Photo Skills

High-ISO exposure test

If you’re shooting in a particularly dark area using a small aperture and a low ISO, the exposure may last for multiple seconds, or even minutes. Doing test shots to work out the optimum exposure to use can be a frustrating experience if you have to wait such a long time to check each shot, but there is a solution: set a higher ISO for the test shots so that you can use shorter exposure times. You can then work backwards to calculate the length of exposure for your optimum ISO setting.

Each time the ISO is halved 
(one stop less light), 
the shutter speed can be doubled) 
one stop more light) 
to maintain the same overall exposure.

To use a simple example, if a shutter speed of 30 seconds at ISO 400 gives the correct exposure, you know you can achieve the same result using 60 seconds at ISO 200. This would reduce the noise but would result in a longer expose and possible camera shake or movement in the image (but that could be what you want)

Stops making sense

In this example, using ISO 6,400 gives an exposure time of four seconds. So if you wanted to use ISO 100 to take the final shot, you’d need to double the length of exposure six times.

ISO 6,400 4 sec
ISO 3,200 8 sec
ISO 1,600 16 sec
ISO 800 32 sec
ISO 400 1 min 4 sec
ISO 200 2 min 8 sec
ISO 100 4 min 16 sec

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