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EXPOSURE
Noise between ISO100 & ISO800 is acceptable, between ISO800 & ISO1600 noise is tolerable; between ISO1600 &
ISO6400 noise is objectionable. Dark frame subtraction reduces thermal noise and this can be done in two ways. A set
of dark frames can be shot before and after the session, averaged, and then subtracted from each frame. The
camera's BIONZ processors can also be set to do a dark frame subtraction after each exposure. Because dark frames
must be taken at the same temperature as the image frames, it is better to use the in-camera dark frame subtraction
since it is taken immediately after each exposure and will be at the same temperature. The temperature will fall
throughout a lengthy imaging session, so the averaged before and after dark frames will only be a close
approximation. Where the exposure times are brief, in-camera dark frame subtraction is preferable.The signal to noise
ratio may be decreased using raw file noise reduction and by stacking frames post exposure.
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EXPOSURE
It is tempting to increase the ISO rating and reduce the exposure time so as to "beat the seeing". Where exposure
times are shorter than 1/100s there is no point in doing so. In Seeing AI the image wavefront is ostensibly plane, so
there is no advantage to increasing the ISO rating. In Seeing AII increase the ISO rating to ISO200 or ISO400, and in
Seeing AIII increase it to ISO800.
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EXPOSURE
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ISO SETTING & RESOLUTION (cont.)
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ISO vs SEEING
BRACKETING vs DR-O
The dynamic range of the Exmor CMOS sensor is 12EV
@ ISO100 with DR-O off. Increasing the ISO rating
decreases the dynamic range. The Sony a900 has a
Dynamic Range Optimizer (DR-O). For Solar & Lunar
photography this should be set either to <OFF> or
<STANDARD>. For deep sky photography of nebula set
DR-O to <ADVANCED AUTO>. For deep sky
photography of star fields or clusters set DR-O to
<ADVANCED Lv5>.
High dynamic range imaging of the Moon is best done by
stacking bracketed exposures during the processing
phase.
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