Somehow i doubt you would be so sensitive about its methodology and findings if LetTheRightLensIn's thread title was "Zomg 5d3 has 14DR u guys!"
In my case i tend to believe him; his approach -with the amount of data we have so far- seems reasonable, his results fits other people tests, and since Canon itself hasnt bothered to announce something about low iso performance, tends to point in the direction LetTheRightLensIn is.
If he insisted his means of arriving at such a conclusion was "exactly the same as DXO's", yes, I'd have issue with it. Its not so much about what he is saying as much as it is about the fact that he is claiming his approach and results are just as good and trustworthy as an institution that has a demonstrated track record of using consistent, repeatable, verifiable techniques to produce valid results.
I'd figure most of the people on this forum realistically
expect about a 1 stop improvement in DR, but hope
for a full two stop improvement that would maximize the potential of a 14-bit sensor. Realistically, I think the 5D III will exhibit around a 1-stop improvement in DR once tested by DXO (12.7-13 stops). However...thats all just expectation and hope
. If someone told me they had run some kind of test that used "masked off" dark pixels from a RAW file and was able to prove with as much accuracy as DXO that the 5D III achieved exactly 14.0 stops of DR...I'd be extremely skeptical. (Having an understanding of the CR2 format, sounds a bit sketchy to me as the masked pixels are intended to be used to set black level by RAW converters, and they are purposely fixed at around 1024, rather than around 0 (don't ask me why Canon does that...its just what they do)...I don't believe they are a valid basis to use for measurement of DR.)
At BEST, I think Canon might be able to achieve the same as Sony, 13.86 stops of DR, which is within the margin of error for 3 electron read noise and quantization error in ADC with a 14-bit sensor. Assuming Canon has not actually moved to a column parallel on-die ADC system in their sensors like Sony did (since Sony currently holds all the patents for such technology), realistically I'd be doubtful that DR will improve much past 13 stops, let alone 13.8. Given Canon's track record with improvements to DR, they have consistently improved it with each successive camera in any one of their lines. Taking the 1D APS-H series as an example using DXO data, the II had 11.1 stops, the III had 11.7 stops, and the IV had 12 stops. Read noise for those cameras was 29.7, 22.3, 16.6 e-, respectively. Canon has shown a notable ability to improve their low ISO dynamic range with reductions in read noise, and they have made some significant claims about improved DR that could bite them in the ass if it turns out to be false, or worse, the 5D III exhibited worse DR than the 5D II.
Realistically, I'd expect the 5D III to approach 13 stops of DR, especially if they reduce read noise to 7D levels (8.7 e-). If Canon has been able to reduce read noise to Sony Exmor levels of about 3 e-, I'd expect the 5D III to push 13.5 stops of DR or more. However its all just educated expectation
...even if I thought I could get accurate measurements from .CR2 files from sample images on the net, I wouldn't make any bold claims about my measurements being accurate to any degree, and especially not as accurate as DXO. I have no idea what kind of lighting was used to illuminate any sample image I might get my hands on, whether the camera was a pre-production model or not, whether the exposure settings were set exactly correct to measure DR for the type and intensity of the bulb used to illuminate the scene. So far no sample image on the net that I've come across is really ideal for measuring DR with any degree of accuracy as there are no guarantees about the viability of the information contained within them.