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01-28-2012, 12:23 AM   #1
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Reciprocity failure in dSLR's?

I have a K-5 and I love it's ability to take low-light shots at fairly high ISO speeds. It is a very good camera for low-light handheld work. But I started wondering, do modern dSLR's like the K-5 suffer from reciprocity failure at long shutter speeds? As a long-time film photographer, we always had to compensate for reciprocity failure when shooting film with long shutter speeds, and I wonder if this still affects dSLR sensors.

For those that do not know what reciprocity failure is, it's when you reach a certain slow shutter speed, such as 10 sec, in order to add one more stop of exposure, you must increase it more than a stop. For example, starting from my 10 second example, 20 sec would be one stop more exposure but when shooting film, you would have to increase the shutter speed to 30 sec in order to achieve one stop more on film. The film loses sensitivity as longer shutter speeds are used.

I haven't yet done many long-exposure shots with my K-5, so I figured it would be easier to just ask!

01-28-2012, 12:32 AM   #2
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From what i understand, light hits a photodiode and creates a signal. As long as light keeps hitting the photodiode, you will get signal. This acts in a linear fashion, so if you double the exposure, then you double the signal.

As for film, rather than photodiodes, we have grains. As light hits the grain, a chemical reaction occurs and the grain changes. Because it is a chemical reaction, its efficiency does not increase in a linear manner due to annoying things that effect the reaction over time, like temperature.

i could be wrong though :P
01-28-2012, 12:33 AM   #3
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Digital sensors do not have reciprocity failure. For most exposures they have a linear response. It's only as you get to near saturation that they tend to go non-linear. Also, areas of amp glow appear to be non-linear. Lastly, thermal conditions are a much bigger factor than with film. The warmer the conditions, the worse the noise.
01-28-2012, 06:44 AM   #4
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smigol has essentailly said it all.

I would only add that in fact, you can actually fairly easily saturate the sensor elements completely and thus blow highlights in low light with long exposure times (for example in astrophotograhy).


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