46

This is a featured called Long Exposure Noise Reduction. To cancel out noise the camera will close the shutter and take an equally long exposure again, this time capturing a black image with only the electrical noise on it. This information is then used to reduce the noise on the original exposure. In the camera settings you can disable the Noise Reduction ...


20

To reduce the processing time for long exposures, you want to turn off Long Exposure Noise Reduction. However, you may not want to give up the benefit of LENR. Long Exposure Noise Reduction (LENR) is Canon's nomenclature for in-camera dark frame subtraction. When you take a photo the camera will expose the image normally and then use the same settings to ...


19

Look up Long Exposure Noise Reduction in your camera's manual. Depending on the model, you may or may not be able to continue taking photos. Some models do the processing immediately after the exposure. Some allow you to continue to take exposures, and then do the processing. This feature uses dark frame subtraction. The idea being that if you expose a ...


11

The long exposure Noise Reduction takes a "dark frame" after the real shot, and then uses the noise pattern from that image to reduce the noise generated by the sensor. That's why the exposure takes twice as long as this NR were disabled. Long time exposure can increase the sensor temperature, and increased temperature also means more noise. So you should ...


10

For exposures longer than 1 second, you can enable Long Exposure Noise Reduction (LENR). This is Canon's nomenclature for in-camera dark frame subtraction. When you take a photo the camera will expose the image normally and then use the same settings to create a dark frame with the shutter left closed. The readings for each pixel in the dark frame will be ...


9

You can't really prevent hot pixels on long exposures, you can only deal with them. For a single four minute exposure the easiest way is to use what is known as Dark Frame Subtraction. Different manufacturers have different names for in camera versions of it. Canon, the brand I shoot, calls it Long Exposure Noise Reduction. After an image is taken the ...


8

I believe it may be a flaw in your testing. Raising ISO should not be getting you a higher signal to noise ratio (or usable dynamic range). You are raising the noise floor, without any corresponding increase in dynamic range at the bright end. Thus you should be left with overall lower dynamic range. It would not be possible to go the other way. Is it ...


7

It's not actually processing for most of that extra time. It is taking a second exposure with the shutter closed, for dark frame subtraction. This removes sensor-based pattern noise. Of course, there is a bit of processing involved in the subtraction itself, but most of the time is in taking a second exposure with the same shutter time as your actual one, to ...


5

Long Exposure Noise Reduction takes a dark frame of the same duration to subtract the noise from the image. That's why there is a long delay; a 30-second exposure plus a 30-second dark frame plus processing time plus the time to write the image to the memory card. There will be obvious gaps in the star trails. If the gaps are minor, they can be corrected in ...


5

On the quality front, in general, it will be better to do it in camera when the sensor is particularly subject to hot pixels (those that only appear stuck when heated) or when there is other heat related noise introduced in the image. At that point, the current conditions in the camera are more accurately represented by a dark frame taken at the same time. ...


5

The best way to deal with noise in the situation you describe is to use a form of dark frame subtraction. If your camera doesn't offer such a built in feature, take a few frames during your session with the lens cap on (using the same ISO, and shutter time). Then use an application in post processing to subtract the dark frame from each segment from the ...


5

I do not know that specific camera, but generally sensors will have a pattern of thermal noise which is not uniform. The "Long Exposure Noise Reduction" feature, if your camera has it, is intended to offset this somewhat. Enabled, it does much as you did, taking one image with the shutter open, and one with the shutter (or in your case the lens cap) shut. ...


5

Ideally, dark frame subtraction should be done with raw images before demosaicing. Then the resulting black spot is 1 pixel, and after demosaicing it will typically be invisible in the result due to the interpolation during processing. You seem to have used converted (jpeg?) files, in which the stuck (hot) pixels have already been smeared over the ...


