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[Jamie Roberts]

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Jamie,

 

There are several DSLRs that can shoot at 10 frames per second (fps). There are some cameras that can shoot much faster such as the Casio EXILIM EX-FH20 ultra zoom at 40 fps. Therefore, all I am suggesting is that they COULD perform dark frame subtraction as a noise reduction technique, not that they do, since I don't know what algorithm they use. My point being that the reading of one dark frame doesn't have to take 1/3 or 1/4 second but could happen in less than 1/10 th second with many still cameras as long as the readout logic is fast enough.

 

John, I'm aware of the cameras that shoot 10fps or more

 

To the best of my knowledge, none of them use dark frame subtraction because it's not an efficient way of doing noise reduction except for extremely noisy exposures (meaning, usually = very long exposures which means no more 10fps ) ).

 

So you may be right, but I don't think it's a very efficient approach.

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[barjohn]

As I stated, I don't know if this is a useful technique or not. What might be done or possible (again I don't know) is to take multiple dark frames while the camera is idol, look for pixels that are consistently generating a signal above the black level (not the random ones) and subtract those averaged pixel levels out of the scene that is shot to account for noise due to thermal characteristics that is consistent and not random.

[t024484]

Jamie,

 

There are several DSLRs that can shoot at 10 frames per second (fps). There are some cameras that can shoot much faster such as the Casio EXILIM EX-FH20 ultra zoom at 40 fps. Therefore, all I am suggesting is that they COULD perform dark frame subtraction as a noise reduction technique, not that they do, since I don't know what algorithm they use. My point being that the reading of one dark frame doesn't have to take 1/3 or 1/4 second but could happen in less than 1/10 th second with many still cameras as long as the readout logic is fast enough.

 

Reading of a frame goes as follows

A frame is mostly divided in 2 or more parts. In case of an M8 it is 2, in case of a Canon 1D III and 1Ds III it is 8 and the S2 will probably has 4 outputs.

Each division is read out serially.

 

For a 4000*2500 Sensor with two divisions, 2000 pixels per line are being shifted out with a very high frequency, something like 24 Mhz for the M8 and the S2, and at least twice this speed for both Canons.

This means that a 14 bit A/D conversion has to be made every 20 to 45 nsec, depending on the Senor.

This is extremely fast, but Analog devices supplies A/D converers even working at 80 Mhz, so the Sensor is still the limiting factor.

For each line an integration period has to be included, to calculate the dark current, which is substracted from the value of each pixel in that line.

Integration time is in the same ballpark of he time to read a line.

 

So in our example sensor, ((2000 * 45nsec) + Integr Time) * 2500 = 0.45 sec (for an integration time of 0.225 seconds).

Ad to this 0.05 second for the shutter, and you have roughly the time that an M8 needs per frame, thus 2 FPS.

 

Since the 1D III has eight outputs, and a shifting frequency of probably something like 50 Mhz, the same calculaton gives ((2000 * 20) + Integr Time) * 625 = 0.05 Seconds (for an integration time of 0.025 seconds). Add to this a shutter time of 0.05 sec.

That is why this camera performs at 10 FPS.

 

The 1Ds III having the same construction, but twice as many Pixels, needs twice the time, hence 5 Fps.

 

Now the S2. If it really has 4 outputs, the time to read a frame will be something like ((4000 * 42 nsec) + integr Time) * 2600 = 0.87 seconds if again the integration time is equal the the read out time of a line. Add to this a shutter time of 0.05 sec to get 0.92 sec.

With these assumptions the S2 will be able to shoot with 1.1 FPS. If integration time can be made shorter, it could go as high as 1.5 FPS, that that will probably be the limit.

 

Hans