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Author Topic: DxO Mark interpretation and application  (Read 4146 times)
Ray
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« Reply #20 on: April 15, 2012, 08:48:17 AM »
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Underexposing by 3 stops at ISO 100 can hardly be called ETTR. You're arguing for argument's sake.

Cheers,
Bart

Bart,
I never claimed it was. My use of ETTR with my Canon cameras was to fully expose to the right of the histogram whatever the ISO chosen. This is not an argument. It's a statement of fact. My argument would relate to the reasons for doing this. And the reasons are, when the required or desired shutter speed is too fast to enable an ETTR at base ISO, the best results are obtained at the highest ISO which is consistent with an ETTR exposure at that higher ISO, up to ISO 1600 at least.

However, this does not apply when using cameras such as the Nikon D7000 and Pentax K5. The reasons for choosing a higher ISO with such cameras would be so one can review the shot on the camera's LCD screen.

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ETTR is all about optimizing (= maximizing without compromising required highlight data) the number of recorded photons. This is most effective at base ISO where the DR (engineering definition) is the largest. Increasing ISO has nothing to do with that, although it may have other useful effects.

It's certainly true that the lowest noise and highest DR is achieved by recording the greatest number of photons, which is only possible at base ISO. But consider this. Would you agree that an exposure which is one stop underexposed at ISO 100 (in relation to a correct ETTR exposure) will have one stop lower DR as a result of that underexposure? Likewise, would you agree that an exposure at ISO 100 that is 2 stops underexposed will have 2 stops (or 2EV) lower DR than the fully exposed ETTR at ISO 100?

If you check the DXOMark DR figures for the Canon 5D2, you will find that at ISO 100 DR is 11.86EV. At ISO 200 it is insignificantly lower at 11.82EV, which is virtually the same. At ISO 400 is down just a little at 11.62EV, which is just 1/4th of a stop less.

It is clear from these figures that in circumstances where a shutter speed is required that would result in a 2 stop underexposure at ISO 100, the same exposure at ISO 400, which would be an ETTR exposure at ISO 400, will result in a significantly higher DR by about 1.75EV. Can't you see this?  Grin
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BJL
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« Reply #21 on: April 15, 2012, 10:00:19 AM »
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Ray,
    to address your new-found concern about T-stop variations:
Please reconsider my example with "equal T-stop" in place of "equal f-stop". This could be achieved by using the same lens in all tests via mount adaptors. However, for all practical purposes, witu good prime lenses there will be negligible variation in transmission efficiency and thus a negligible discrepancy in T-stop at equal f-stop. Oh, and maybe change the EI values up a few stops, to say 6400 and 3200, because that what I say should be clear even with your examples of those unfortunate old-tech Canon sensors that need to use analogue amplification carefully in order to control noise sources later in the analogue signal path.

In that equal T-stop scenario, can you truly not see why people interested in low-light, high shutter speed performance want to compare images taken at equal shutter speed? (Along of course with optimal choice of exposure index setting and analogue gain levels, to the extent that this helps.)


P.S. When the Dx0 measurement of exposure index at higher "ISO" settings disagrees with that of the camera, it is more likely that the camera maker is conforming to the relevant definition of sensitivity amd exposure index in ISO standard ISO12232:2006, meaning one of the two measurements that the Japanese industry association CIPA requires its members to use:
Standard Output Sensitivity (SOS) or
Recommended Exposure Index (REI).
These are both based on getting appropriate levels in the final (JPEG) output when the exposure level (combination of shutter speed and aperture ratio) is as stanard light metering indicates. DX0 instead uses another measurement, based on so-called saturation based sensitivity, which is _not_ recomended for use by CIPA. We could debate why CIPA rejects that saturation-based measure, but to me it is clear: that measure is what used to be called base ISO speed, and is intended primarily for measuring the minimum safely usable EI setting (maximum amount of exposure sent to the sensor), not at all for calibration of the higher EI settings.

Why might that be? At high enough EI levels, quantization noise from the ADC is overwhelmed by dark noise and photon shot noise, so worrying about placing the signal as close as possible to maximum raw output levels is misguided. Maybe with Canon's technology, "high enough" is not till about EI 1600, but with newer sensor technologies as used by Sony and Nikon (and maybe Panasonic and Olympus), this is true from about 400 and up, and with some MF cameras with their combination of somewhat noisy CCDs, no possibility for on-chip gain, and excellent ADCs, this is possibly true at all EI levels, so that there might be no point in vatying the analogue gain at all.

