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Author Topic: Reciprocity Failure  (Read 1856 times)
Mike Guilbault
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« on: July 19, 2013, 09:29:21 PM »
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Is it a consideration when shooting digital? 
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Schewe
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« Reply #1 on: July 19, 2013, 10:51:04 PM »
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Not really, but there are other problems with long exposure times for digital...notably noise caused by heat and having the sensor charged for capture. Some cameras/sensor are better/worse than others.
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Eric Myrvaagnes
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« Reply #2 on: July 19, 2013, 10:55:27 PM »
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I think not.
Most present-day photographers probably don't have the foggiest idea what reciprocity failure is anyway.

Jeff beat me to it.
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-Eric Myrvaagnes

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BartvanderWolf
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« Reply #3 on: July 20, 2013, 02:32:23 AM »
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Is it a consideration when shooting digital? 

Hi Mike,

No, reciprocity failure as we (or some don't) know it from film is not an issue with digital capture that you would need to correct for. When you e.g. require a 100x longer exposure, you just expose 100x as long. Also the colors won't shift as a result of that longer exposure time.

As mentioned, you may instead run into noise issues with exposures longer than 1 second though. Most cameras offer a DarkFrame subtraction (or long exposure) mode for such exposures, which helps, but adds a long waiting period (equal to the original exposure time), usually before you can take the next shot. It is possible to avoid such long waits by shooting separate DarkFrames, and use a Rawconverter like RawTherapee to apply the separate darkFrames to the lequally long actual exposures.

Cheers,
Bart
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Mike Guilbault
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« Reply #4 on: July 20, 2013, 04:44:39 AM »
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I thought as much but wanted to verify.  I have LE nr turned on with my D800e - but yah... that wait between shots for the DarkFrame is annoying!  From what I understand though (and I could be wrong), shooting separate darkframes yourself to apply later works only if you are using the same exposure and settings for all the shots. 
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BartvanderWolf
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« Reply #5 on: July 20, 2013, 07:24:15 AM »
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I thought as much but wanted to verify.  I have LE nr turned on with my D800e - but yah... that wait between shots for the DarkFrame is annoying!  From what I understand though (and I could be wrong), shooting separate darkframes yourself to apply later works only if you are using the same exposure and settings for all the shots.  

Hi Mike,

DarkFrame subtraction works best if you use the exact same exposure time as for the actual images, because with the same duration, the 'same' amount and locations of pixel-response-non-uniformity (PRNU) can build up as time progresses. Clever software can cheat a bit by correcting for different exposure times, but it's a compromise. Even better software than that can use an average of multiple DarkFrames, to reduce random noise in the DarkFrame and only use pattern noise to remove from the actual images.

I'm not absolutely sure if RawTherapee does both of those clever optimizations (although it wouldn't surprise me), but it does allow to use a single DarkFrame for multiple images, which is more than most other RawConverters.

A separate DarkFrame is typically shot with lenscap or bodycap on, and the viewfinder covered, and the exposure time obviously set to manual.

Since heat buildup in the camera during long exposures can play a role, it also helps to use similar outdoor temperatures for the DarkFrames and try if shooting a DarkFrame mid-session works better than at the end of a shooting sequence. If the session is only a few images, it is also possible to shoot the DarkFrames at a different time.

Cheers,
Bart
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Mike Guilbault
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« Reply #6 on: July 20, 2013, 04:16:55 PM »
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My images from last night with LEnr worked beautifully. I'm going to just leave it on unless there's some compelling reason to turn it off.  Thanks for the inside info
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NikoJorj
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« Reply #7 on: July 22, 2013, 05:05:27 AM »
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It is possible to avoid such long waits by shooting separate DarkFrames, and use a Rawconverter like RawTherapee to apply the separate darkFrames to the equally long actual exposures.
A LR-addict question : do you know of any software that does the dark frame substraction at the raw level and then saves a DNG or other raw format?
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Nicolas from Grenoble
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BartvanderWolf
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« Reply #8 on: July 22, 2013, 05:23:14 AM »
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A LR-addict question : do you know of any software that does the dark frame substraction at the raw level and then saves a DNG or other raw format?

Hi Nicolas,

Once it is corrected, it is no longer Raw. But RawTherapee is one of the few applications available for multiple OS platforms that does this at the Raw conversion stage (i.e. before demosaicing, in Linear gamma space). Converters for specific MF camera backs (Sinar, Phase One) can do it when the camera automatically records BlackFrames (often without the user knowing).

That will produce (much) better quality 16-bit/channel images for further processing, but it won't allow any other Raw conversion stage tricks that converters may have up their sleeves. RawTherapee can e.g. also attempt CA correction before demosaicing, which obviously help color accuracy and luminance resolution.  

As far as LR is concerned, there is an experimental plug-in from Adobe for LCC (Lens Cast Calibration), but I'm pretty sure that DarkFrame subtraction functionality was not included. Who knows, it might be in the future.

Cheers,
Bart
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Hans van Driest
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« Reply #9 on: July 22, 2013, 07:12:27 AM »
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dark frame subtraction adds some noise. With my Sony a99 I never use it up to 30s when sensitivity is in the iso 50-100 range. when it is hot you sometimes see one or two specs in the final image (depending on how much shadow lifting is needed), easily removed in post. It might be a good idea to just test it with your own camera to see when it is really needed. and only use it when needed. saves a lot of time as well.
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BartvanderWolf
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« Reply #10 on: July 22, 2013, 08:48:36 AM »
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dark frame subtraction adds some noise.

Hi Hans,

That's correct for subtraction of a single DarkFrame , and that is why one would preferably use the average of several DarkFrames. It also depends on camera model, because some behave better than others, and may need less correction.

However, even the subtraction of a single DarkFrame will remove pattern noise and hot pixels. The noise that remains (part of which was added by the subtraction of two random noise sources) is much more random and well-behaved, and should be relatively easy to remove with a noise reduction plug-in.

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It might be a good idea to just test it with your own camera to see when it is really needed. and only use it when needed. saves a lot of time as well.

I agree. It's always useful to do some testing on the actual equipment at hand.

Cheers,
Bart
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