Astronomical Observing and Photography


ISO bracketing in Astrophotography
by Joe Cali

The photograph below was taken using ISO bracketting.
Taken with a Vixen VC200L 200mm f6.4. Pentax K1.

ISO6400 4 x 55s
ISO3200 4 x 55s
ISO1600 4 x 55s
ISO800 4 x 55s
ISO400 4 x 55s
ISO200 4 x 55s

Adobe Lightroom HDR Merge low ISO images
Adobe Photoshop , Statistics Stacking [median]

The Orion nebula has an enormous brightness range.  The usual approach of astrophotographers is to expose at many different exposure times. The Adobe HDR merge can equally be used with different exposure times, apertures or ISO.  It will work with any images with exposure. So that consideration didn't come into it. 

I used the more traditional "different exposure times" method and HDR via stacking and alpha masking in this photo. 

Dynamic Range varies with ISO.
My camera has a photographic dynamic range(PDR) of about 6eV at ISO6400 and 10eV at ISO200, 12eV at ISO100 as can be seen in the graph below taken from from the "Photons to Photos" website.

Using ISO bracketing, I was able to take advantage of the higher dynamic range at the core of the nebula near the trapezium where the light levels vary most rapidly.  I had to burn in the trapezium core and dodge the outer regions  so when I repeat this, I would probably do a series of  2 minute exposures at ISO6400 or 1 minute at ISO12800 to get more outer detail and a series of ISO100 at 1 min and a series of 30s exposures at ISO100 to get into the core. 


The other obvious advantage is that at lower ISO's the read noise is lower, at ISO100 on the k1 it is very small.  My next run at this technique will involve more subs at higher exposure values for the outer faint reaches of the nebula and less subs when capturing the core with the low ISO low noise characteristics of the sensor.


So my next exposure sequence might look something like this:-

Vixen VC200L 200mm f6.4. Pentax K1
ISO6400 12 x 120s
ISO6400 12 x 55s
ISO3200 8 x 55s
ISO1600 8 x 55s
ISO800   4 x 55s
ISO400   4 x 55s
ISO200   4 x 55s
ISO100   2 x 55s
ISO100   2 x 30s
ISO100   2 x 15s