General > General Technical Chat
Digital camera sensors and the "film analogy"
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paulca:
I keep finding myself making faces when photographers talk about things like ISO and shutter speed on digital sensors like they mean the same thing as they do in film.  I know that camera makers have been desperately trying to make that true also.  But it's just not. 

ISO is just analogue gain on the sensor.  The noise resulting from it, is not "digital noise", there is no such thing, it's the same analogue noise that was present on your sensor, just amplified.  More expensive sensors are less noisy so higher ISOs work better.

Shutter, is irrelevant.  Sensors have no accumulative effect like film when exposed.  They would only last one photo is they did.  People talk about them like the pixels are buckets that catch photons.  They don't.  They sample an instant.... or on a CMOS sensor a skewed slice across an instant in time.  To create a long exposure the DSP (Digital signal processor) in the camera, rapidly samples each instance in time, I assume as fast as it can or as much buffer/processing memory it has and ADDs them together like photo stacking in astrology.

I have seen advice to turn off "Long exposure NR" in my camera by several YouTube photographers.  However I feel it is likely the ONLY time NR can be done on long exposures is in the camera while those individual samples exist or are at least being stream processed.  Otherwise you will be adding together all the noise from all the samples and trying to remove all of it in post editing.  It is better to remove the noise while compositing the exposure rather than add it all together for the end surely?

In fairness the primary reason they give that advice is, after a long exposure your buffer/processing memory will be 100% full of samples and it will take, however long it takes to clear that buffer before it will shoot again.  On the A6100 this can be 10 seconds for example.

The other aspect would be the sensor sample rate.  On long exposures it can't just take an infinity number of samples per second and have the processing power to immediately composite them.  So there has to be a compromise and the longer the exposure the more it will be impacted by memory and processing limitations. 

It is however very likely HIGHLY optimised only adding differences and using ASIC electronics and "microcode" to process samples at incredible rates.

If I'm correct, sensors should have certain properties such as "Frequency" and "Response time" and other digitiser-IC-style parameters.  For example, a pixel stands zero chance of sampling an LED blinking at 10KHz if the sensor frequency is less than 20kHz.  You would effectively get a pseudo-random result long exposure shooting it.  I'm sure people like Nikon/Cannon/Sony et. al. have billions of IP invested into the real processing DSPs and ASICs to do this.  However, digital cameras with variable "shutter speed" have been around for decades in some forms, so the DSP required to do it must also be fairly old.

If I'm wrong, I probably need to look further into how exactly CMOS/CCD/* sensors manage to produce a cumulative electrical effect in the sensor over time and then reset it.  Optical-accumulative-electro-static?
TimFox:
Technically, the ISO (or ASA) rating on a photographic film refers to the "toe", where the emulsion starts to expose.  Many transparency films, and proper users of the Zone System, refer instead to "EI", which is in the same units but measures the mid-point of the S curve from blank to exposed, but this depends on the development.  ISO is supposed to be a parameter of only the film.  Note that the photographic literature rarely discusses this.
Now, for a digital sensor, there is an inherent maximum level, where the charge on each pixel saturates as the diode forward-biases.  Typically, a camera sensor saturates at an exposure level that is typical of approximately ISO 100 to 200 film.  The very first line-sensor device from Fairchild that I encountered in the early 1970s was specified as equivalent (in that sense) to ISO 100.
The ISO setting in a digital camera is, in fact, the gain from the sensor to the ADC input, as you stated.  The quantization noise of the ADC remains constant with respect to the digital word put out, but as you crank up the gain the noise in the dark current of the sensor becomes larger with respect to the full-scale of the ADC and the image appears noisier.
Modern cameras may have built-in tricks to average multiple exposures by computation to improve the SNR, but this requires a constant object and a good tripod.
Exposure is the product of light input rate (determined by the object brightness and the f-number of the lens) and the exposure time (which in a SLR may be determined by a physical shutter).
"f-number" is merely the effective aperture diameter of the lens divided by its focal length, which is why it is written "f/16", where f is the focal length on the aperture scale.  Traditionally, a lens has click stops at "1/2 stop", where a "full stop" is a factor of 1.4:1, or an area factor of 2:1.  A traditional shutter has click stops at full stops (2:1 in time), and ISO settings on the light meter at "1/3 stops", or 1 dB (in terms of light energy).  Again, this last statement confuses some, since twice the light energy develops twice the charge in a digital camera, which is 6 dB at the ADC voltage input.
Another important difference is "reciprocity", as in "reciprocity failure".  With film at very long exposures, some of the charge on the photographic grains while the chemical state of a grain is still metastable, and not fully exposed, will drain off, so it takes more total light-induced charge on the grains to achieve the stable "latent image" chemical state.  I have not seen any good descriptions on any analogous conditions with photosensors at very long exposure times, but I'm sure the astronomers have carefully looked into this with their cooled sensors.
Once, when using my 8x10 inch view camera downtown, shooting a cityscape, a tourist asked me how many megapixels I had.  I did a mental calculation and replied "about 500".  Repeating that calculation at home, I found that was approximately correct, although the sampling is random with finite resolution, as opposed to periodic in a digital sensor.
Sal Ammoniac:
Digital sensors don't have any reciprocity failure that I've been able to measure. I use a monochrome CMOS camera (with filters) for astrophotography and have never seen any effect that I could attribute to reciprocity failure like film has. My camera has an in-built Peltier cooler. Back in the bad old days of film, reciprocity failure was a big problem and people mitigated it to some extent in two ways: cooling the film (typically with dry ice) and by soaking the film in hydrogen gas before exposure.

Amateur astrophotography was horribly primitive back in the film days compared to today with digital cameras, stacking software like PixInsight, autoguiding, plate solving, etc. Nowadays I can tell my telescope what I want to image, push a button, go to bed, and come back in the morning to close everything up.
eti:
Destin has all you need to know, in his usual, incredibly detailed, well-explained style:

TimFox:
Reciprocity failure in film is physically interesting:  it indicates that more than one photon is required to "flip" a single grain from its initial state, through one or more metastable states, to the final stable state.  Clever photographers can use it to reduce the contrast between highlights and shadows (tricky).  Astronomers were the first to investigate it seriously with photographic emulsions, but I'm sure they are glad to avoid it now.  There should be some resistive leakage of charge during a long exposure, as well as biased-diode dark current (adding exposure to shadow regions), but the cooling reduces the dark current (and its noise) greatly.
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