Astrophotography with Canon EOS 40D
This is an astrophotography review of Canon EOS 40D by Christian Buill. It’s interesting to see that you can use ISO 100 with 40D to get “the same” astro image as ISO 400 on 400D. It also seems that he confirms the live view’s usefulness in astro work, as there are clearly enough stars visible in the live view image to achieve perfect focus. Also 40D does not appear less or more sensitive than the preceding DSRL generation for faint flux applications. But the 14 bits coding, the possibility of long exposure starting from USB interface and the LiveView mode are real advances. The 40D approaches CCD cameras, even exceeded on certain points (transfer speed of the images, real time Live View).
The main thing for astrophotography is to take exposures long enough to get the sky background signal well above the noise, so when averaging tens of images together, the signal will rise out of the noise floor. Since signal and noise are scaled together when increasing ISO, you can see that it does not effect the minimum exposure time. The ISO just determines how the recorded signal is distributed /amplified over the 14 bits of range. Higher ISO amplifies signal, but it also amplifies noise. With ISO 1600, you also saturate/clip the brighter stars sooner, making bright stars appear more bloated and pancake-like. Over all conclusin with 14 bits in Canon EOS 40D, digitization noise should be less of an issue, making ISO selection even less critical. Lower ISO will allow greater recorded dynamic range.
Reference : Dpreview Canon EOS 40D forum, AstroSurf/Christian Buill.
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