How to recover a 10 stop underexposed K-5 image.

Started Nov 2, 2010 | Discussions thread
Veteran MemberPosts: 6,283
Re: Yep
In reply to Jeff Charles, Nov 8, 2010

Jeff Charles wrote:

Iliah Borg wrote:

K5 is a typical representative of a newer camera generation, with typical colour problems by the way.

What are those color problems?

Look at the metameric failure indexes.

I don't fully understand this concept, except for a quick Wikipedia lookup. How does it apply to the K-5?

Can you show examples?

What sort of examples, loss of resolution in foliage or in skin tones? It is well-discussed already ("mushy greens", "plastic skin").

By "examples", I mean photographs that show the problems. Also, I cannot find posts in this forum about those problems. Where have they been discussed?

I think a little bit too much has been blamed on metamerism indexes on any perceived wrong colours.

The Metamerism Failure Index is linked to the Colour Rendition Index (CRI), which has been depreciated in favour of the Sensitivity Metamerism Index (SMI), and that is what is measured by DxOMark in "Colour Response". Most recent Pentax DSLR cameras including the highly rated colour-wise K10D have had a SMI of about 81 to 82 at the D50 daylight illuminant with the K-5 having a score of 78. DxOMark have this to say about this index: "SMI is an index quantifying this property (metamerism or confusion of colours - GBG), and is represented by a number lower than 100 (negative values are possible). A value equal to 100 is perfect color accuracy, and is only attained when Luther-Ives conditions hold (which, as previously stated, never happens in practice). A value of 50 is the difference in color between a Daylight illuminant and an illuminant generated by fluorescent tubes, which is considered a moderate error.

" and "In practice, the SMI for DSLRs ranges between 75 and 85, and is not very discriminating. It is different for low-end cameras (such as camera phones) which typically have a SMI of about 40. For this reason, we give this measurement as an indication but did not integrate it in DxO Mark." at: .

While colour metamerism or the incidence of two particular colours becoming the same colour when captured by a particular sensor under a particular illuminant can be demonstrated both mathematically and practically, I sincerely doubt that it is causing any "mushy greens" and "plastic skin", etc. in the K-5 as the score is just not nearly different enough.

What I see from the Colour Response of the K-5 as compared to other DSLR's is that the blue channel is somewhat weaker - meaning a darker blue filter on the Colour Filter Array (CFA) - but also that the colour gamut of the CFA has been tweaked so that the green gamut is slightly reduced while the red and blue gamuts have been slightly increased. The reason for a slight change in the SMI is likely due to just this slight difference in the weighting of the input from the three RGB channels in determining the 18 colours required for the SMI test procedure. The new colour gamut is still more than enough to show all the normal colours in nature and most of the ProPhoto colour space especially as to most of the reds and all of the blues, only cutting out a very few intense reds and greens shading toward cyan that rarely if ever occur in nature.

Practically speaking, I think one can get every colour one wants out of the K-5 with an appropriate colour matrix for the colour temperature and you will likely never notice that some of the colours may have metamerism in that they may appear to be identical when they shouldn't. Without a link, we can hardly investigate further, but I would suspect that the reports of "mushy greens" and "plastic skins" may well be attributed to other causes such as the over application of Noise Reduction (NR), inappropriate raw conversions, etc., etc.

Regards, GordonBGood

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