Ok, this is my current workflow that seems to be working:
ImageMagick for conversion
HDRShop for pixel average (I like it because it gives the average value [0-1] to several decimal places rather than as whole integers [0-255])
1. Take DNG photo series with a constant light source of a white background. Try to get the highest exposure close to the top end without blowing out highlights.
2. Convert all DNGs to 16 bit TIFs (ImageMagick: "convert -depth 16 -type grayscale FILE.dng FILE.tif" convert does them individually, so I just made a batch file to do the series. mogrify is supposed to do multiple, but I didn't get it to work and didn't bother trying to figure out how to.
3. Open all files in HDRShop (or whatever your editor of choice is) and get the averag pixel value or compare histogram peaks.
I use HDRShop -> Image/Info/Average which gives the average R,G,B values (as 0-1). Since it's grayscale, they're all the same. They could also be done in colour as well and compare each channel.
Using this, I seem to get the 2 stop value I am expecting:
I took a series of 7 photos of a white wall: full wide, AV=f2.8, ISO=80, over 6 full stops -> 1/8s to 1/500s
With the following results.
|Shutter ||Ave Pix ||Ratio |
The exposure ratios are as close to 1/2 as I expect to reasonably get. As these are all taken within the normal camera range, it seems like the approach should be working. For higher shutter speeds, a much brighter exposure will be needed, especially for the other and of the aperture range. A higher ISO can also be used to help compensate with the understanding that it will add more noise.
Note that these exposures were taken in a room lit by halogen bulbs, but any frequency effect didn't seem to have an impact here. However, it would likely be best done with a constant light source like DC driven bulbs or daylight.
The key thing to note is to avoid white over exposure and black under exposure areas as they will adversely affect the pixel average weighting.
I will be trying some of the fast shutter speed testing myself another time, as I am currently running through some aperture testing (watch for results on the other thread, they are progressing quite well).