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Long exposures and sensor noise with Canon 5D2

Author/Copyright:

John Jovic

Shooting long exposures with digital cameras can result in images with unusually high levels of background noise. This noise is a side effect of the sensor itself heating up. Some cameras are better suited to long exposures than others. This is a brief comparison of images shot with a Canon 5D2 to see what happens during very long exposures, between 5 and 60 minutes. These tests also look at what happens when Live View is used and if this affects sensor noise.

The test images where all shot with the cameras body cap fitted, ie no lens, and the files are shown below for comparison, both as full frame and 100% crops of each image. All tests where performed at room temperature. The images where shot in RAW and where converted with Phase One Capture One using the default noise setting in Capture One with no exposure, contrast or any other adjustments at all. The images where simply resized for web use.

The following images where shot with a cold camera, ie one which hadn't been used at all for several hours before the exposures where made.

 

 
This 5 minute exposure on a Cold camera looks identical to the reference image.   The 100% crop shows no appreciable noise.

What happens when the sensor gets hot? The following images where shot once the camera had been used for over 1/2 hour so was effectively a normal temperature for the camera/sensor. Although a 60 minute exposure does generate considerable background noise it appears that 5 minute exposures are quite comfortable for the 5D2, even after consistent or constant use, such as immediately after the 60 minute exposure.

 

Not so good, but it could be much worse.    
 
This exposure was made immediately after the 60 minute exposure was made, ie with no delay, so the camera was hot due to exposure, not Live View.

 

The resulting 5 minute exposure shows very little if any noise and is almost the same as the 5 minute exposure with a Cold camera. It appears that the 5D2 does not generate a great deal of sensor heat during the exposure itself.

What happens when you use Live View? The camera certainly draws quite a lot of current as the batteries typically drain quite quickly, and the camera certainly heats up. These tests where performed using Live View Stills only, Silent shoot, Mode 2. The camera was left in Live View mode for a full 25 minutes before each of these test images was shot. The camera warms considerably in Live View, more so than during the 60 minute exposure but this might simply be due to the LCD screen itself rather than due to the sensor being active.

 

This exposure is essentially identical to the one below except that the camera was cooler when this test was performed.   The 100% crop shows significant noise.
 
This exposure was performed when the camera was already fairly warm/hot so using Live View generated even more heat. This 5 minute exposure looks similar to the 60 minute exposure!   The 100% crop shows significant noise and is surprisingly similar to the 60 minute exposure.

It appears from the above tests that Live View contributes to long exposure sensor noise quite significantly so it may pay to minimise using it if noise performance needs to be optimised.

The Table below sets out the same images show above (100% crops) but in a way that allows them to be easily compared.
 
 

This 5 minute exposure on a Cold camera looks identical to the reference image. The 100% crop shows no appreciable noise.

  This exposure was made immediately after the 60 minute exposure was made, ie with no delay, so the camera was hot due to exposure, not Live View. The resulting 5 minute exposure shows very little if any noise and is almost the same as the 5 minute exposure with a Cold camera. It appears that the 5D2 does not generate a great deal of sensor heat during the exposure itself.
  This exposure is essentially identical to the one below except that the camera was cooler when this test was performed.
  This exposure was performed when the camera was already fairly warm/hot so using Live View generated even more heat. This 5 minute exposure looks similar to the 60 minute exposure with significant noise.

 

     
 
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