High or low ISO for night photography?

Simple question and I often get sceptical looks and disagreement on why in the world I would use the lowest native ISO of my camera for night photography when there is no or little light, when, in fact, using a higher ISO, but shorter exposure time, would give me the same noise. Not so for two reasons!

1) ISO / Exposure Time Variation with Steady f-stop

Image 1 shows the scene I shot for demonstration, a view out of my bedroom window, which I converted from raw format and exported to JPG without alteration using Lightroom 4.1, and then cropped the dark shaded portion out of it for comparison between exposures.

Below is a series of these photos, starting with a 1 second exposure at ISO 6400 (I didn't consider higher ISO, which will become quickly unusable for fine art enlargements, even with the latest cameras). I then decreased the ISO by one stop (i.e. half) and increased the exposure time by one stop (i.e. double) until I ended up at ISO 100, my camera's lowest setting, and a 10' 40" exposure, all producing roughly the same "exposing to the right" image. A histogram is shown for illustration why I chose those settings.

Even a cursory look at these photos reveals a gradually decreasing noise going from ISO 6400 to ISO 100, with dramatic differences between the extremes, even though the exposure time increases from 10 to 640 seconds, making a very strong point for using lower ISO and longer exposure time. The slightly higher (apparent) sharpness of the high ISO pictures is an artifact of both the noise itself as well as of less movement of the branches during the shorter exposures.

2) Constant ISO, Varying Exposure Time and f-stop.

Another effect contributing to longer exposures being less noisy can be demonstrated as follows. I took a two pictures at the same high ISO 6400 setting, but varied the exposure time from 15 to 30 seconds (1 stop, compensating for the increased exposure by stopping down the aperture accordingly, again, resulting in approximately the same exposure.

Whereas one would expect significantly higher noise with the longer exposure, the images above are not all that different with respect to noise, even though the latter exposure is twice as long. This is due to the fact that long exposures naturally help reduce the noise: noise is a random effect of digital photo sensors. Essentially, the individual photo sites randomly either produce a higher or lower signal than the amount of light hitting it should produce. Each sensor site can do either, they are not necessarily (but may be) consistently offset in one direction. This happens randomly and exposing for longer times will naturally cancel at least some of the random occurences out, resulting in a less noisy image. This effect is less pronounced compared to changing the ISO (and it is part of the above comparison), but still illustrates the point.

Bottom Line

Low ISO setting and longer exposure time (at comparable exposure) yields less noise, more detail and results in less post-processing and photos that can be enlarged bigger for fine art display.

This was shot with a Nikon D700 full-frame DSLR, a camera design from 2008. Newer cameras may have less noise overall at comparable ISO settings (however, consider pixel size, the D700 will still be better than some newer consumer DSLRs), but the relative comparison will hold true, that higher ISO setting will always result in more image noise.

All of this, of course, should be well known and very apparent to whoever is using the standard 6-stop high-ISO preview to gauge the approximate exposure time needed at ISO 100, common teachings of night photography schools such as those of Lance Keimig...

High-ISO Noise Reduction

High-ISO noise reduction (HINR), which is turned on by default in most cameras only affects JPG images, not RAW images. You should still tunr it off for long exposures, because the image review on your camera screen actually uses the JPG and ont the RAW file, hence, it previews an image that may not be as noisy as the RAW file is, if you use HINR. Shoot RAW file format!

Long Exposure Noise Reduction

Long exposure NR (LENR) helps to keep the already less noisy images shot at low ISO even less noisy on long exposures (say, more than 30 s). However, aside from shooting star trails, where LENR needs to be turned off to avoid delays and gaps in the trails, many photographers use LENR otherwise. Depending on the camera and NR generation, this may help, but I found that NR of any kind typically reduces detail, just like any software NR applied. In addition, noise created in camera can always be eliminated through modern software, such as Photoshop, Lightroom, NIK modules or others, and with much finer control over it, resulting in overall better or more pleasing images. I usually shoot without LENR at all, too much hassle having to switch it on and off betwen star trail and other photos and I, for one, am way too impatient to wait any additional time after an already long exposure. The night has only so many hours... :-) In addition, shooting at low ISO and averaging random noise over time keeps night photos clean. However, if you shoot longer than, say, 4 minutes, your camera may produce "ugly" noise, i.e. fine horizontal and vertrical lines or square artifacts, which can only be removed with LENR (or excessive cloning in post processing). In these cases, and also if you believe you might have a winning image, by all means: wait the extra time and use LENR.

For More Information

If you are interested in more information about high ISO or ISO comparisons, here are some links:
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