Exposure corrections problem

Hello, Im working on project making HDR photo out of RAW images by making 3 samples with different exposures. Overexposing the image by setting values exp_shift between 2.0 and 8.0 works great but the underexposing values just arn't working. Setting them to 0.5 or 0.25 and it makes no difference on the result between it and input image.
Normal process - unpack(), dcraw_process(), dcraw_make_mem_image()

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You also need to turn off

You also need to turn off auto brightness. If not, values will be scaled to fill full data range.

-- Alex Tutubalin