Looked at it a bit more, and I can see that with the nex5.json SSF the dark blue patch on the CC24 gets a bit more red in it than it should. Making a profile from a CC24 shot you don't get as much red in there, then it's very closely to Adobe's matrix result, and looking at a real CC24 it matches other cameras well too. There might still be a tiny bit too much red in there though, which I think can be that the camera filter response has a tendency towards pulling deep blues in that direction.
So what does this mean? I think the SSF curves you've got has some issues in the blue range. I think I've seen this with other SSF curves for my 5Dmk2 for example, that is you see that it's not as accurate on the deep blue patch of a CC24, and maybe this is due to some limitations in measurement when measuring short wavelengths? Or simply that there may actually be some difference between Nex5 and Nex6 so the SSFs is not actually matching? SSF may seem scentific and all, but a badly measured SSF curve set is certainly not better than a good measured CC24 target.
It may be possible to manually tune the SSF curves to make it match the blue better, but then you're on your own
. Personally I'd play a little bit with that, at least to start with. Otherwise you can base your profile on a SSF target, or make an ugly thing in this case when Adobe seems to have a colorimetric matrix -- steal their matrix
. You don't get a LUT correction but this sensor matches very well with just a matrix (see DxOMark's metamerism index, 85 for this). Stealing the matrix makes an almost exact match with your own CC24 target, but the Adobe matrix seems to be designed with some larger target as it's more stable on extreme saturation, like the pink bags.
But still it seems like the camera has some tendency towards getting a little more red into the deep blues, probably it would be nice to have a target with more deep blues than the CC24 has. It seems like Adobe's NEX5 profile is designed with a colorimetric base matrix, and then their look table reduces red component a bit (manual tuning?).
Conclusions:
- The SSFs in nex5.json has an issue in the blue range, and cannot be used for good precision there.
- Not using any relaxation/weighting at all is not recommended, as you can worsen things for colors with higher chroma than the target
- With a CC24-based profile you get less problems with the blue, but possibly there is still some small amount. This is probably hardware related and you
need a target with more deep blue and purple patches to get better precision
- The issues is not due to any bug, but specific to these SSFs and this camera. That is no software changes required.
Note that C1 is known for adding in a good amount of yellow, making saturated reds orange (see the H&M text top of image), and most likely that deep blue is affected too, so I wouldn't use that as a reference.
Solution(s):
- Use traditional CC24 target, add weighting/relax, residual problems are then small
- Use a more saturated target like an IT-8 (preferably combine with CC24), better quality
- Try to manually correct the SSF curves for better deep blue matching
- Manually correct the blue range with look operators.
- Steal matrix from Adobe Standard profile (by eye the base matrix seems to have good colorimetric properties), this is the easiest and most fool-proof in
this case. Note that Adobe's profiles are designed differently profile-to-profile, the base matrix is far from always a colorimetric one like here.
Note that if you copy the Adobe matrix the look of the profile will not at all be like Adobe's profile, as the tone reproduction is totally different. I have attached a native profile with the Adobe D65 matrix you can play with, just run your make-icc commands on that.
