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The photo above is taken in the deepest, darkest Swedish winter night, with only the moonlight helping me. To get it so bright and sharp it’s necessary that the shutter remains open for a long period of time, to gather enough light. In this case, the exposure time was roughly ten minutes. To do this you put the camera on as stable a tripod as you can, and you use the mode called “Bulb Mode”, where you manually decide how long the shutter stays open – the camera will keep the shutter open for as long as the shutter button is depressed. It helps a lot to have a remote control in this scenario.
When I posted the photo to a Facebook group I was asked the question how I knew that I needed to expose for ten minutes and no more or less. When in Bulb Mode with long exposures, you can’t use the camera’s light meter to help you, but there’s a simple trick that can be used. If you put the camera back in manual mode (with the same aperture setting, of course) you can raise the ISO number and measure the light, as well as to a test exposure to see that it looks good. Then you can calculate the exposure time at ISO 100 by counting backwards, so to speak.
Let’s say I raised the ISO to 1600 and got a correct exposure in 13 seconds. Since ISO 1600 means an amplification of the light by 16 times (or four stops), this means that the same exposure at ISO 100 would be gotten in 13 seconds * 16 = 208 seconds, or roughly three and a half minutes.
In this specific case I used ISO 6400 and got a correct exposure in 9 seconds. So the correct exposure at ISO 100 is therefore 9 * 64 = 576 seconds or just over nine and a half minutes. And here’s another trick if you don’t have a calculator with you (or, let’s be honest, can’t be bothered to get your cell phone out to make the calculation). Since 64 seconds is roughly a minute, you can say that every second of exposure at ISO 6400 equals roughly a minute of exposure at ISO 100!