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While comparing the features of Canon 400D and 1000D, I've noticed that Canon 1000D has an improved DIGIC III processor, over the DIGIC II for Canon 400D.

From Wikipedia:

The DIGIC III Image Processor was advertised to deliver superior image quality, faster operation and extended battery life compared to its predecessor.

Will a newer version of the DIGIC processor provide visible improvements of image quality? What about when shooting in RAW?

I understand that newer versions may perform faster (when focusing, or a higher frame rate) and have features such as face detection and live view, but that is of no importance to me.

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The answer is Yes and No. It will indeed provide improvements in image quality when shooting jpeg, but the raw image is unaffected by DIGIC processing, it being the raw unprocessed data. Here is an article produced by Canon about Digic processing A/D conversion and RAW files:

http://www.usa.canon.com/dlc/controller?act=GetArticleAct&articleID=2748

Most importantly, the increase in Bit Depth of the DIGIC IV processor:

Equally important are the image quality benefits provided by the DIGIC 4’s integrated 14-bit Analog-to-Digital (A/D) converter...

Unfortunately for your specific DIGIC III question its mostly about the advantages of DIGIC IV :p

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    \$\begingroup\$ It is difficult to find exactly what the DIGIC processor does, but it clearly does much more than JPEG conversion and is involved in fundamental parts of the raw image creation pipeline, including autofocusing and noise reduction. See dpreview.com/news/0209/02091601canontech.asp or youtube.com/watch?v=jeNRKvX1kLo . Therefore we should conclude that raw image quality will be affected by improvements in this processor. \$\endgroup\$
    – whuber
    Feb 28, 2011 at 3:03
  • \$\begingroup\$ Unless you turn off all the image processing, which is probably a good idea anyways (e.g. no noise reduction, etc...). \$\endgroup\$
    – Fake Name
    Feb 28, 2011 at 4:09
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    \$\begingroup\$ @Fake The quotation specifically mentions the DIGIC 4 includes the ADC. Without that, there would be no raw image, period. This causes me to suspect that some of the advertised "noise reduction" occurs within the digitization of the analog signal itself and therefore cannot be turned off. \$\endgroup\$
    – whuber
    Feb 28, 2011 at 15:50
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    \$\begingroup\$ The latter. An ADC has basically one main characteristic - It's noise (there are others, linearity, sample-rate, etc... but they effect picture quality much less), but there is really no noise-processing done in the ADC. There are other techniques, like double-correlated sampling, but those are done in processing after the ADC.. \$\endgroup\$
    – Fake Name
    Mar 2, 2011 at 2:10
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    \$\begingroup\$ Basically, an ADC does one thing. It converts an analog voltage into a digital number. There are many ways to do this, but there really no noise reduction in the actual process. You may work to reduce the noise on the analog voltage, using analog filters, or post-process the digital output, but the actual conversion is pretty much a single-step operation \$\endgroup\$
    – Fake Name
    Mar 2, 2011 at 2:16

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