Why no camera supports very low ISO?

Klorenzo

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Hi, I was just wondering: why no camera (that I know) has 50 ISO, 25 ISO, 12 ISO, etc.? Ok, maybe I've seen an ISO 50 somewhere, but not much less for sure.

I think this should be quite easy to do (?) and would be an alternative to ND filters. Why nobody does it?
 

agentlossing

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I think the use of slower zooms to sell kits started a trend towards higher base ISO as people traded smooth, high detail for convenience and versatility. We are seeing a return to an image-quality weighted public consciousness, so we might start to see a reversal of that.
 

alex66

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I think the old Kodak DSLR's went down to 25 or less, my Ricoh GX100 is 64 native, I have thought of this as either Panasonic or Olympus could make a high resolution sensor i.e. 40mp but with a lower iso range of say 25-200 as a trade off. I frequently come in to the problem of there not being a low enough iso for a series I am doing of clouds, shooting when the sun is behind a cloud and have no issue with using a tripod at other times. Oh and I think some of the older prosumers like the Minolta 7i A1 started at 50.
 

Petrochemist

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I wonder how hard it would be to offer a high and low iso version of some cameras.
The only issue in a pseudo low ISO would be sensor heat. The camera can easily read a line of pixels, then reset that line & start collecting photons again, while reading the next line etc. When it gets to the end, read them all again & average results for each pixel. Interlacing the pixel read patten would probably give smoother results.but doesn't add any complexity. I'm sure much of this technology is used in video output.

As it stands a burst of images could be taken and averaged, in post but this could give a staggered appearance to the final image, rather than a smooth blur.
 

OzRay

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From my readings, all sensors have a 'native' base ISO at which the sensor performs at it's optimum, it's something that can't be significantly altered. Lower and higher ISOs are created artificially and so every step requires compromise. It's how much compromise people are willing to accept.
 

GFFPhoto

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From my readings, all sensors have a 'native' base ISO at which the sensor performs at it's optimum, it's something that can't be significantly altered. Lower and higher ISOs are created artificially and so every step requires compromise. It's how much compromise people are willing to accept.
Interesting. I had assumed it was like film where a lower ISO was cleaner and richer, but if this is accurate, the only benefit of an artificially low ISO is that you can shoot in bright sun without having to go into diffraction territory. If thats the case, then give me a ND filter and engineer for high ISO capability.
 

alex66

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Interesting. I had assumed it was like film where a lower ISO was cleaner and richer, but if this is accurate, the only benefit of an artificially low ISO is that you can shoot in bright sun without having to go into diffraction territory. If thats the case, then give me a ND filter and engineer for high ISO capability.
When I pulled some films (mono) in the same dev some would behave differently to expected, though this could be solved by using a different dev, but Ray is right the sensors tend to work optimally at their native iso and pulling can increase noise and lower the dynamic range. Some would like a sensor that has a lower native iso but as high iso capability has become a boasting point like mega pixels its unlikely to happen.
 

David A

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From my readings, all sensors have a 'native' base ISO at which the sensor performs at it's optimum, it's something that can't be significantly altered. Lower and higher ISOs are created artificially and so every step requires compromise. It's how much compromise people are willing to accept.
That's easily demonstrated with the new "Low ISO" setting on the E-M5 after the last firmware update. The speed rating is equivalent to ISO100 but the menu calls it "Low ISO" because it isn't a real setting. Effectively it gives you a one stop overexposure but the cost of using it is slightly less dynamic range than you get at ISO200 which is the "native ISO". Higher than ISO200 settings also reduce dynamic range progressively as you increase the setting. You can't get better results than you get at ISO200 by going to the new Low ISO setting. There's really no advantage to using the E-M5's Low ISO setting over simply using exposure compensation to give a bit more exposure at ISO200, "exposing to the right", provided you can do so without clipping highlights you want to save.
 

bassman

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Ray is spot on. AFAICT, a sensor has one ISO. This is the sensitivity of the sensor to light, expressed in an international standard measurement. All higher ISOs set in the camera are just boosting the signal coming off of the sensor, which also boosts the noise coming off of the sensor. Modern sensors are wonderful in allowing for more of a boost before noise becomes apparent at usual magnifications.

Try this experiment: on a tripod and under unchanging light, shoot a scene in raw at (your camera's base ISO) 200, 400, 800, etc. increasing ISO as far as you like. Load them into your favorite raw processor. Make copies of the ISO 200 and increase the exposure on the copies by 1, 2, ... stops. See how the pairs compare: shot at 400 vs +1 stop, shot at 800 vs. +2 stops, ... . Report back to us about your camera and results. I think we would all learn something about what ISO mean (myself included, for sure).
 

