Why is there a *lower* limit to ISO values?

Question

Well, it's easy to see why high iso values degrade image quality, but why is there a lower limit of 100 or 200 in most DSLRs?

Could the exact same result of, say, 10 ISO not be achieved by taking a photo at the sensor's ideal sensitivity (e.g. 200 for the sake of this argument), then dividing all measured values by 20 in the memory while the sensor takes another measurement without any delay in between, dividing its measured values by 20 again and adding them to the values of the already taken photo in memory.
Repeat this another 18 times and the sum of the values should be those of a normal exposure that took 20 times as long as one at 200 ISO, i.e. an exposure at effectively 10 ISO.


This way, one wouldn't need expensive ND filters, and due to the much higher number of photons that hit the sensor the measurements would likely be even more accurate than those of a photo taken with a ND filter with much fewer photons per unit of time.

So i'm wondering...why isn't it done that way? If there seems to be a simple solution to a problem that appears to elude professional engineers working in the field, then either all those engineers are idiots or one underestimates the complexity of the problem. The latter is much more likely, of course - but i'd like to understand the actual difficulties.
Could it be that the sensor is not instantly ready to take another measurement as the data of the previous has been read? Then again, recording video is possible, so achieving a frame rate of 25 or even 60 per second apparently is possible. So this shouldn't be a problem for a several second or even minutes exposure.

Answer 1

I can only imagine it's something to do with the physics of the electricity. Maybe a threshold type of thing, but I'm just pulling things out of my bum right now.



I know, it's strange to get an old camera and see the ASA reminder that goes from 10-100. And 200 was considered high speed.

Answer 2

Sensors are designed with a native sensitivity and if you go too many stops over or under, you sacrifice dynamic range.



If the native ISO is 200, as it is on current Nikons, raising the ISO to 3200 (4 stops) results in a perceptible loss of image quality. Lowering the ISO from 200 to 10 is also 4 stops, and you can expect a similar loss of image quality. Better to keep the ISO at 200 and slap on an ND filter.



Here it's explained fairly simply but with an inkling of maths: http://www.picturecorrect.com/tips/iso-setting-in-digital-cameras/

If you Google for "below native iso" or so I'm sure you can find a more in-depth explanation.

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added:



What the native ISO means, is that the sensor only has one way of recording. One intensity. You can change the shutter speed and aperture to determine how much light hits the sensor, but raising or lowering the ISO is done by boosting or reducing the signal in software.



When you boost the ISO to 1600, the in-camera software is actually pulling details from extremely underexposed images. Conversely, if you lowered the ISO to 10 the camera would have to salvage details from extremely overexposed images. And dSLRs don't handle overexposed details gracefully - not like print film does. They tend to clip overexposed details and give you 100% white pixels. No matter how good your software is, you can't extract information if the sensor only transmits (certain details as) pure white. So upon further thought, I expect that lowering the ISO to 10 would produce horrible results.

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also added:



If you look at the DXO website (I think they list this stuff) you'll see that image quality takes a hit well before you reach Nikon's extended ISO range. They further you stray form the native ISO, the worse things get. It's the same with all brands. As far as I can tell, and I'm quite serious, Nikon uses the High/Low nomenclature just to throw unexpected information on the LCD for the casual user: better to confuse & thus dissuade new users from experimenting with these settings than to trigger angry emails and word of mouth about 'faulty' equipment.



I wish you the best of engineering & marketing luck with your endeavor. Personally, I'd gladly forgo +$70 ND filters.

Answer 3

As to anything existent in this universe, there is an upper limit and a lower limit. ISO measures the light sensitivity range of the digital sensor.



It's not that simple. The digital sensor has physical and electronic limitations. There are ISO 64 MF dSLRs but that would explain the very high cost of those cameras (or camera parts). Below ISO 100 is not just a replacement for ND filters. The colors are richer, more vibrant and more saturated. It's almost like having your eyes treated with laser while at the same time the atmosphere is cleared of all pollutants suspended in it. It's better than HD! If you haven't shot with ASA 64 (or lower) slide film, you wouldn't know what it's like.