.[a href=\"index.php?act=findpost&pid=63508\"][{POST_SNAPBACK}][/a]

This is back-to-front. ETTR is about increasing exposure level to get as much light as possible on each photo-site, which goes with _reducing_ amplifier gain: lower ISO speed setting = lower amplifier gain.

But you are still talking about the scenario where decreasing ISO speed goes with decreasing shutter speed. I am discussing the different, high speed/low light situation where I am up against the minimum acceptable shutter speed, and might choose an ISO setting other that the "normal" one suggested by the light meter.

[BJL]

I did an analysis of the following exposure situations according to the methods used by Roger Clark for the Canon EOS 1D Mark II. Interested readers should refer to Roger's web site for details.

http://www.clarkvision.com/imagedetail/eva...-1d2/index.html

1. Scene exposed for ISO 100 with camera set to ISO 100

2. Scene exposed for ISO 1600 with camera set to ISO 1600

3. Scene exposed for ISO 1600 with camera set to ISO 100

[a href=\"index.php?act=findpost&pid=63558\"][{POST_SNAPBACK}][/a]

Thanks, I am talking about a milder case of the comparison between the last two cases: same exposure, different ISO setting (In my case, maybe exposing for ISO 800 but with camera set to ISO 800 vs cameras set to ISO 1600, in a situation when the later will not ruin highlights.)
As the table show, setting the camera to a higher ISO (with exposure levels based on the same ISO) improves the S/N ratios by reducing the effect of read noise.

I am not sure about dark current noise being negligible though; for Kodak's FF CCD DSLR sensors, dark current noise can contribute about 16-22 electrons of noise, comparable to or greater than the read noise figures that Clark gives. AFAIK, CCD dark current noise is roughly constant for all but very long exposures (1s or longer) because it continues to accumulate after the exposure is finished but the electrons are still waiting to be read out.

Maybe though, Canon's CMOS sensor do immediate pre-amplifyication "on site" while waiting for read out, avoiding this extra accumulation of dark current noise. That might be part of their good shadow noise handling.

[bjanes]

Thanks, I am talking about a milder case of the comparison between the last two cases: same exposure, different ISO setting (In my case, maybe exposing for ISO 800 but with camera set to ISO 800 vs cameras set to ISO 1600, in a situation when the later will not ruin highlights.)
As the table show, setting the camera to a higher ISO (with exposure levels based on the same ISO) improves the S/N ratios by reducing the effect of read noise.

I am not sure about dark current noise being negligible though; for Kodak's FF CCD DSLR sensors, dark current noise can contribute about 16-22 electrons of noise, comparable to or greater than the read noise figures that Clark gives. AFAIK, CCD dark current noise is roughly constant for all but very long exposures (1s or longer) because it continues to accumulate after the exposure is finished but the electrons are still waiting to be read out.

Maybe though, Canon's CMOS sensor do immediate pre-amplifyication "on site" while waiting for read out, avoiding this extra accumulation of dark current noise. That might be part of their good shadow noise handling.
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According to Roger's tests, dark current averages for the EOS 1D Mark II at ISO 1600 were 0.013 to 0.02 electrons/second, but some pixels have dark currents as high as about 0.25 electrons/second. For exposures of a few seconds, this noise is neglibible, but for exposures in minutes, the dark current then would be significant. The camera most likely has a dark current subtraction option, at least the Nikon's do, but it is not needed even with the Nikon (which has poorer noise characteristics) under normal circumstances. See Roger's site for details. Also, these methods could easily be adapted to your situation.

Increasing the ISO may reduce read noise, but that is a relative minor component of noise in most situations it is outweighed by photon noise as shown in the spreadsheets.

Your point about the dark noise accululation post exposure is a good one; Roger waited for the data to be written out to memory from the buffer before taking more shots, so the delay should be minimal.

[a href=\"http://www.clarkvision.com/imagedetail/evaluation-1d2/index.html]http://www.clarkvision.com/imagedetail/eva...-1d2/index.html[/url]

[bjanes]

More specifically, one should use the lowest shutter speed that is feasible (assuming fixed f-stop), because that maximizes the among of light received by each photo-site, and so maximizes the S/N ratio of the signal coming out of the photo-sites (reflecting combined dark current and photo noise).
This is back-to-front. ETTR is about increasing exposure level to get as much light as possible on each photo-site, which goes with _reducing_ amplifier gain: lower ISO speed setting = lower amplifier gain.

