Common mistakes with camera settings: 5. Long exposure noise reduction
In-camera long exposure noise reduction works by taking a second ‘dark’ exposure immediately after the image has been recorded.
This dark image is taken with exactly the same exposure time as the first, but the shutter doesn’t actually open so no light reaches the sensor.
The idea is to record the non-random noise which is caused by variations in pixel sensitivity and hot pixels, and that becomes visible with longer exposures.
Because in-camera long exposure noise reduction effectively doubles the exposure time for each shot many photographers are tempted to turn it off – especially with exposures that are several minutes long.
However, the results of using it are well worth the wait.
It is possible to perform your own dark frame extraction using image editing software, but it is still advisable to shoot several dark frames throughout a shoot as the level of noise tends to increase as the sensor warms up during its heavy use.
The most foolproof approach is to use the in-camera system.
Common mistakes with camera settings: 6. Shutter speed
Many novice photographers overestimate their ability to keep a camera still and consequently shoot handheld with relatively long exposures.
A general rule for getting sharp images from a hand-held full-frame camera is to use a shutter speed that is at least one second divided by the focal length of the lens.
This means that if you are shooting with a 100mm lens, the shutter speed needs to be at least 1/100sec.
This rule can be adapted to work for sub-full-frame sensors by taking into account their focal length magnification factor.
For example, a 100mm lens on a Canon APS-C format SLR, like the EOS 700D, has a 1.6x focal length magnification factor, so the shutter speed would need to be at least 1/160sec.
Many lenses and some cameras now have image stabilisation systems built-in and this allows slower than normal shutter speeds to be used when the camera is handheld.
Some lenses now claim a 4EV compensation which enables the shutter speed to be reduced by up to 16x. That’s the difference between 1/125sec and 1/8sec.
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