I know in the film days there was ASA, which evolved into ISO in digital cameras, and that there is a actual ISO that dictates the standards. But how exactly does Canon (or Nikon for that matter) determine that 51,200 is one stop less than 102,400 and that 204,800 is one stop more? I guess I'm looking for a lay person explanation if it's possible.
F stops represent a 100% change in light. So 1 stop is either 1/2 the light when closing or 100% when opening. Same shutter speed, one stop closed equals 1/2 the light hitting the sensor. One stop more open doubles the light hitting the sensor. So when working the algerbra to ISO, when you want to move the Fstop one down (closed) then you have to double your ISO to get the same exposure. ISO 200 would then have to go to 400, then 800, 1600, etc.
Last edited by Busted Knuckles; 02-16-2012 at 12:36 AM.
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That's basically it. ISO 100 is an approximation to ISO 100 film in sensitivity (the same aperture and shutter speed should give the same exposure, in theory). Of course it's slightly more complicated than that due to sensor noise, marketing "statistics", and the like, but some sites like DPReview will even go out of their way to test the accuracy of the ISO ratings of each camera. I suppose the idea is that if ISO 51,200 film existed, it would have the same sensitivity to light as a digital sensor set to ISO 51,200.
Thanks BK and Positron, okay I understand that. But how does Canon determine that for example the 1D X's sensor sensitivity to light is actually ISO 102,400 and not 204,800, if we were to set it to those settings? I'd say, well because one has more noise in the image. But is that how do Canon measures the ISO differences and determines uniformity between models, etc.
@Bigblue That's a function of what's called the quantum efficiency of the detector. Basically, what's the probability of creating an electron when a photon strikes the detector. Then there's the amount of charge that each pixel can hold before it's saturated (the value won't go up anymore as more photons hit). I'm guessing that the 50% mark is hit at similar exposure conditions that would give 50% gray on equal ISO film. There may be some other fraction of the total amount that's agreed upon by the digital camera manufacturers but the idea is the same. If you could magically create an array that could store 100x more photons then you could declare it to be ISO 1 instead.
Good question - at what point does noise overtake signal to some agreed upon level. WIth Magic Lantern you can push the sensor to stupid effective ISO that creates about 50% noise and 50% signal.
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As sensor technology as well as algorithms for interpolating data (in firmware) improve, the ability to squeeze more usable pixels out of the camera at high ISO improves. If Canon wanted, they could release a camera today with ISO ten billion just by taking the image at base sensitivity (assume ISO 100) and ratcheting the exposure up +27 EV (give or take). It would look similar to what would happen if you tried the same thing in Photoshop -- not pretty. They always play a balancing game between pushing the highest numbers possible for marketing purposes and trying to only give the user the option of what's vaguely usable so customers don't get disgruntled when they buy a camera that can go up to ISO 13,421,772,800 and the images look terrible... it's bad for business.
@bigblue1ca. Well I asked the same question some years back of Chuck Westfall (Canon) and he supplied me with the Standard Specification "Sensitivity of Digital Cameras CIPA DC-004 (2004)". I would be happy to send it to you (400K PDF) as a PM, but I don't think its possible, any ideas?
Last edited by Raid; 02-16-2012 at 02:59 AM. Reason: syntax
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Interesting all, thanks. Raid, sure that would be great, you could just add @gmail.com to my username and email to me if that works for you.
Or, you could click HERE for the pdf.Originally Posted by bigblue1ca
As for, 'how does Canon know that ISO 102400 is one stop more than ISO 51200', put in lay terms, it's simple math. The sensor detects photons of light, each pixel 'counts' those photons for the selected exposure time, storing them as electrical charge. If the ISO is set to 100 (let's assume that's the 'base' - any sensor has only one base ISO), that amount of charge is converted to a digital signal of a certain numerical value, which is the value assigned to that pixel for brightness (the position under the RGB mask determines the color that's assigned). If the ISO is higher than 100, up to the top 'native' ISO (i.e. non the expanded 'H' or 'L' settings), the sensor applies an analog gain to the signal prior to converting it to digital. Analog gain is like 'turning up the volume' on the signal electrically, and the amplifiers are precise, unlike turning the know on your stereo, it can apply a 2-fold amplification for ISO 200, a 16-fold for ISO 800, etc. If you've selected an expanded ISO like ISO 102400 on the 1D X, that is a digital gain applied after the analog signal has been converted to digital. So for the ISO 102400 case, the camera is applying the maximum analog gain (to get to ISO 51200), then it just takes the numerical value assigned and doubles it to reach ISO 102400.
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