Looks like we will disagree on this. It is kinda hard to measure supposedly 14.5 stops of DR on a 13.7 strip, with really 13.3 EV to work with. The way I see it, the slider does allow us to recalibrate the analysis, clipping the first point off starting from 2 stops in, Id love to understand why that is an unreasonable assumption. Add that two stops back in after the calculation and both cameras are well within the ballpark of much more expensive methods.
Again, if you have a better suggestion I am all ears.
It is not a disagreement, Michael. I don't mean to be rude, so with all due respect, you are misunderstanding how to use the strip. You don't need 14.5 stops, because you are not even making full use of the 13.7 stops the strip you have now currently offers. Until you clip swatch 1, you are under-utilizing the Souffer 4110 Wedge. It isn't a matter of opinion here, it is a matter of proper application. It is necessary to clip that first stop. The rest is a simple matter of mathematics.
I checked out the statistics in PI, calibrated to 14-bit. In 14-bit number space, the Max (Swatch 1) and STDev (Swatch 36/40) values are:
5D III (Red): 12282/10.5
D750 (Red): 13644/2.5
5D III (Green): 11300/5.8
D750 (Green): 12144/1.3
5D III (Blue): 12923/7.5
D750 (Blue): 12941/1.5
In terms of dynamic range, the formula when using digital numbers is 10 * log(Max/Stdev) * 3 (similar but slightly different to the formula when using electron charge) , which takes the maximum (saturation) over the standard deviation of noise (of a swatch where SNR reaches a ratio of ~1:1...I used the first swatch where the seam between it and the next swatch was not visible in each image, as every swatch after that looked the same (for some reason the PI statistics window doesn't actually directly report SNR, so this was my only real option)...this was swatch 36 for the 5D III, swatch 40 for the D750). I ran the above formula, and got the following:
5D III (Red): 10.22 stops
D750 (Red): 12.45 stops
5D III (Green): 10.96 stops
D750 (Green): 13.23 stops
5D III (Blue): 10.78 stops
D750 (Blue): 13.12 stops
The red channel in the D750 image is exposed a little bit more than the other two channels, and that seems to weight the noise, hence the reason that channel has 12.45 stops instead of 13.something. These numbers seem to be pretty in line with what I was expecting, given the exposure level of swatch 1 in both images.
To actually reach 13.7 stops of dynamic range, we need the maximum level to be 2^14 - offset. In a Canon camera, the bias offset is 2048. In a Nikon camera, the clipping offset is 600 (this is based on work by some guys at Nikon Hacker who figured out how to restore a bias offset to Nikon cameras for astrophotography purposes, which after the black point clipping was removed, resulted in a bias offset of 600). So, for the 5D III, the maximum level that can be reached in a given pixel is 14366. For the D750, it is 15784. At those levels, to reach 13.7 stops of DR, you would have to have read noise as low as 1.113 for the 5D III (not going to happen), and 1.23 for the D750 (well, it got as low as 1.3 in the green channel...so, maybe.)
Anyway, if we run the math with those numbers:
5D III (Max Stouffer DR): 10 * log(14366/1.11) * 3 = 13.7067 stops
D750 (Max Stouffer DR): 10 * log(15784/1.23) * 3 = 13.69437 stops
With maximum saturation numbers like 12282 and 13644, you are exposing WELL below the clipping point on swatch 1. Swatch 1 needs to be clipped, then swatch 2 would be used as the maximum saturation swatch, and whichever the first swatch where the seam between it and the next becomes invisible would be used for the standard deviation (SNR 1:1) swatch (this, at least, is the only way it could be done using PI statistics...I am honestly not sure which swatch is truly where SNR 1:1 would first occur...but I don't have any better way at the moment given the tools at my disposal, and once all the dark swatches reach a point where they all look the same and blend into each other, you HAVE reached the minimum usable level.). That is the only way to properly use the Stouffer T4110 Step Wedge. Otherwise you are under-utilizing it.
Based on the numbers above, your about two thirds of a stop or so under-exposed, which is fairly significant.