[Walter Dnes]

I checked out the "Superdogs" show at the CNE this week. I have a Pentax KX with a Sigma 17-70 zoom lens. Since lighting was "rather low", I cranked up the ISO to 3200, to see how well it worked. I also "cheated" a bit. The camera shoots images up to 4352x2868, which is overkill for photosharing.

However, that does allow you you to "bin" the photos down by a factor of 4 (1/16th the area) and still have a 1088x717 output. The theory is that because noise is random, and signal is additive, the result will look like it was shot at 1/4 the ISO (i.e. ISO 800) as far as noise is concerned. Running under linux, I used GIMP with the Ufraw plugin to import the DNG raw file and save as PNG. Then I ran Imagemagick to do the resizing/binning.

I've uploaded a dozen shots, ranging from 1.1 to 1.5 megabyte in PNG format. The URL is http://www.mediafire.com/?ak0y2gkz16b8s You have to click on the PentaxKX_ISO3200 folder to see the individual images.

 

[gordonb]

Walter,

The images appear to be fairly clear & not have a lot of noise. I think you are probably correct in your assertion that reducing the size by a power of two would reduce any visible noise. In your case by reducing by 4 your resizing program (ImageMagick) would interpolate 4 pixels from the original image down to one pixel in the target image. Whether this gives you the same quality of 1/4 of the ISO is an interesting question.

The JPEGs you posted don't seem to have any EXIF information so I can't see the shutter speed and aperture you were shooting but the speed must have been a little slow as I do see motion blur in the dogs but not in the handlers or judges. Last time I tried to use PNGs as an intermediate file format (I use TIFF usually) I was frustrated by PhotoShops lack of ability to save EXIF information to PNGs even though the latest specifications for the PNG format supports EXIF information in the header.

I also noticed that on the right hand side of all your images a thin band of noise?? Is that on your downsampled images too or is it an artifact introduced by the file sharing service?

I'm glad to see someone using Linux for image manipulation. I'm a big fan of Linux having been using it for 17 years now but I must admit that these days I use Photoshop as the last time I played with the GIMP I found the lack of common tools (such as red eye removal) frustrating. This was several years ago though and I imagine things have changed. ImageMagick is a really nice tool - the linux image manipulation Swiss Army Knife. Some of my Linux servers have gallery software on them which uses ImageMagick to scale photos and it appears to do a nice job.

 

[Walter Dnes]

The images appear to be fairly clear & not have a lot of noise. I think you are probably correct in your assertion that reducing the size by a power of two would reduce any visible noise. In your case by reducing by 4 your resizing program (ImageMagick) would interpolate 4 pixels from the original image down to one pixel in the target image. Whether this gives you the same quality of 1/4 of the ISO is an interesting question.

There is a lot of theoretical work on the subject of pixel-binning. See http://www.noao.edu/outreach/aop/glossary/binning.html for a beginner's explanation.

The JPEGs you posted don't seem to have any EXIF information so I can't see the shutter speed and aperture you were shooting but the speed must have been a little slow as I do see motion blur in the dogs but not in the handlers or judges. Last time I tried to use PNGs as an intermediate file format (I use TIFF usually) I was frustrated by PhotoShops lack of ability to save EXIF information to PNGs even though the latest specifications for the PNG format supports EXIF information in the header.

The images I uploaded are PNG files. My problem was that exiv2 can't read EXIF data from .DNG raw files. Anyhow, I've now found exiftool, which can read EXIF data from .DNG files. I've uploaded text files corresponding to the DNG file that each image came form. I notice that the first image was actually shot at ISO 2000, but I wasn't happy with the shutter speed, so I dialed up to ISO 3200 for the rest of the shots.

I also noticed that on the right hand side of all your images a thin band of noise?? Is that on your downsampled images too or is it an artifact introduced by the file sharing service?

It shows up on the full-size images I've imported from the DNG raw. I'll have to crop that area in future.

ImageMagick is a really nice tool - the linux image manipulation Swiss Army Knife. Some of my Linux servers have gallery software on them which uses ImageMagick to scale photos and it appears to do a nice job.

The nice part is that it runs via commandline scripts, so I could automate stuff like cropping the noise on the right-hand edge. exiftool and ufraw-batch are also commandline scriptable. I can see myself setting up a script that goes from .DNG to downsized PNG or JPG all by itself.