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Assuming that the camera does not shake during long exposure photography, How does the sharpness of the image get affected? Would it be more sharp, less sharp, or remain the same?

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5 Answers 5

up vote 8 down vote accepted

In a perfect camera with a perfect sensor, sharpness would remain the same. No such thing exists so you have to consider two possibilities with whichever camera you have. The long exposure causes the sensor to heat and increases image noise:

  1. Your camera does not like that and therefore applies long shutter noise-reduction to clean things up and you get a less sharp image.

  2. Your camera is oblivious to the increase in noise and leaves it there. Speckles caused by noise destroy some fine details but make the image appear sharper sometimes.

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Though to note: the noise reduction applied would be different depending on shooting in raw/JPEG and the camera's capabilities. –  Nick Bedford Jan 27 '12 at 2:50
    
Indeed. On some cameras you can even turn NR off. Yet on other cameras, it still applies NR when NR is off! –  Itai Jan 28 '12 at 3:58

A long exposure allows light to accumulate over each imaging sensor for a longer period. This can create a saturated (bright) image which doesn't have anything to do with image sharpness in the technical sense. However, the perceptual effect is that of a less sharp image.

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I think this is a good answer because of the perceptual bit, however I think that could be expanded a bit to provide some examples. I think it might also help to expound upon how a very slight loss in sharpness may actually benefit some images. –  jrista Jan 27 '12 at 2:46
    
Saturated doesn't mean bright. By bright do you mean over exposed? Do you have a citation for the claim that saturated or 'bright' images appear less sharp? Not saying you're wrong; just that I've never heard this claim. –  Poldie Jan 27 '12 at 12:21
    
@Poldie: By saturation, I believe he is referring to "pixel saturation", or how much light each pixel in the sensor has collected relative to its maximum capacity. In technical circles, its not uncommon to hear the word saturation used that way. –  jrista Jan 28 '12 at 17:15

If the distance to the subject is long (10s of meters) you might experience atmospheric distortion which can reduce the sharpness over time. It's not a big problem unless the sun is shining or you have other heat sources between you and the subject.

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I guess it depends on what you consider a long exposure. I know on photographer who considers 1/20th a long exposure. Me, I consider 4 seconds, or 15 seconds a long exposure.

I saw a big increase in sharpness when I upgraded my tripod. The old one just wasn't up to the task.

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Digital image sensors can also exhibit a blooming effect after long exposure times. This is particularly acute in astrophotography, where exposure times of many minutes are normal. The bloom usually radiates out as vertical and horizontal lines. This is more of a problem in a scene with sharp contrasts (e.g. stars against black sky) than it would be in say a terrestrial night scene. Not all sensors are designed with this in mind, since not all manufactirers think of astronomy as a target demographic for their products. EOS Rebel was one that did. Older cameras are worse than newer ones too.

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