That is, analogue amplification for the sake of place the midtones as far to the right as is safe in the raw files (this seems to be roughly what you are meaning by "ETTR") is of little of no relevance at sufficiently high EI, and the final tonal placement in output files (JPEG, TIFF or whatever) might best be done largely or completely in the digital domain, with litle or no variation in analogue gain.

At that point, Dx0 style callibration based on level placement in raw files is a red-herring: it is a measurement part-way through the process, which I consider half-baked. By "the process", I mean getting from a required high shutter speed to a final image (JPEG, TIFF or whatever) with correct placement of mid-tones and such and as little noise as possible.
« Last Edit: April 15, 2012, 10:07:37 AM by BJL » Logged
BartvanderWolf
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« Reply #22 on: April 15, 2012, 10:57:12 AM »
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It's certainly true that the lowest noise and highest DR is achieved by recording the greatest number of photons, which is only possible at base ISO. But consider this. Would you agree that an exposure which is one stop underexposed at ISO 100 (in relation to a correct ETTR exposure) will have one stop lower DR as a result of that underexposure? Likewise, would you agree that an exposure at ISO 100 that is 2 stops underexposed will have 2 stops (or 2EV) lower DR than the fully exposed ETTR at ISO 100?

No, not necessarily on all cameras.

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If you check the DXOMark DR figures for the Canon 5D2, you will find that at ISO 100 DR is 11.86EV. At ISO 200 it is insignificantly lower at 11.82EV, which is virtually the same. At ISO 400 is down just a little at 11.62EV, which is just 1/4th of a stop less.

I find it more instructive to view the ratings for the sensor itself than for a downsampled print. But indeed, ISO 100=11.16, ISO 200=11.12, and ISO 400=10.92, showing a similar pattern. The penalty for underexposing and ramping up the gain to boost the signal level is mild in this camera. It has to do with the actual gain factor and the number of electrons per DN (data number, also referred to as ADU) at a given ISO, amongst other things. So by all means use it if a faster shutterspeed or a wider aperture are more important than the modest loss of DR. Of course one probably also boosts the visibility of pattern noise, but photography is also about making compromises. TANSTAAFL.

But the principle remains that one still needs to optimize the exposure to get as many photons in as possible without clipping required highlights. Comparing a 2 stop underexposed ISO 100 versus a correctly exposed ISO 400 makes no ETTR sense.

Cheers,
Bart
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bjanes
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« Reply #23 on: April 15, 2012, 11:37:34 AM »
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My other, secondary, issue is a complaint about all low-light performance comparisons I have seen, not specifically of Dx0: some cameras (Canon is often accused, for example) give lower shutter speeds than others at the same "ISO speed" setting. This is hard to explain as an error, and so sounds very much like deliberate gaming of high ISO noise level testing, by giving the sensor a bit more light than the competition. Real world comparisons with moving subjects might be a good way to expose this dodge! If ISO speeds were corrected in that way, there would be a different horizontal movement of the curves.

As I will keep saying, it mystifies me why the very useful ISO standard definitions for noise based sensitivity, S40 and S10, are so rarely measured, published or discussed, despite the almost morbid obsession of so many camera forum participants with noise comparisons.

Some references on ISO12232:2006 noise-based speed measures:
http://en.wikipedia.org/wiki/Film_speed#Noise-based_speed
and pages 18 and 19 of this useful slideshow overview of many aspects of photography related standards and measurement methods from an ISO participant.
http://www.rps-isg.org/DF2008/DigitalPhotographyStandards.pdf

DXO uses the ISO12232:1998 saturation standard for the ISO exposure index where Ssat = 78/Hsat. Hsat is the exposure in lux.s needed to reach sensor saturation. This means that if one exposes a gray/white target according to the reading of a standard light meter, the sensor will be at 12.8% saturation and there will be a half stop of highlight headroom. This is what I find with my Nikon dSRLs.

