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Long exposures bring a way to capture time in a photograph. As time goes by the camera keeps recording and the changes in the scene are reflected in the final photo as a single static capture of what happened while the shutter was open. There are some common myths about long exposures in photography so let’s review a few of them.
1) Long exposures can damage your camera or sensor
This is heard around beginners or people that just start using a DSLR camera. The fact is that hundreds of photographers do long or very long exposures with their cameras every day and astrophotographers for example only do long and even very long exposures. The lifetime of cameras that often do long exposures is the same or even longer than normal cameras. This is because the very short exposures and bursts are the ones that create some degree of mechanical stress in the camera shutter and moving parts, so shoot longer and prosper!
2) It has to be dark to take a long exposure
Nothing further from the truth as you can use ND filters to shoot long and even very long exposures during the daytime, even at noon with the sun high in the sky. For long exposures during the daytime you will need a very dark filter like a ND400 or ND1000 filter. The common filters like the ND4 or ND8 just won’t do it, they will increase your exposure time but not enough to create a distinctive effect.
3) Long exposures decrease noise
In a long exposure the sensor temperature increases and if you leave the shutter open long enough thermal noise will be a factor. In general terms exposures under 1 minute are normally free of thermal noise but from 1 minute and up and depending on the ambient temperature things can get ugly. A five minute exposure with a DSLR in a Summer night will really look very bad due to noise.
This is why for star-trails you should better take several short exposures and then stack them instead of a very long, say 20 minutes exposure.
4) Long exposures increase noise
Wait… what is going on here? Are both wrong? Yes indeed because a long exposure will in fact decrease noise as long as thermal noise is not a factor. So while keeping your exposures under 1 minute you should always try to expose as much as possible to get a better effect and also maximize the signal to noise ration. You should think this “The longer the exposure the less the noise until thermal noise kicks in” that’s the rule.
5) You should leave the Camera Long Exposure Noise Reduction Off
Yes and No. Modern cameras usually have two types of noise reduction functions. One is called “long exposure noise reduction” and you should leave that “On” or “Auto” if you are taking single shots without thinking about stacking. The camera will take a second exposure with the same exposure time as the photo but with the shutter closed, then the camera just subtracts what it finds in this “dark” exposure from your photo decreasing the noise.
This is difficult or impossible to do with noise reduction software so let the camera do its job.
The second type of noise reduction is called “high ISO noise reduction”. This only applies to JPG files and is similar to what noise reduction programs do so you can leave this type of NR off and do your own processing if needed.
6) The longer the exposure the better the result
This is a very wrong assumption. In many examples the exposure time depends on what you are shooting, distance, the wind and several other factors. For moving water a short exposure usually brings more detail in the flow of the water while creating a long exposure effect at the same time. I’ve found that exposure times ranging from 1/8 to 2 seconds produce nice results with the ocean and water streams of waterfalls. Longer exposures usually just flatten everything creating a silky effect that can also turn out well but it’s always a good idea to experiment different times.
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