Gordon, did you see this (in the other forum)?

Started May 3, 2013 | Discussions thread
GordonBGood
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Re: Gordon, did you see this (in the other forum)?
In reply to GordonBGood, May 4, 2013

GordonBGood wrote:

Kerusker wrote:

ewelot wrote:

.....I found - to my big surprise - that the black point issue didn't show up and dark frames exhibit a nice gaussian intensity distribution! It appears that accessing the in-memory raw image data of the camera via PC control software offers data at a less processed state and - most importantly - the image pixel mean intensity is not shifted to zero level. ......

This should be the link:

wwwDOTpentaxforumsDOTcom/forums/pentax-k-5/217407-blackpoint-issue-k5iis-raw-not-raw-3.html#post2366621

Gordon, I would like to know what you draw from the thread over there.

From my K-5, it seems to me that the black point is uncompensated up to ISO 1600, but it is a while since I have checked it and it may have changed with the latest firmware updates.  All of these cameras will definitely have a zeroed black point for ISO's of 3200 and higher where the ISO's are achieved by shifting of underexposed ISO 1600 captures, and I assume that is why the ISO 1600 and higher sensitivities are black point zero compensated:  to prepare for the shifting operations.

I didn't know that the K-5 II(s) would be any different and don't have access to one, but I guess I can check from the raw images posted on Imaging-Resource.

When I get some time in the next day or so, I will do a quick check of K-5 II(s) raw images and report back here.

Interesting that the PKTether type programs would be able to capture the raw data at a different stage in the processing pipeline.

I took some time to download ISO 800 and ISO 1600 raw DNG K-5 II images from Imaging-Resource (IR) at K-5 II ISO 800 Raw  and K-5 II ISO 1600 Raw and checked the black offsets.  For these images I found they were at firmware version 1.00 and had an offset at ISO 800 of about 512 (out of a full scale of 16384) and at ISO 1600 image had a black offset of zero, just as for my K-5 (other than that the offset for low ISO's is at about level 64 out of 16384) for the reasons as explained in my last post above.  As the K-5 IIs is identical to the K-5 II except for no Anti-Aliasing filter, I didn't bother burdening IR's bandwidth in checking those images.

Now, it is an interesting question whether newer firmware versions have changed this black offset compensation behaviour, but I see no reason why they would.  If you would like to check this for yourself, you can use the Beta version of PhotoMe to see the Black Level (for each of the RGGB quad of channels) in the Additional Image Data (1) tab or just filter for Black Level or Black Point (same information contained in the Manufacturer notes section of the metadata).

So the interesting questions are"At what ISO level was this black point issue identified on the other forum experienced for normal raw captures?" and "Is the PKTether capture really any different given the ISO sensitivity constraints?"

I don't really think that the main CPU of the K-5's has access to the raw data pipeline as the black level offset compensation is likely more a function of the PRIME2 hardware imaging engine, but it is possible that black level determination and application are done using separate passes of the PRIME2 engine and that the uncompensated raw data may be available for access by the PKTether hook even for higher ISO sensitivities, especially if it is able to hook into some debug facilities for the camera(s).

For my use with the K-5, I haven't dabbled with PKTether or its variants, and find that if I want to have uncompensated black levels I just stick to ISO sensitivities below 1600, underexposing and boosting the raw brightness as necessary.

Regards, GordonBGood

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