Following are a pair of image comparisons between several current-at-review-time Digital SLR cameras - the first of which incorporates detail that shows resolution differences. All images were taken with identical settings (including the important RAW file format) and under identical conditions.
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As you likely already know, the amount of light recorded in a specific amount of time (shutter speed) and at a specific lens opening (aperture) doubles with each doubling of the ISO sensitivity value. Because image quality degrades as the ISO setting is increased, using the lowest ISO setting the particular shot scenario will allow is desired. Not enough light is the usual reason for selecting a higher ISO setting.
There are many comparisons that can be made from the above samples. What is noticeable to me is that the higher resolution sensors are pulling more detail out of the scene (as expected) and that the Canon EOS Rebel XS / 1000D image is very sharp - maybe too sharp for this finely-patterned subject.
While high ISO noise can be hidden by noise reduction, detail is also removed. The XS / 1000D images remain sharp right up to its highest ISO setting - 1600.
Knowing that details are not being removed by noise reduction, evaluating noise is now easy in the nearly-detail-void color block comparison below.
I know, it is a stretch to compare the 5x more expensive Canon EOS 5D Mark II to the Rebel XS, but it helps maintain perspective.
The low end Rebel XS does not give up anything in high ISO performance (within its available range of settings) to the higher end Rebels in this comparison. Throw a point and shoot into this comparison and the advantage of the Rebel XS would be very clear (I last included a Canon PowerShot model in the Canon EOS Digital Rebel XSi / 450D review).
Back to the Canon EOS Rebel XS / 1000D Review.
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