But you are still talking about the scenario where decreasing ISO speed goes with decreasing shutter speed. I am discussing the different, high speed/low light situation where I am up against the minimum acceptable shutter speed, and might choose an ISO setting other that the "normal" one suggested by the light meter.
[a href=\"index.php?act=findpost&pid=63585\"][{POST_SNAPBACK}][/a]

I don't use your shooting methods and have not read your posts in detail. However, as I said, for the best S/N one should use the lowest ISO possible, since that will maximize the photon count--this is entirely different from exposing to the right as discussed by Mr. Reichman and others. Exposing fully to the right at ISO 1600 will give a more noisy image than 1 stop underesposure at ISO 100 as shown on the spreadsheet that I posted previously. Of course, for best results, one should expose to the right with ISOs of 100 or 1600.

Of course amplifier gain is less at low ISO and I don't know why you even bother to mention this. ETTR per se, does not involve choice of an ISO, but merely adjusting the exposure so that the AD converter output is near maximum (DN 4095 for a 12 bit device). This can occur at ISO 100 or 1600.

I don't know about your light meter, but with meters I use one enters the ISO and the meter then indicates the exposure. One could work backwards, but I normally do not do so.

[Ray]

More specifically, one should use the lowest shutter speed that is feasible (assuming fixed f-stop), because that maximizes the among of light received by each photo-site, and so maximizes the S/N ratio of the signal coming out of the photo-sites (reflecting combined dark current and photo noise).

[a href=\"index.php?act=findpost&pid=63585\"][{POST_SNAPBACK}][/a]

BJL,
As I suspected, you are really suggesting a procedural approach for achieving the optimum combination of shutter speed, aperture and ISO, consistent with ETTR.

I guess we all have a slightly different approach to getting the most out of our camera, and that approach will also be influenced by the design of the camera.

The 2 cameras I've used most to date are the D60 and the 5D. The 20D was an interim purchase which I would never have made if I'd known at the time that the 5D was in development.

A frustrating factor in using the D60 resulted form the fact that ISO could not be changed whilst looking through the viewfinder. One had to to press a menu button, scroll down to the ISO settings, press another button, turn a wheel, then press a button again. If you are long sighted, as I am, then that would also involve putting on one's reading glasses. The whole process was a major interruption to the act of taking a photo. I therefore did a series of tests to determine just how much benefit to image quality and tonality was achieved by using a higher ISO setting, as opposed to using the same exposure at ISO 100 and in effect underexposing the image.

To my surprise, there was essentially none, or very little at best. The deepest shadows showed marginally better detail and tonality, but such shadows are usually black on a print. In general, not only were the differences marginal but they were only apparent at a pixel-peeping level at 200% enlargement on screen, which is equivalent to a far bigger print than I would ever make from a 6MP image.

The consequence of this is that I didn't use the higher ISO settings on the D60 as much as I should have. I tended to rely upon IS to reduce camera shake and too often forgot that a shutter speed that is adequate for camera shake is not necessarily adequate for even a slowly moving subject.

Old habits die hard and this is perhaps one reason why I'm so relaxed about underexposing a shot at ISO 100. One gets the benefit of a faster shutter speed and the greater certainty of retaining all highlight detail. Essentially, the D60 was a one ISO camera with different settings for brightening the LCD review. If you couldn't get a fast enough shutter speed at ISO 100, then you were basically stuffed. There was no alternative to a noisy image, outside use of a tripod.

The 5D however is a very different camera. As far as I can tell, image quality at ISO 100 is not one whit better, appart from the greater resolution afforded by more pixels. My first impression was that it is actually worse, in terms of S/N and DR, than the D60. This is because the 5D suffers from noticeable banding which the D60 is free of. Underexpose a high DR scene by one stop with the 5D and objectional banding is very apparent when the shadows are lightened. Underexpose the same scene by one stop with the D60 and the shadows are far less objectionable (but still degraded of course in terms of detail and tonality).

However, I need to do more tests to confirm if this. There's a slight discrepancy between the true ISO values for both cameras at ISO 100. On the 5D, ISO 100 is actually 125 and on the D60 I think it's around 150 or 160.

[Ray]

I did an analysis of the following exposure situations according to the methods used by Roger Clark for the Canon EOS 1D Mark II. Interested readers should refer to Roger's web site for details.

[a href=\"index.php?act=findpost&pid=63558\"][{POST_SNAPBACK}][/a]

bjanes,
It's taken me a while to wrap my head around these results you've shown in the table. I'm not specifically technically orientated. As a layman, I sometimes struggle to find a meaningful interpretation from tables of figures, so perhaps you can help me out here.

Let's do an analysis of a single row (zone 7) taken from the exposure situation equivalent to my previous test images in the thread.

I've reproduced the two lines of data for easy reference.

