From DXOMark: More pixels offsets noise3

Started May 30, 2009 | Discussions thread
John1940 Senior Member • Posts: 2,820
Re: Bob..


I think that one of the issues with regard to comparing the 50D and 40D is that the pixel spacing (horizontally or vertically) only drops by about 18% in going from the 40D to the 50D.

Here is a test that might prove something regarding the effectiveness of doing Gaussian blur before down-sampling with a series of successive doubling pixel-to-pixel spacing changes. It’s one that I have done before many times but I did not do the blurring first. However, I still have the evidence and may do some studies if I ever get the time.

I have scanned many 35-mm Kodachrome slides using a Canon 4000 dpi film scanner. When I first got the machine almost six years ago I decided to do quite a few tests with film and digital cameras I had at the time. My newest camera was a Canon 300D (Rebel) DSLR. I also had (and still have) a 35-mm Canon film body (which takes EF lenses) and had a Minolta 35-mm system with six lenses.

In one series of tests (using several slides but no DSLRs) I scanned each slide at 4000 dpi, 2000 dpi, 1000 dpi and 500 dpi successively without touching or moving the slide at all. In round numbers I got a 20 Mp, 5 Mp, 2.5 Mp, and 1.25 Mp image in Tiff-8 or Tiff-16 format (or RGB format). The scanner can do 8 bits or 12 bits per color at every pixel location. Each pixel therefore uses either 3 bytes or 6 bytes of memory. I then did various (but not scientifically rigorous) comparisons by down-sampling but did not do Gaussian blur with the higher resolutions first.

The potential value of this test method (which I’m sure has been used by many people in many organizations going back decades) is that all other variables are removed since the source of the image is the same and the scanner is the same. Also, the steps in resolution are large enough to show the primary advantage of the higher pixel count (detail). I never found any increase in noise in my unscientific experiments. But I never looked at whether Gaussian blur before down-sampling would make what I thought was a good result even better in favor of the higher-resolution scans.

I think that the scanner fires the same number of photons (as such things go) for each pixel location. If I’m right, that takes another variable off the table, or at least makes it knowable.

At this point I’m in the 50D-is-better camp for detail and for being no worse for noise if down-sampled to 10 Mp with Gaussian blur done first. But, unlike with the scanner, it’s not easy to do accurate side-by-side tests with the 40D and 50D. There are too many variables that cannot be controlled easily.

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