This brings up the ISO 2721 standard for calibration of light meters, which dates back to film days. In effect, the light meter is calibrated to an effective subject reflectance of 12.8% (see Thom Hogan). Digital camera makers differ in the amount of highlight headroom that they allow for their sensors. If they choose to allow more than the standard 0.5 EV of headroom, they have two choices. They can adjust the calibration of the meter so that less exposure would be given than according to ISO 2721. However, in this case a photographer using a hand held meter would have to use this different calibration, which would be confusing. The other alternative would be to leave the meter calibration unchanged so that there would be concordance between an external meter and the camera meter and then assign a higher ISO to the sensor than would be warranted according to ISO 12232.  In the case of the Phase One IQ180, a large amount of highlight headroom is allowed. For a nominal ISO of 100, DXO measures a value of 29.

Regards,

Bill
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Ray
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« Reply #24 on: April 15, 2012, 12:00:12 PM »
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But the principle remains that one still needs to optimize the exposure to get as many photons in as possible without clipping required highlights. Comparing a 2 stop underexposed ISO 100 versus a correctly exposed ISO 400 makes no ETTR sense.

Cheers,

Bart

It makes perfect sense to me, Bart. ETTR means Expose To The Right, doesn't it? 'The Right' refers to the right of the histogram, doesn't it? Setting aside the unavoidable inaccuracies of a histogram based on a jpeg conversion, the histogram and warning flashing is still a guide for correct exposure. Whatever the ISO setting, if one exposes beyond the right of the histogram, one will unavoidably blow some highlights. If the exposure is such that the histogram doesn't extend fully to the right, the image will be noisier than it otherwise might have been.

The principle of ETTR applies to all ISOs, even those which of themselves don't provide any image quality advantage, such as ISO 3200 on a 5D. If I choose ISO 3200 on a 5D and use a 1/2000th exposure when I could have used a 1/1600th sec without blowing highlights, I will have a lower quality image, noise-wise, because I failed to expose to the right.
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BJL
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« Reply #25 on: April 15, 2012, 12:06:31 PM »
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Bill, that is basically right, except that Dx0 is clearly not using _sensor_ saturation, which would give a value independent of the EI setting or any amplification applied between photosites and ADC. Instead, Dx0 is using "ADC" saturation, requiring that the levels in the raw file are at a standard placement, with mid-tones about three stops below maximum ADC output level. The 2006 standard is perhaps ambiguous about whether it must be applied to maximum signal in photosites or can alsomapply to maximum signal after analogue amplification and ADC comversion but before any further levels adjustment in conversion to JPEG.

My feeling of course, is that this measurement mid-wy through the process is neither what the ISO standards originally intended nor or much practical relevance to assessing the low light performance of a camera.

For one thing, once at high enough EI that the dynamic range of the signal is several stops less than the bit depth of the ADC, there are several stops of placement in ADC levels that give equally good handling of noise levels, and then a lower than standard placement improves highlight handling without making noise worse, and so could be a good thing. Yet the Dx0 measurement "punishes" that approach by declaring the true sensitivity to be less and shifting the noise and DR graphs to the right.

In an exteme case, a future sensor with column parallel ADC or even with ADC done right at ach photosite (such designs do exist) might have no reason to apply variable analogue gain at all, with the effect of each doubling of the EI setting being simply to double the shutter speed (at equal f-stop and lighting), halving the raw level placement of a mid-tone part of the scene, and then shift one bit to the left in conversion from raw to JPEG (or flagging this intent in raw files). In this scenario, DX0 would declare the "true ISO" to be the same regardless of the EI setting, and produce a graph with all measurements stacked vertically at the left!
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Ray
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« Reply #26 on: April 15, 2012, 12:10:57 PM »
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Ray,
    to address your new-found concern about T-stop variations:
Please reconsider my example with "equal T-stop" in place of "equal f-stop".


I have and it's not practical. There are no T-stop markings on any of my lenses.

 
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This could be achieved by using the same lens in all tests via mount adaptors.

That's even less practical.

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However, for all practical purposes, witu good prime lenses there will be negligible variation in transmission efficiency and thus a negligible discrepancy in T-stop at equal f-stop.

Most people use zoom lenses. My best zoom lens, the Nikkor 14-24/2.8 seems to have T-stops that differ quite significantly from the F/stops, according to my own comparisons with other lenses

 
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Oh, and maybe change the EI values up a few stops, to say 6400 and 3200, because that what I say should be clear even with your examples of those unfortunate old-tech Canon sensors that need to use analogue amplification carefully in order to control noise sources later in the analogue signal path.

