Gonz wrote:
Ok. I've done some experiments to test this whole ISO invariance subject, which some ascribe almost mythological healing powers to underexposed images. I'll reveal one pair of experimental images after I have my methodology down. Hence this post. The results were so surprising, that it made me question my methodology.
Which body are you using?
Here is what I did. 1. take an image with ISO 1600 properly exposed. Use manual and set shutter and aperture for image result that takes up most of the histogram (avoid blowing highlights) 2. take a second image with same shutter and aperture but at ISO 100. I.e. 4 stops underexposed. 3. Import into lightroom, compensate +4 exposure on the underexposed ISO 100 image. My lightroom has a limit of +4, hence the selection of 100 and 1600 for ISO values. Images should look roughly the same if this methodology is right? Are the ISO values correct? 100 * 2^4 = 1600, or is this wrong?
It looks right to me. Before I had heard of ISO invariance, I sort of discovered it accidentally. When not shooting action, and when I don't want to bother chimping the histogram for every shot, or when I'm shooting wide dynamic range images I'll just bracket, generally three exposures +/- 2 or three stops.
My preferred auto exposure mode is usually Tav. I noticed that when bracketing in Tav, after correcting my exposure in lightroom, I generally couldn't see much difference between the three shots. Do note that in the "over" shots I'd get a bit more clipping, and in the "under" shots, I'd sometimes get a bit more noise in the shadows. Also, there seem to be some nonlinearities in the way LR processes files, so they wouldn't always all come out exactly the same after processing, but yeah, modulo some channel clipping, they'd look pretty close.
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