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Author Topic: G10: Michael vs DPREVIEW  (Read 8910 times)
Ray
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« Reply #20 on: December 02, 2008, 10:18:04 AM »
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Pom,
To drive my point home, I'll quote from a DPR review of the Canon 1Ds. I think Dpreview might have forgotten their own history. Below is a jpeg of the luminance noise comparison of the 1Ds and the D60. That a previous generation cropped format should be less noisy than a current full frame format really is preposterous. Yet the graph shows that the 6mp D60 has less noise, up to about ISO 640, than the famous 1Ds.

Note the explanation at the top of the graph. I'll quote it, in case you missed it. "It's important to remember that the 1Ds produces larger images than the D60. This means that at the same print size, noise will be less visible for a 1Ds image than a D60 image."

Are there any owners of both the D60 and 1Ds who would claim that this is not correct? I'd like to hear from them.

[attachment=10067:Dpreview...60_Noise.jpg]
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ejmartin
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« Reply #21 on: December 02, 2008, 12:58:57 PM »
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Quote from: Dale_Cotton2
Ben/Pom writes:I hear a lot about downsizing to match noise but have never seen the truth of it in the real world myself,

You've got company: DPReview Blog

Unfortunately, DPR understands very little about image processing, and many other technical aspects of what they do in their testing.  This blog post has been thoroughly debunked in a couple of threads over at DPR:

http://forums.dpreview.com/forums/read.asp...essage=30176643

Moreover, the DPR blog post isn't even a test of what it claims to be.  For the test they should have done, see
http://forums.dpreview.com/forums/read.asp...essage=30190836
http://forums.dpreview.com/forums/read.asp...essage=30211624

Bottom line: downsizing eliminates fine scale structure from the image.  If there is no fine scale noise (the "grain" is coarse; noise reduction has been performed), then there is nothing in terms of noise for downsizing to eliminate -- it has already been taken out of the image.  This is why often one sees little effect on noise from downsampling an image; that is certainly what happened in the examples studied in the DPR blog post.  When I ran a controlled study of downsampling a 50D test image to the dimensions of a 40D image (the second set of links above), I found results consistent with theoretical expectations -- the std dev of noise reduced by roughly the ratio of pixel dimensions of the original and downsampled image.

BTW, it is not so much that downsampling reduces noise.  Rather, when one pixel peeps one is looking at a coarser scale in the image, and the noise at that scale.  Since noise rises at finer scales (unless it has been scrubbed out by NR), one removes that part of the noise spectrum with the downsample and what is left over is the lower level noise at the coarser scale.  But that noise was always there prior to downsampling, and wasn't changed by the downsampling.  It was simply made into the "pixel level noise" by changing what "pixel level" means.  So in this sense downsampling is not reducing noise; on the other hand, it is reducing noise since one is throwing away the high frequency noise (well, one is throwing away high frequency everything, including image detail) via the downsampling.
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emil
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