How does pixel brightness correlate to camera exposure?

Started Nov 10, 2013 | Questions thread
bobn2
bobn2 Forum Pro • Posts: 66,302
Re: How does pixel brightness correlate to camera exposure?

gbarrett03 wrote:

Thank you very much Bobn2 for your helpful answer. Your answer leads to another series of questions that hopefully you'll also be able to help with.

If the output data from the sensor is Linear is there a way to access that data before it goes through gamma correction and raw conversion?

Yes, have a linear raw converter, or none at all. You can use toold like dcraw or raw Digger to access the original raw pixel values.

Also does the photo have to be in Raw format or could it be in any format directly from the sensor or from a specific frame of a video?

The raw format should be just the numbers taht the sensor records, give or take a bit of signal processing.

To further clarify what I am trying to ascertain I'll give you more background. For a school project I am investigating changes in light level over time. Using HDR photography there is program called photosphere that allows you to measure the overall luminance of the photo based on one calibration point. http://web.mit.edu/SustainableDesignLab/projects/TeachingResources/HDR_II_Photosphere.pdf
Since my research is time based I would like to use video of light changes as a starting point to gather research metrics. I would like to use a specific frame as my calibration frame for luminance and then determine a formula for the change in cd/m2 through the duration of the video based on camera settings and sensor output. I realize that without the fully dynamic range of HDR each frame is limited by an upper and lower threshold of the frame's range but it should allow me to start establishing data regarding contrast ratios and changes in cd/m2 over time.
Thank you once again.

I would recommend for that kind of job you work out a script using dcraw and Image Magick to produce images as you want.

http://www.cybercom.net/~dcoffin/dcraw/

http://www.imagemagick.org/script/index.php

http://www.rawdigger.com/

If you want to work in video, you generally can't access raw files, you'd need to degamma and de S-curve the video. So long as you know what those curves are, Image Magick an do it.

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Bob

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