The wider apertures such as f/1.4 allow for lower ISO settings and less noise, but there is a trade-off. The wider the aperture, you’ll see increased optical aberrations of the stars that make the stars look like they have wings and tails. In addition, there is a general softness that becomes more apparent. Both of these issues are especially noticed at the edges of the frame. This is less of an issue with professional lines of lenses. On these lenses, photographing using f/2.0 will help significantly.
WHAT ISO SETTING IS BEST?
It is important to set the ISO high enough to get a good exposure. You don’t want to have to lighten the images in post-processing. This will dramatically increase the noise when you are using too low of an ISO. Conversely, darkening the image in post-processing will result in lower image quality due to higher ISO used. Use Manual shooting mode and set the exposure so that it is on the plus side by one or two stops over the standard exposure reading of the sky. For a very dark night without the moon, settings of f/1.4 at ISO 1600 to 3200 or f/2.8 at ISO 6400 will get you in the ballpark for a correct exposure.
REVIEW THE HISTOGRAM
On a trip to the Bristlecone Pine Forest, the morning after shooting the Milky Way, I reviewed my images. I discovered that the images were underexposed. When I reviewed the images at night while I was still shooting, they looked so bright on the LCD screen because my eyes had adjusted for the dark night making the image appear to be a proper exposure. That night was the only evening with a clear sky so I did not get any usable night images on that trip. This is why histograms are so beneficial!
The histogram shows the values of digitized light from black to gray to white in these images, with black represented on the far-left side and white on the far-right side. These are examples of an ideal histogram on a dark night. Notice where the information comes in from the left to the second bar on the Canon model and about 1/3 of the way into the frame for Nikon or 1 1/2 bars into the frame. This provides a good exposure of the sky. You can have spikes on the left or right as well as reaching up and past the top of the histogram. That is fine for night sky photography.
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