S
simon_t
Guest
Hi all,
Over the past week or so, I have been researching which pocketable RAW-shooting camera I should buy. While playing with the raw files from a number of cameras, I noticed something strange in the Ricoh files: the highlights can take on a subtle magenta tint that gets stronger as you try to recover blown highlights. As an example, here is a crop from a GRDII DNG file taken from http://ricohforum.com/phpbb/viewtopic.php?f=39&t=553&sid=01ee3c9f30f11212b2d737bed7d5e71c
[Exposure has been set to -4 to exaggerate the effect.]
This effect seems to be caused by the following. The sensor outputs a 12-bit signal that is roughly proportional to the amount of light that has been recorded. If too much light is collected, the signal clips and the output maxes out at a value of 4095. The RAW converter detects this and corrects for any color shifts that may happen because one channel (usually green) clips before the others.
In practice, however, the sensor's response first levels off before it hits the maximum value. Often, the green channel will reach this plateau first, which causes the green channel to become weaker relative to the red and blue channels. If the RAW converter doesn't know this is happening, it incorrectly assumes that there is less green in the original scene, leading to a magenta color cast - and this is exactly what's happening with the Ricoh DNG files.
However, there is a fix for this: the DNG specification allows one to specifiy the point at which the camera's response is no longer linear by means of the LinearResponseLimit tag. It seems that Ricoh has left this tag at its (incorrect) default value of 1.0 (fully linear response). I definitely think Ricoh should re-assess this (and the defined WhiteLevel) in the next firmware upgrade.
Luckily, users are not completely dependent on Ricoh for a fix. ExifTool allows you to manually adjust the LinearResponseLimit tag. Running it on the above DNG file with the "-LinearResponseLimit=0.9" option gives the following result:
I hope this is of any use to you. It will be for me, as I have decided to get the GRDII once I have time to drop by my camera store.
Cheers,
Simon
Over the past week or so, I have been researching which pocketable RAW-shooting camera I should buy. While playing with the raw files from a number of cameras, I noticed something strange in the Ricoh files: the highlights can take on a subtle magenta tint that gets stronger as you try to recover blown highlights. As an example, here is a crop from a GRDII DNG file taken from http://ricohforum.com/phpbb/viewtopic.php?f=39&t=553&sid=01ee3c9f30f11212b2d737bed7d5e71c
[Exposure has been set to -4 to exaggerate the effect.]
This effect seems to be caused by the following. The sensor outputs a 12-bit signal that is roughly proportional to the amount of light that has been recorded. If too much light is collected, the signal clips and the output maxes out at a value of 4095. The RAW converter detects this and corrects for any color shifts that may happen because one channel (usually green) clips before the others.
In practice, however, the sensor's response first levels off before it hits the maximum value. Often, the green channel will reach this plateau first, which causes the green channel to become weaker relative to the red and blue channels. If the RAW converter doesn't know this is happening, it incorrectly assumes that there is less green in the original scene, leading to a magenta color cast - and this is exactly what's happening with the Ricoh DNG files.
However, there is a fix for this: the DNG specification allows one to specifiy the point at which the camera's response is no longer linear by means of the LinearResponseLimit tag. It seems that Ricoh has left this tag at its (incorrect) default value of 1.0 (fully linear response). I definitely think Ricoh should re-assess this (and the defined WhiteLevel) in the next firmware upgrade.
Luckily, users are not completely dependent on Ricoh for a fix. ExifTool allows you to manually adjust the LinearResponseLimit tag. Running it on the above DNG file with the "-LinearResponseLimit=0.9" option gives the following result:
I hope this is of any use to you. It will be for me, as I have decided to get the GRDII once I have time to drop by my camera store.
Cheers,
Simon
