If you do take a high ISO shot to test exposure and composition, don’t throw it away. For those who don’t know, usually when freezing outside in the middle of the night attempting to do some night photography, you will set the aperture (usually to its widest point) then choose a low ISO to keep the image clean and use the shutter speed to dictate the length of time that the sensor needs to be exposed to light. First, a high ISO test shot will often be taken, then assuming the composition is correct and the level of light is appropriate, the ISO would be lowered and the shutter speed increased at the same rate of changes in exposure stops e.g. f4 ISO 1600 at 1 second will result in the same exposure as f4 ISO 400 at 4 seconds. When halving the ISO, double the shutter speed (assuming no change in aperture) to keep the same exposure.
The thought process for this image and blog post was that since raising the exposure introduces artefacts, what would happen if I reduced the exposure in post. The result, a pleasing image of the night sky.
Here is a sample shot of the night sky shot at a max ISO 25600 (for the Canon 5D MkII), this was taken just to confirm where the tree was in the frame.
At this ISO, the image is unusable, however, before deleting this image, I decided to reduce the exposure and tweak the shadows to show more of the Milky Way which (without pixel peeping) appeared to clean up the image.
The last decision I took was to shift the white balance towards blue which although subjective, provided a more pleasing tone.
And here is the final image
So before deleting test shots, give them a tweak to see if they can be used. Ironically, when looking for other images taken at this vantage point at a different ISO for comparison, I couldn’t find any, so luckily I kept this one as it turned out to be the best one of the night. There may be part two of this blog post when I try again – aiming to compare two images, one shot at a relatively low ISO and another at high ISO for the same scene.