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Output image file quality / JPEG artifacts


Standard Test Scene
The *ist DS provides three different image sizes in combination with three JPEG compression settings (as indicated by a number of stars). In addition (as you would expect) there is a RAW file capture option (to Pentax's own .PEF format), however the *ist DS doesn't have a RAW+JPEG mode.

Below you will find crops of the same 240 x 180 portion of the center of a sequence of images taken at some of the available combinations of image size and quality. Crops shown are at 100%, saved as extremely high quality JPEG. The RAW file was converted to a TIFF using Pentax Photo Laboratory Version 2.10.

3008 x 2008 RAW
10,689 KB .PEF (not for download)
3008 x 2000 JPEG
2,789 KB .JPG
3008 x 2000 JPEG
1,667 KB .JPG
3008 x 2000 JPEG
909 KB .JPG
 

2400 x 1600 JPEG
2,015 KB .JPG

 

The *ist DS's three star mode (named 'Best' in their browser software) provides extremely clean images with no noticeable JPEG artifacts, indeed you need to drop down to the one star setting before artifacts become obvious (at 100% magnification). By the file size we can assume that Pentax isn't compressing the RAW data (other manufacturers use lossless ZIP like compression to cut down the RAW file size), the variation in size is due to differences in the size of the embedded JPEG image (used for thumbnail / magnified view in-camera).

Color space

The *ist DS as independent selection of output color space, you can select from sRGB or Adobe RGB, the only downside is that instead of having this option on the first record menu this feature is buried in the Custom Functions.

Images taken in the Adobe RGB color space have their filename prefixed with an underscore (_) this complies with DCF 2.0 (Exif 2.21)*. As you can see if you move your mouse over the labels below there's only a subtle difference in color rendition between sRGB and Adobe RGB with the later having slightly richer blues and browns.

sRGB Adobe RGB (converted to sRGB)

* This has nothing in particular to do with Pentax but this renaming system makes it difficult to keep your images in the correct order if you don't routinely rename them by date/time.

Color space: CIE u'v' Color Distribution chart

Note that in these samples the Adobe RGB image has not been converted to sRGB and so to view it correctly you will have to load it into a color space aware photo application and assign the Adobe RGB color space. Below each sample is the CIE u'v' Color Distribution chart; larger gray triangle approximately represents the range of color which the human eye can resolve, the inner triangle the available gamut in each color space (sRGB or Adobe RGB).

sRGB Natural Adobe RGB
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