[attachment=507:attachment]

1. From left to right the total number of electrons and shot noise is the same, 414 and 20.3 (the square root of 414).

2. The read noise is significantly higher at ISO 100, 16.6 as opposed to 3.9, yet the total noise does not reflect a simple addition of both noise figures. At ISO 100, the total noise is only 5.6 electrons greater than at ISO 1600. 5.6 electrons greater noise in relation to 414 does not seem significant.

3. The S/N of 15.8 as compared with 20 for the ISO 1600 shot might seems significant, but this figure is a ratio and might be misleading.

4. The huge discrepancy I see, comparing  these figures, is in the actual quantity of data numbers (DN) available to describe or quantify the signal during A/D conversion.

Whether we've set the camera to ISO 100 or 1600, the possible total variation in electron count in zone 7 (roughly mid-tones) is 414. My simple mind tells me that within that range of exposure (roughly one f/stop, but probably a bit less), there are 414 possible values, but only 31 numbers to describe those values at ISO 100.  At ISO 1600, however, there are 511 numbers to describe the 414 electrons.

This I would suggest is the major reason for the dramatic reduction in noise at ISO 1600.

However, having said that, there is the problem of the D60 showing only marginal noise improvement in the same circumstances. We can therefore assume that the D60 employes 'noisy' preamplifiers, whereas the 5D and the 1D2 employ less noisy preamplifiers. Alternatively, we could surmise that it's a combination of superior components plus a better arrangement of processes on the chip.

Either way, it becomes an argument in semantics to quibble about whether or not additional noise reduction has taken place. Noise in the D60 at ISO 100 is no worse than noise in the 5D at ISO 100. Noise in the D60 at ISO 1000 (the highest setting) is very much worse than noise in the 5D at ISO 1000.

[bjanes]

bjanes,
It's taken me a while to wrap my head around these results you've shown in the table. I'm not specifically technically orientated. As a layman, I sometimes struggle to find a meaningful interpretation from tables of figures, so perhaps you can help me out here.

Let's do an analysis of a single row (zone 7) taken from the exposure situation equivalent to my previous test images in the thread.

I've reproduced the two lines of data for easy reference.

[attachment=507:attachment]

1. From left to right the total number of electrons and shot noise is the same, 414 and 20.3 (the square root of 414).

2. The read noise is significantly higher at ISO 100, 16.6 as opposed to 3.9, yet the total noise does not reflect a simple addition of both noise figures. At ISO 100, the total noise is only 5.6 electrons greater than at ISO 1600. 5.6 electrons greater noise in relation to 414 does not seem significant.

3. The S/N of 15.8 as compared with 20 for the ISO 1600 shot might seems significant, but this figure is a ratio and might be misleading.

4. The huge discrepancy I see, comparing  these figures, is in the actual quantity of data numbers (DN) available to describe or quantify the signal during A/D conversion.

Whether we've set the camera to ISO 100 or 1600, the possible total variation in electron count in zone 7 (roughly mid-tones) is 414. My simple mind tells me that within that range of exposure (roughly one f/stop, but probably a bit less), there are 414 possible values, but only 31 numbers to describe those values at ISO 100.  At ISO 1600, however, there are 511 numbers to describe the 414 electrons.

This I would suggest is the major reason for the dramatic reduction in noise at ISO 1600.

However, having said that, there is the problem of the D60 showing only marginal noise improvement in the same circumstances. We can therefore assume that the D60 employes 'noisy' preamplifiers, whereas the 5D and the 1D2 employ less noisy preamplifiers. Alternatively, we could surmise that it's a combination of superior components plus a better arrangement of processes on the chip.

Either way, it becomes an argument in semantics to quibble about whether or not additional noise reduction has taken place. Noise in the D60 at ISO 100 is no worse than noise in the 5D at ISO 100. Noise in the D60 at ISO 1000 (the highest setting) is very much worse than noise in the 5D at ISO 1000.
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Ray,

1. The noises are expressed in standard deviations and you may remember from statistics that SDs do not add, but variances do. So the total noise is the square root of the sum of the two standard deviations squared.

3. The S/N is the standard for evaluation of noise. If you look at the ISO 100 exposures, you will see that they have the highest noise levels, but since the S/N is high, noise is not visible.

4. I did look at the histogram of the tests for you repeat test showing the trunk of a tree. The ISO 100 shot shows clipping of the shadows as predicted by the spreadsheet and the shadows are replaced by noise. The noise does look higher than predicted by the model, but I don't really know how the 5D is responding here or if there is additional noise reduction at ISO 1600. However, at ISO 100 with a 12 bit data number of 254, you are making use of only the lower 8 bits of the AD converter and are thorwing away resolution. With ISO of 1600, the DN is 4090 and you are making use of the full range of the AD converter.

To me, it does not make sense to set the camera for ISO 100 and then expose as for ISO1600.