30 sec max. exposure for Q-S1

Started 1 week ago | Discussions thread
OP manual_focus Senior Member • Posts: 1,233
Re: 30 sec max. exposure for Q-S1

masklayer wrote:

My guess is it might be the size of the memory that it buffers image data into, possibly?

I believe Pentax engineers decided to err on the side of image quality conservatism and in essense disabled the Bulb mode by not allowing exposure time to go beyond 30 secs. Since the limitation is one of firmware, it would be nice if we had the option to increase the Bulb exposure time (as long as there was no risk of actual physical damage to the sensor/camera) to something of one's own needs/desires.

Here is what I've found so far and it is quite interesting.

There are two different levels of concern dealing with sensors under long exposures.

1) Damaging the sensor from too much heat.

While this can happen it takes a very long exposure >10 minutes and if it's hot outside.

2) Too much Dark Noise which degrates the image as it adds to the overall noise thereby decreasing the S/N ratio. This appears to be the primary concern.

Dark noise it equals the square root of the Dark Current times the Exposure Time.

"Dark current is a current that flows even when no photons are incident on the camera. It is a thermal phenomenon resulting from electrons spontaneously generated within the silicon chip (valence electrons are thermally excited into the conduction band). The variation in the amount of dark electrons collected during the exposure is the dark shot noise. It is independent of the signal level but is dependent on the temperature of the sensor as shown in Table 1."

See: https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=10773

Now this is interesting.

Apparently small pixels heat up faster than a larger pixels thereby generating more Dark Noise. (From a discussion on maximum exposure times).

"First of all, yes, eventually the sensor would overheat and fail, in theory. But that would presumably take a much longer exposure than anyone is going to be making with a smartphone.

The more immediate problem is thermal noise. Every sensor, particularly those with smaller sensels/pixels such as a smartphone's, is limited by signal-to-noise ratio which is basically what percent of the data it's recording is actually the light its detecting as opposed to background noise from its own electronics. As a sensor heats up, thermal noise arises and shifts that ratio in the wrong directly, often rapidly (again, particularly for smaller sensels).

Typically, the maximum exposure time your camera/smartphone's software allows will be the point at which thermal noise will push the SNR to unacceptable levels, rather than the point at which your sensor will overheat and damage itself. Nonetheless, I would make absolutely sure those apps didn't void my smartphone's warranty before I used one of them."


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