Promit

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Ray is spot on. AFAICT, a sensor has one ISO. This is the sensitivity of the sensor to light, expressed in an international standard measurement. All higher ISOs set in the camera are just boosting the signal coming off of the sensor, which also boosts the noise coming off of the sensor. Modern sensors are wonderful in allowing for more of a boost before noise becomes apparent at usual magnifications.
Yes and no. Different sensors work differently. Some sensors are single ISO and some are not. A sensor can be designed up front for a low base ISO, but you lose the high ISO in the process. I don't know that the trade-off is balanced either; imagine that you gain ISO 25 but lose usable 1600+. That's a very harsh trade off.

My Sony A77 goes down to ISO 50 and detail is definitely very impressive at that setting.
 

T N Args

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I'm curious about this, too. My iPhone 5 has a low iso setting of 50 and I was surprised the native iso for my GX7 was 200.
But note that the GX7's ISO125 setting is, unlike some cameras, a 'real' setting. 200 is simply the 'default' setting.
 

bassman

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Yes and no. Different sensors work differently. Some sensors are single ISO and some are not. A sensor can be designed up front for a low base ISO, but you lose the high ISO in the process. I don't know that the trade-off is balanced either; imagine that you gain ISO 25 but lose usable 1600+. That's a very harsh trade off.

My Sony A77 goes down to ISO 50 and detail is definitely very impressive at that setting.
Ok, so I just did what I should have done in the first place - do the research. So I went to my second favorite information source (after mu-43 :wink: ), Wikipedia. Here's the article: http://en.wikipedia.org/wiki/ISO_5800#ISO_12232

They define ISO in the context of a digital camera as the interaction between the physical sensor and it's physical sensitivity, and the analog amplification of that signal on it's way to being transformed into a digital representation of that data collection, and then often in reference to the an sRGB jpeg created from that digital image. That resulting jpeg should wind up at 18 percent grey (across the entire image). So the effective ISO reported for a camera (not the sensor) is affected by the light sensitivity of the sensor, the amplification applied, and the processing done by the ASIC which constructs the jpeg. Obviously, the raw image file is constructed as well, presumably along the way. The manufacturer has choices to make in how much amplification they apply, which may vary at different ISO settings on the camera; but amplification jacks both the signal and the noise. The ASIC can then apply a variety of algorithms - which may also vary depending on the selected ISO - to manage the noise; we often read reviews or commentary which says things like "even the raw image has had the noise reduced, and the image looks smeared". The highest ISO which a manufacturer will make available is based on their assessment of an "acceptable" image quality, given that the noise apparent in the image increases along with the ISO. That's also why some cameras have settings like "HI-1" or "HI-2" - they create output images which are more sensitive to light, but the manufacturer doesn't feel passes the ISO test for acceptable quality.

And since the output file has a fixed maximum brightness it can represent (blown whites), it seems to me that amplifying the signal reduces the total dynamic range available to the image - the lowest brightness reported goes up, but the max stays the same.

Therefore I stand corrected: the camera or at least the imaging system in the camera may have multiple ISOs available to the user. However, I still stand by the statement that the underlying sensor has a fixed light sensitivity which is just part of it's design. And, by the standard, the sensor itself has no ISO; ISO can only be applied to the output of the imaging system, and is often measured by reference to the jpeg.

The standard gives a manufacturer a number of ways to do the calculation aside from comparing the sRGB jpeg against 18 percent grey, but I don't think that changes the underlying conversation. If anything, some of the choices (one choice measures the point at which the sensor becomes saturated, i.e., blown whites) seem even more physically driven to me.

So to refer back to my experiment from earlier, that would highlight the difference between the analog signal amplification and ASIC processing within the camera, and the digital amplification that your raw processing uses. They may or may not yield significantly different results at different levels of amplification, for different cameras, and for different camera ISO settings.
 

Promit

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Therefore I stand corrected: the camera or at least the imaging system in the camera may have multiple ISOs available to the user. However, I still stand by the statement that the underlying sensor has a fixed light sensitivity which is just part of it's design. And, by the standard, the sensor itself has no ISO; ISO can only be applied to the output of the imaging system, and is often measured by reference to the jpeg.
Well, ultimately you have photosites with photon wells, and X photons falling on that point will generate some voltage. This is a physical property and cannot be changed except at the design stage. You're right that ISO is a sum total of a lot of things; it has no meaning at this level. But what cameras do in going from that voltage to the raw image data varies wildly. Some convert to digital on the spot, while others apply analog processing. The ones that convert to digital right up front are called "ISO-less" from what I've seen, and there are fascinating technical discussions on the DPReview forums about them. Most sensors now seem to have analog processing that helps them get up to ISO 400 or 800 cleanly, and the rest is done in digital. Any time you see a linear drop off in DR with ISO (see DXO graphs), you're probably looking at the digital processing part of the camera. Those ISOs can always be recreated in RAW software from pushing a lower ISO image.

Low ISO is going to be all about reducing the voltages on the sensor as much as possible, or increasing the voltage headroom, to avoid saturating. That gets into the physical design of the sensor itself; don't know what the issues are there.
 
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