Didn't I mention? A number of Nikon cameras behave in a similar way, including the D3, D3s, D700. Also, if the graphs presented on this forum showing the DR of the new 5D3 are reasonably accurate, then the 5D3 has similar characteristics in this respect.

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In that equal T-stop scenario, can you truly not see why people interested in low-light, high shutter speed performance want to compare images taken at equal shutter speed?

Of course I can, and that's precisely what I'm able to determine from the results that DXO already provide. I can't understand why you have a problem in this respect.
Consider a comparison between the IQ180 and the D800. The DXO Measurements tell us that at a base ISO of 74 the D800 can provide a DR of 14.33EV.

At a base ISO of 29, the IQ180 can provide a slightly lower DR of 13.56, which is still quite good, but at ISO 29 the shutter speed needs to be less than half what the D800 needs for an ETTR at its base ISO of 74.

If I want to determine the noise and DR at equal shutter speeds, the graphs provide a fairly accurate indication, plus or minus say 1/10th of a stop. To use the same shutter speed with the IQ180 as I would use with the D800 at its base ISO of 74, I would need to either increase the ISO of the IQ180 by 1 & 1/3 stops to reach approximately ISO 74, or underexpose 1 & 1/3rd stops at ISO 29. Either way, DR is going to be about 12.2 EV, significantly worse than that of the D800 at the same shutter speed.

However, SNR at 18% grey will be about the same at 45dB, for both cameras. Same for tonal Range and Color Sensitivity.What's the problem? All this is quite apparent from the existing graphs.


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BJL
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« Reply #27 on: April 15, 2012, 12:22:11 PM »
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Ray, we seem to have wandered off onto a different topic, and you for some reason keep quoting data about Canon sensors at lowish EI settings like 100, 200, and 400, even though I keep pointing out that my concern is comparisons of performance at high EI, like 1600 and up.

So let me ask again a simple question:

Why should I care about comparison of noise levels in low light situations where one camera is using a longer exposure time than the other while being at equal f-stop (or equal T-stop if you insist), regardless of whether either the camera maker or DX0 declare the cameras to be at the same ISO sensitivity?

I ask because the main thing that I care about in that situation is performance when I a m forced to use a high shutter speed by subject movement or such, and I will then need to use that _same_ shutter speed with any camera.
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BJL
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« Reply #28 on: April 15, 2012, 12:36:46 PM »
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Of course I can, and that's precisely what I'm able to determine from the results that DXO already provide. I can't understand why you have a problem in this respect.
Ray, your T-stop issue is a red herring: I was talking about professional testing of cameras such as that done by Dx0, where it is perfectly practical to use prime lenses for which f-stop is a thoroughly accurate measure of the intensity of illumination reaching the sensor.

Why on earth do you think that the Dx0 measurement based on highlight headroom in raw files is making the correct comparison even when it mismatches shutter speeds? As I indicated in my previous post, a camera with no significant noise introduced beyond the photosite could quite reasonably have the same Dx0 "ISO" rating at every setting, correcting levels for final display digitally instead of with analogue gain. More modestly, different cameras using the same sensor can choose to have different raw level placements with no significant affect on SNR or on noise levels in the final displayed images, so that the only difference is that the camera choosing the lower placement would have better handling of extreme highlights, and yet Dx0 would rate it as having worse noise, due to pushing its noise vs "ISO" curves to the left. (In fact, Olympus and Panasonic do seem to differ in their raw level placement when using the same sensors, and I believe that Pentax, Nikon and Sony also vary in their raw placements when using the same Sony sensors.)
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bjanes
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« Reply #29 on: April 15, 2012, 02:42:05 PM »
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Bill, that is basically right, except that Dx0 is clearly not using _sensor_ saturation, which would give a value independent of the EI setting or any amplification applied between photosites and ADC. Instead, Dx0 is using "ADC" saturation, requiring that the levels in the raw file are at a standard placement, with mid-tones about three stops below maximum ADC output level. The 2006 standard is perhaps ambiguous about whether it must be applied to maximum signal in photosites or can alsomapply to maximum signal after analogue amplification and ADC comversion but before any further levels adjustment in conversion to JPEG.

In an exteme case, a future sensor with column parallel ADC or even with ADC done right at ach photosite (such designs do exist) might have no reason to apply variable analogue gain at all, with the effect of each doubling of the EI setting being simply to double the shutter speed (at equal f-stop and lighting), halving the raw level placement of a mid-tone part of the scene, and then shift one bit to the left in conversion from raw to JPEG (or flagging this intent in raw files). In this scenario, DX0 would declare the "true ISO" to be the same regardless of the EI setting, and produce a graph with all measurements stacked vertically at the left!

