@russell2pi, jbinpg
Yes, I turned off any high iso noise reduction and long exposure noise reduction features. Maybe it was not stated clearly in my earlier posts.
Originally posted by smigol Thomas,
Regarding the K10D, I've decided to use ISO 100 with 1200 sec subs as my default setting. When I used the ISO setting before I started calibrating with Maxim, I was disappointed in how much of the dark detail was missing in my shots. For a year, I used ISO 400 with 600 second durations to get the data out of the lowest ranges. After
re-running the numbers on the camera, I decided to go back to ISO 100 and use much longer subs. This has given the effect of getting the signal out of the lowest areas while leaving a maximum amount of headroom *plus* I'm adding a shoulder of 300 ADU in Maxim.
This combination has finally allowed me to get the amp glow in the K10D under control. No other calibration routine seemed to properly handle the bad areas.
At this point, if Pentax does do funny stuff with the data before it's written to the RAW file, at least I have only one variable to manage: temperature. Everything else has been held constant. Also, the K10D is a CCD based camera, so some of the noise reduction requirements are not required like with CMOS devices.
Someday I'll have to see if I can reconcile my camera's numbers with Craig's method.
Everytime I think if you can trick the K5IIs into not clipping the blacks by using a hot environment it becomes moot because you would not have the same environment in the field. Thus, the camera would clip.
What do your bias files look like? Are they clipped?
I don't think you could trick the camera by using a hot environment. I guess the in-camera pre-processing routines analyze the data from unexposed sensor area (overscan area, which is on the left side in the K5IIs) to determine some kind of average count. This value - which depends on temperature - is then subtracted from the whole image before writing the raw data to file resulting in approx. 50% clipped pixels in any dark exposure.
For a better understanding of the black point issue I'll now present a few results obtained with my K5IIs at ISO 800, 10 seconds exposure time, 0°C air temperature under complete darkness.
1. Master dark (average of 30 dark exposures) in manual mode.
The image above shows the whole sensor area. You can clearly see vertical stripes and some amp glow or heat source at top and right.
This is a very typical and perfectly usable dark image for a low dark signal environment (cold, short exposure in terms of astrophotography).
2. Histogram plots (from a 4000 pixel area) of a single exposure.
From left to right:
- exposure in manual mode
- exposure in bulb mode
- exposure in bulb mode with the bin at 0 ADU omitted (for better comparison to the first one).
The effect of clipping the left side of the gaussian noise (about 50% of all pixels) in the bulb mode is shown. The original black point near 512 ADU is preserved only by exposures in manual mode and at 10 seconds and below.
3. Average Row (from master dark images comprising of 30 individual exposures)
The vertical dark image pattern as seen in 1. is measured by averaging many rows. The left figure shows ADU vs. column number for the master dark obtained in manual mode, the right figure shows similar measurements (same sensor area) using images in bulb mode.
It is very obvious that the pattern visible in unbiased dark images (mainly columns with
lower ADU, visible in left figure) has vanished in the noise-clipped bulb mode images to a large degree! But it will appear in any light exposure because counts are much higher then and no clipping happens. The goal of removing the underlying dark pattern by subtractiing a master dark image can not be achieved anymore for bulb exposures.
Another interesting effect is that by averaging noise-clipped images we get a mean value above zero! This will definitely affect image reduction when it comes to vignetting correction (known as flat field reduction). When you model this effect then it appears that darks with higher intrinsic noise will result in a higher "false mean". This might well account for Stephen's observation that shooting at higher ISO will make dark frames even more inappropriate. There is both missing information and statistics is fooling you too.
What makes me burning my head is: Why does Pentax apply this shift of the black level to zero? If it were to ease the average user in processing their raw images then they would have implemented this shift for all exposure settings or at least for the most common setups which are with short exposure times. But Pentax engineers have decided to do the reverse ...
Any thoughts?
Thomas