5

Regarding Bulb Mode If you use a wired remote there is generally not a time limit regarding the length of an exposure using bulb mode with most current DSLRs. Pressing the button, halfway or fully, on a wired remote is pretty much identical to pressing the camera's shutter button (except you don't physically touch the camera). There have been some DSLRs (e....


3

Shutter time is only one of many variables that affect when dark frame subtraction would be beneficial. There is no single answer to your question. Since what we call 'noise' is present in all digital images, it doesn't just begin to appear at some point in time. We tend to notice it when the signal (that is, the light entering the camera) is sufficiently ...


3

The SLT A58 has the so-called "long exposure noise reduction" function which performs the dark frame subtraction mentioned in Micheal's Clark's answer. You should choose the shutter speed a lot less to prevent start trails (A few seconds at most, depending on the focal length) and crank up the ISO instead. That will dramatically increase the noise, but you ...


3

I'll explain the image stacking method here. Image stacking can yield better results, you then take multiple images at lower ISO and/or expose for a shorter time. A practical way to go about this is to just take the first picture at a high ISO and expose for long enough until you see the details you want to see, but possibly with a lot of noise and a lot of ...


3

Long exposure noise reduction is actually dark-frame subtraction done automatically in-camera. If you are taking many shots and would like to avoid the time delay, you can take just one dark frame, with the lens cap on in the same conditions and exposure time, at the end of the shooting and then use dark-frame subtraction in Photoshop, Lightroom, GIMP and ...


2

Since there are many variables that affect the sensor read noise, taking the dark frame at the same time increases the chances that it is also taken under the same conditions as the exposed frame, particularly with regard to the temperatures of various areas of the sensor.


2

In order to match the type and character of noise accurately, the dark frame subtraction should be done at the same time as the exposure. So doing it in camera should yield better results.


2

The sitting around for another 30s is called dark-frame subtraction. You can turn it off in the menu under Long-Exposure or Slow-Shutter Noise-Reduction , I do not remember which it is called on the D5100. Essentially, the camera takes another shot with the shutter closed to characterize noise and then it substracts it to give you a theoretically cleaner ...


2

The extra time that the camera takes to store long exposure shots is caused by the extra processing that the camera has to do to reduce noise. It may help to turn off long exposure noise reduction if you are using that.


2

Noise reduction is concerned with maximizing the Signal-to-Noise Ratio (SNR) by differentiating between actual signal and signal created by noise. With a lens cap on the camera there is no signal in terms of electrons excited by photons belonging to light entering the camera that are striking the imaging sensor. It is all noise. How the camera acts under ...


2

To address the "for beginners" aspect of the question, most recent-generation Olympus Micro 4/3 cameras (which are mirrorless, not technically DSLRs) have a feature called "Live Bulb" which allows you to actually see the progress of the exposure on the LCD screen at preselected intervals. This can be a very user-friendly way of approaching long exposures, ...


1

I am confident that the exact method used by each camera manufacturer is different and company-confidential. All manufacturers have a large vested interest in getting the maximum image quality using methods that are undetectable and therefore not reproducible by their competition. In the case of Nikon, several sources describe a simple subtraction; for ...


1

Some very good answers about long exposure times, but for astrophotography I thought it was worth adding another consideration - the rotation of the earth. If you're looking for star trails (https://en.wikipedia.org/wiki/Star_trail), the rotation of the earth becomes a feature, but if you want stars to appear as sharp points on longer exposures, you're also ...


1

There are many sources of noise in the CMOS imaging process. Fixed-pattern noise (FPN) refers to image noise caused by nonuniformity in the sensor. For example, differences in manufacturing might cause some pixels to be slightly more sensitive to light. Or interference from other circuitry might cause pixels in some areas to report a different value ...


1

First, let's distinguish two kinds of noise. One is temporal random noise, such as thermal noise, photon shot noise, or electron shot noise in the analog electronics. These tend to follow Poisson distributions, and vary from shot to shot. Since Poisson -based signal-noise ratio goes as the square root of total signal, the noise tends to be observable only ...


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