You make a good point: the saturation standard applies only when the sensor reaches saturation at base ISO. The increased amplification used for ISOs above base do not increase the sensitivity of the sensor but merely scale the signal so that it occupies the full range of the ADC. I understand that some recent cameras (e.g. the Nikon D7000) have already reached the ISO less stage that you mention.

Regards,

Bill
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BJL
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« Reply #30 on: April 15, 2012, 04:12:54 PM »
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Bill,
I just thought of another simpler way to state this: one ideal, that we might not be so far away from, is that the raw output could be essentially a count of the photo-electrons accumulated in each photosite. Subsequent transformations in the digital domain could take care of appropriate "tonal placement". Or perhaps (for mere convenience) the raw values could be modified from these electron counts by a suitable scaling factor, to reflect an intended tonal placement.
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Ray
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« Reply #31 on: April 17, 2012, 01:28:46 AM »
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Ray, your T-stop issue is a red herring: I was talking about professional testing of cameras such as that done by Dx0, where it is perfectly practical to use prime lenses for which f-stop is a thoroughly accurate measure of the intensity of illumination reaching the sensor.

You seem to have misunderstood my point, BJL. Of course DXO is quite capable of either finding a prime lens with a T-stop rating similar to the F/stop, or making some adjustment to the measurements to compensate for any discrepancies. That's their business.

The issue is not that your idea cannot be implemented, but rather, how useful the data about sensor performance in relation to shutter speed might be for the camera-buying public who are mostly unaware of the T-stop values of their lenses?

DXO seem to be one of the few lens-review companies who test the transmission quality of lenses, if not the only one. For example, I can't find any T-stop values for my Nikkor 14-24/2.8. DXO haven't posted any test results yet for this lens, as they haven't for many other popular lenses.

However, during my own comparisons of this lens with the Sigma 15-30, both used on the Canon 5D with an adapter for the Nikkor lens, I was surprised to find some significant variation in shutter speed requirements, at the same focal length and F/stop, shooting the same scene with same lighting. As I recall, the differences sometimes seemed to be as great as 1 full stop. In other words, for an ETTR exposure on the 5D, I needed to use F5.6 with the Nikkor lens, at 24mm, but F8 on the Sigma lens, with the same shutter speed.

I wish I could find some verification for this observation, but I can find no T-stop results for either of these lenses on the internet.

However, searching the lens data base of DXOMark, I can find T-stop results for some of the lenses I already own and use, and the results are surprising.

For example, the Canon 24-105/F4 that I have used a lot, has a T-stop rating that differs by 0.8EV, at 24mm and full aperture. That's the equivalent of almost one full F/stop in exposure.

The Nikkor equivalent of this lens, the AF-S 24-120/F4 VR which I also use a lot, has T-stop values which are much closer to the F/stop ratings. The difference is only 0.2EV at 24mm.

In other words, when these two lenses are used at 24mm and the same f/stop, either on the same camera through use of an adaptor, or on the appropriate camera brand, both of which happen to have the same ISO sensitivity (let's assume), the Canon system will require almost 2/3rds of a stop more exposure to compensate for that additional transmission loss.

Now it may be the case that prime lenses tend to have less transmission loss, on average, than zoom lenses. But such transmission loss can still make the difference between something that would not be noticeable, and something that suddenly becomes noticeable through choice of lens.

DXO state that a difference in DR of less than 1/2 a stop is probably not going to be noticeable (depending on degree of pixel-peeping). However, a difference of 1/3rd of a stop in noise and DR, coupled with another difference of 1/3rd of a stop due to T-stop differences, could result in an over all 2/3rds of a stop noise difference at the same shutter speed, which could be noticeable and significant, depending on the nature of the scene and the degree of print enlargement.

Checking DXO's lens database, I notice there are a number of prime lenses with a T-stop rating that differs by 0.4EV. The Nikkor AF-S 24/1.4 differs by 0.6EV.

Why you think this is a red herring, beats me.  Grin

Another good reason for DXO not adopting your suggestion of relating sensor performance to shutter speed, is the fact that they have already tested hundreds of camera models using a consistent methodology and consistent format for the presentaion of the results, which allows for easy comparison between the latest Nikon model, say the D800, and the earliest Canon model, say the 6mp 10D.

