Reciprocity failure in Digital Cameras

Dear Members,

 

can you please confirm that,is there any any reciprocity failure in digital

cameras?. I have tried in all combinations in my D-200 and I found that there

is no limitaion up to which the sensors can obsorb the light . What ever the

exposure time given , the sensors are continue to repond to the light until it

becomes whitei.e complete washout. . Is it right?

Reciprocity failure is a characteristic of the chemical make-up of film emulsion so it is not relevant in digital photography.

There are issues of dark current and noise that could be considered similar, and you'll be quite limited in how long you can expose your sensor before noise becomes unbearable, whereas hours-long film exposures can be made with ease.

Noise tends to build in digital images, which varies with the camera. My Nikon D1x would develop "snowflakes" after about 15 seconds. The D2x is good for at least 30 minutes without noticeable dark current noise. Nikon D2 cameras have a noise reduction scheme for long exposures which is very effective.

 

Digital sensors tend to be very linear in their response to light, in both time and intensity. Film reaction to light is a partially reversible chemical reaction. As light gets less intense, the silver halide molecules tendency to "heal" after exposure becomes relatively more significant.

 

Hardly anyone in professional astronomy uses film because CCD sensors are more efficient, faster and suffer no reciprocity failure. They are often cooled with liquid nitrogen to minimize thermal noise.

The best illustration of film exposure reciprocity is this: imagine you have an empty bucket and you want to fill it up with water using a tap.

<p>

<li>Normal situation (with no reciprocity): you turn your tap to a dripping pace and the bucket gets filled up over a period of time, t1.

<li>Imagine you now go to a room where the humidity is extremely low. So, the root issue here is that while your tap is dripping, you're also losing water thru evaporation due to the low humidity. The bucket will eventually fill up but it will take a period of time, t2. And, t2 is certainly larger than t1.

<p><p>

Same thing with film reciprocity, when the exposure time goes beyond a certain limit (specified by the manufacturer), the increase in exposure time is not linear anymore. It takes longer to "fill up the bucket", so to speak.

<p><p>

Now, ccd/cmos sensors, although called digital, is analog when you consider the reception of light into the light-gatherng wells. I suspect there's some reciprocity but it is not as pronounced as film and the processor chip in the camera is adjusting all this for you automatically based on its knowledge of the sensor's reciprocity.

No, there is no reciprocity failure effect in a digital sensor. Reciprocity failure in film refers to the effect where the response of the film is no longer linear at long exposure times. In contrast, the response of a digital sensor remains consistently linear across its entire usable range (up to the maximum charge handling capacity of the pixel).

 

Dark current and read noise have no bearing on reciprocity failure. Dark current is linear with time provided the sensor temperature does not change, so it can be subtracted out with a blank frame of the same exposure duration as the actual shot. Read noise does not change with the integration time.

For more information about sensor characteristics than you probably care to know, I'd suggest <a href="http://www.clarkvision.com/imagedetail/index.html#sensor_analysis">Roger N. Clark's site</a>.<br>

<br>

Cheers,<br>

<br>

Geoff S.