08-23-2015 02:39 PM
Bob from Boston,
This is the way I understand it;
"The only one that affects a RAW file is Long Exposure NR, it takes a dark frame after the exposure and subtracts that from the RAW file before it is written to the CF card. Not reversable!"
Virtually any of these alterations can be applied with great success or the lack thereof in PS or LR. But prefferably in ACR 9.
I know you are not as big on post as I am so you will have to determine if the 'fix' is better or worse.
08-23-2015 07:46 PM - edited 08-23-2015 08:02 PM
The only one that affects a RAW file is Long Exposure NR, it takes a dark frame after the exposure and subtracts that from the RAW file before it is written to the CF card. Not reversable!
This is not to say that Long Exposure NR is a bad thing to do even when shooting RAW.
'So, if you make a 30 second exposure, Long Exposure Noise Reduction will take another 30 seconds of processing time, during which time you will be unable to take another photo with your camera.
What's happening is this: your camera makes its exposure for the image. Long Exposure Noise Reduction then kicks in, and charges the camera's imaging sensor for the same amount of time, making a "dark frame." The camera then compares the noise in the dark frame with the noise generated in the image, and removes any noise that is the same. This method is known as dark frame subtraction.'
The interesting thing about this process is that it should make no difference when or where you create the dark frame. All you're doing is establishing a point on a graph of your sensor's noisiness as a function of exposure time. (To the actual mathematicians reading this: Yes, it's a many-valued function, so the result is an ordered set, not a point. But that's just an implementation detail.) So in principle you could create the dark frame any time after the exposure; all you need is the exposure time that was used for the original. Then you'd apply it in post-processing, with the option of ignoring it if it didn't give the desired effect (i.e., reduce the apparent noise). It might even be more accurate than the way it's done now, which practically guarantees that the sensor will be hotter for the dark exposure than it was for the original, thus exaggerating the amount of noise to be removed.
Or am I, in my profound ignorance of a feature I've never used, missing something significant?
Yes, you are missing something significant.
The temperature of the sensor impacts the amount of noise it produces, by taking the dark frame right after the long exposure it ensures the sensor produces the same amount of noise. For example if Long Exposure noise reduction is set to Auto and you are taking a 30 second photo in the winter, the camera may not even do a dark frame, because it is not needed. However, a 1 second exposure on a hot summers day the camera may do a dark frame because it is needed.
This is the example of one factor that I know of that impacts the sensor's noise that varies from the time the shot is taken.
There may be other factors* that vary the noise, but, the sensor temperature is one that I am sure of.
* humidity, electronic interference, ???
As far as 'exagerating' the amount of noise removed being good, even dark frame subtraction can reduce detail. So as always the least amount of noise is best.
08-23-2015 07:55 PM - edited 08-23-2015 07:59 PM
I found this note about using the 'always on setting' interesting:
Off: No noise reduction is applied. This setting is the default.
Auto: Noise reduction is applied when you use a shutter speed of 1 second or longer, but only if the camera detects the type of noise that's caused by long exposures.
On: Noise reduction is always applied at exposures of 1 second or longer. (Note: Canon suggests that this setting may result in more noise than either Off or Auto when the ISO setting is 1600 or higher.)
(from How to Use Long Exposure Noise Reduction on a Canon EOS 70D)
08-23-2015 08:17 PM - last edited on 08-25-2015 11:25 AM by Stephen
One of the true experts on dark frame subtraction is Roger Clark, PhD NASA. Here's his take on using in camera long exposure noise reduction compared to tradition post processing dark frame subtraction (for astrophotography):
'Conclusions: The use of modern digital cameras with on sensor dark current suppression (that means on sensor dark frame subtraction during the exposure!!), combined with modern raw converters that use lens profiles (that means flat field corrections on the linear raw data) and reading the hot/dead/stuck pixel list from the raw file to correct bad pixels means astrophoto image processing is simpler, producing a better result than traditional methods. The example shown here used a top quality lens. If the lens had more aberrations, like chromatic aberrations, modern raw converters will correct that and can also improve other aberrations, making the difference between traditional and modern processing even greater.'
from Astrophotography Image Processing Using Modern Raw Converters [Mod note - removed link per FORUM GUIDELINES - send a PM if you have questions]
08-25-2015 10:51 AM
When it's enabled, LENR will only operate on shots of 1 second or longer.
You want the LENR "dark frame" to be made immediately after the primary exposure and under the same conditions, so that the noise generated in the dark frame is as close as possible to the same as what was generated in the original exposure. There are various things that might effect image noise, such as the ambient temperature. So it's best done right after the original exposure.
One important thing to remember with LENR is that if you cancel the "dark frame" image for some reason, both it and the original image it was going to be applied to will be thrown away by the camera. So if you forget you have LENR engaged and think the camera has "locked up" during the dark frame exposure, so turn it off or pull the battery... you'll lose both that image and your primary or 1-second + shot.