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Stacking RAWs

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The sun popped its head out briefly yesterday so I went outside and took some photos with the new lens including this one.

Stack of Milton War Memorial

What you're seeing there is a three image stack constructed by the RAW files off the camera and I'm very pleased with what I'm getting. Well, until you look very closely. Then you start to see problems. Below is a crop of John's chimney taken from that photo and you can see that it's a bit blurred, not that surprising as the focal point was the memorial, and there's a bit of blue fringing on the right and at the top.

Apparently this is a known artefact of the algorithm ufraw uses to process RAW files.

You may have already discovered that if you put your mouse over that crop it changes: it both gets sharper and the fringes disappear. The second crop is from a stack constructed from the JPEGs off the camera. Nikon are being smarter about their processing1.

Interestingly I tried processing the 0EV RAW file from that stack using an old release of RawTherapee and got a much better result, so ufraw isn't the best at this, but it does have the advantage of working from a shell script (unlike RawTherapee).

But the truth is that the artefacts really aren't noticeable at all on most images and even where they are, as on the image above, I bet you didn't see them at the size I was displaying it: it's only if you want to print it at the full 3264 x 4928 pixels that it starts to become an issue. So the only change I can see me making to my image processing path as a result of this voyage of discovery is knowing that I can fall back to a JPEG stack if I'm not happy with the RAW stack. That loses me some dynamic range, but might be better in some situations.

  1. It's also worth noting that Nikon and lensfun have a different opinion as to what correction to apply to the distortion of the image produced by the lens, which is why the chimney seems to change shape.

Tags: photos Written 19/03/13

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