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Why on earth do you think that the Dx0 measurement based on highlight headroom in raw files is making the correct comparison even when it mismatches shutter speeds?


DXO doesn't even mention shutter speeds in its results. How can it mismatch them?

As regards the correctness of their comparisons, my confidence is based upon two factors.

(1) My own tests of sensor performance in different models of cameras, some of which were carried out before DXO published their test results, concur very closely with the DXO results.

(2) I've never seen any published tests from other parties which demonstrate inaccuracies in the DXO results. But I've seen lots of 'blah, blah, blah' which has not been backed up by any testing employing rigorous methodology.

I always remember the occasion in Bangkok when I compared the high-ISO performance of the Nikon D3 with my Canon 5D, before the DXOMark site existed. I couldn't hire or borrow a D3 because they were in short supply at the time, a bit like the current D800. But I found a camera shop that allowed me to use their demonstration model inside the shop, for comparison purposes.

The reason I took the trouble to do the comparison was because the claimed improvement in high-ISO performance of the D3, up to 2 stops better than any other existing DSLR, just seemed too good to be true. If they were true, I wanted the camera, since I often find myself in situations where flash and/or tripod are not appropriate.

However, I suspected that most reviewers of the D3 at the time, had been duped by the fact that the D3 had a couple of stops higher ISO settings than, for example the 5D. I recall statements in some reviews along the lines, 'We weren't able to compare performance at ISO 12,800 with the 5D because the 5D doesn't have such a setting'. I guess they just weren't smart enough.

My procedure was to shoot the same dark scene in the corner of the shop with both cameras on a tripod, using a number of different exposures varying by 1/3rd of a stop, underexposing the 5D at ISO 3200 to simulate ISO 6400 and 12800, and using the same aperture and focussing, and the same FL of lens, but different lenses.

Now, at the time I didn't even consider differences in T-stop ratings, didn't even think about them, but I did notice something which, at the time, I incorrectly attributed to differences in ISO sensitivies. The D3 seemed at about 1/3rd or perhaps even 1/2 a stop more sensitive. Looking now, at the DXO T-stop ratings for the two lenses that I used, the Nikkor 24-70/2.8 on the D3 and the Canon 24-105/F4 on the 5D, I see that the Nikkor lens has about 1/2 a stop lower T-stop rating than the Canon lens I used. (0.8Ev for the Canon as opposed to 0.3EV for the Nikkor).

That means that the Nikkor lens lets pass about 1/2 a stop more light than the Canon lens at any given exposure of the same scene at the same aperture.

The fact that I incorrectly attributed such differences to ISO sensitivity instead of T-stop differences, should not affect the results provided I took into consideration such differences during my comparisons, which I did.

In fact, according to DXO results, the ISO sensitivities of these two cameras is about the same, the D3 being only about 1/5th of a stop at the most, less sensitive. Offsetting the fact that the Canon glass was 1/2 a stop less sensitive (ie, more opaque), we get a nett effect of the D3 appearing to be about 1/3rd of a stop more sensitive.

Making adjustments for such apparent differences, I was able to determine that the D3 had a high-ISO advantage of the order of 1/3rd of a stop. Since 1/3rd of a stop was the smallest increment I could use, and since I was relying upon the impartiality of the ACR RAW converter I was using, it was impossible to be absolutely precise about such differences. With more precise testing using more sophisticated procedures and equipment, I would have accepted any differences of values between 1/4 of a stop and 1/2 a stop as being valid and consistent with my own testing.

Checking the DXOMark results which were published much later, and allowing for that slight difference of around 1/5th of a stop (or less) in ISO sensitivities, the DR of the D3 at ISO 3200 (actually 2512 as opposed to 2710 for the 5D) is shown as being about 0.4EV better, or slightly less than 1/2 a stop better.

This is one of the reasons why I have confidence in the accuracy of DXOMark test results. I've also carried out other comparisons of other cameras, which have also concurred with DXO results, such as comparing the DR of my D7000 with that of my 5D.

The crucial point when making such comparisons is that one must always start off from a comparison of equal exposures in terms of equal sensor saturation, such as ETTR, so that one can make allowances for any apparent differences in ISO sensitivities and/or T-stop differences.
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