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FORUMS Photography Talk by Genre General Photography Talk 
Thread started 23 Dec 2011 (Friday) 12:57
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Why is ISO on Digital Cameras?

 
Miki ­ G
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Dec 23, 2011 17:06 |  #16

I'm guessing that altering the ISO value will change the voltage / current & resistance values in the sensor & will effectively effect the signal to noise ratio.




  
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Old ­ Baldy
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Dec 23, 2011 17:20 |  #17

Miki G wrote in post #13591482 (external link)
I'm guessing that altering the ISO value will change the voltage / current & resistance values in the sensor & will effectively effect the signal to noise ratio.

My understanding is that the actual capture of photons and thus photo-sensitivity of the sensor is NOT changed at all. It is only the calculated multiplication of the counts by the analog amplifier, before the capture is written to RAW that is changed. Thus, the photo-sensitivity APPEARS to be increased by increasing the ISO, but in reality, the photon capturing mechanism in the sensor is fixed, and the end result is multiplied by the ISO factor before the RAW is written. The end result appears to be an increase in the sensitivity of the sensor to light, but it's really a straight calculation/multiplica​tion of a fixed sensitivity to cause an increase in output brightness.


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Miki ­ G
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Dec 23, 2011 17:42 |  #18

I'd agree that the actual capture of photons and the photo-sensitivity of the sensor would not change, but rather that, because of the changes brought on in the electrical circuit within the camera itself, by changing the ISO value, these values (voltage/resistance/cu​rrent) would also have to be altered and balanced. Just like turning up the volume on your stereo, the values change & the S/N ratio is effected. This may explain why some cameras (with the same sensor) perform better than others when dealing with noise.




  
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bcd01
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Dec 23, 2011 17:42 |  #19

Ralph III wrote in post #13590577 (external link)
Hello All,

I understand the relationship of ISO, Aperture and Shutter Speed in getting correct exposure. But what is the purpose of ISO on modern digital camera's?

...

As with any technical device, there are compromises to be made. When you chose a film for a camera, you select the ISO for the level of grain versus the type of shooting you will be undergoing. Of course the higher the ISO the more grain for film. Fortunately, with a DSLR it is dial-able and you don't have to wait until you finish the roll of film to change it. But the electronic image suffers greater noise (the somewhat equivalent to grain) with the higher ISO numbers. The nice thing is you get to select the level of compromises in image quality. Better yet there are software products that help reduce the noise on your image.


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Ralph ­ III
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Dec 24, 2011 00:26 |  #20

I cannot answer everyone but there have been some excellent replies since my last post. Thanks to Curtis N. and Old Baldy (and others) for their technical explanations, as I was looking for!


Just FYI, I shoot a lot of sports (mostly Full Auto) so I'm thoroughly familiar with how ISO, Aperture and Shutter Speed work in conjunction to control exposure. That wasn't the question though....

My question was basically two part:
1) EXACTLY HOW IS ISO CONTROLLED ON A DIGITAL CAMERA BECAUSE THERE IS NO APPARENT PHYSICAL CHANGE A SENSOR CAN UNDERGO; WHERAS FILM UNDERWENT A PROCESSUAL DIFFERENCE. THEREFORE YOU COULD PURCHASE FILM WITH DIFFERENT ISO RATINGS.

2) WITH THAT IN MIND IS "ISO" EVEN NECESSARY ON A DIGITAL CAMERA, GIVEN WE ALREADY HAVE APERTURE AND SHUTTER SPEED, AND THE CAMERA IS CAPABLE OF ALGORITHMS TO ACCOMPLISH WHAT WE DESIRE.

I somewhat answered the second part myself because aperture has an effect on both light and depth of field. So something as ISO is necessary to help control light alone.


DwainRowe: “To my knowledge the sensor itself is unchanged by increasing the ISO setting on the camera. Only the electronic "gain" of the "computer" is changed”.

This is exactly what I was looking for and concluding. Thanks DwainRowe.
AntonLargiader hit the nail on the head though as to what I was thinking.

AntonLargiader wrote in post #13591362 (external link)
I know what you're asking, and I think the retention of the ISO numbering system is just a holdover from film days. If we had to create a digital exposure system from scratch I'm sure several things would be different, among them the 'third leg' issue (if it still existed at all). We could replace it with EC (for auto exposure) and an arbitrarily labeled exposure sensitivity (for manual) and we'd learn it just fine. But everyone knows ISO, so it stays...

God Bless


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RAW ­ RAW ­ RAW
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Dec 24, 2011 01:16 |  #21

Ralph III wrote in post #13591141 (external link)
A) I want to know exactly HOW a digital camera's "sensor" can/is changed with different ISO settings?

B) Being that your camera is digital (computerized) why is ISO even necessary? It would seem your camera could understand a slow shutter setting and wide open Aperture, is a request for more light? Or is that the case, it's just a request to digitally enhance exposure? In that case, it wouldn't be correct to say ISO affects the sensor.

A) "turning volume up on stereo" is exactly how this works on a digital sensor it is simply an amplification of the signal in exactly the same way as a stereo sound system. The sensor is not changed, the signal from the sensor is electronically handled differently.

B) ISO is not necessary on a digital camera, hence the auto ISO modes on some cameras, ISO control can be easily dealt with by a simple algorithm in the camera's operating system. ISO control is available and used: 1) because it is a known and familiar principal from film photography, 2) because it is technically possible to emulate the ISO settings of film photography and 3) if there were no ISO control in digital photography the art of digital photography would be much the poorer.

HTH, RAW




  
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digital ­ paradise
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Dec 24, 2011 01:53 |  #22

Curtis N wrote in post #13591239 (external link)
Higher ISO settings are possible by amplifying the voltage produced by photons hitting the photocells. However, just like when you turn up the volume on your radio to hear a weak station, you get "noise." You get the voltage produced by photons and you get voltage produced by other things. This reduces image quality.

I have tried to follow forums where they talk about this at the molecular level. I find it difficult to follow mostly because I'm not that interested. I'n sure if I took the time I could learn all about it. This is one of the best basic explanations I have heard so far. Makes sense. I have no idea what read noise is and I don't care. As long as it works. I'm going keep this one in my hip pocket.


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tzalman
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Dec 24, 2011 05:04 |  #23

One more thing remains to be said:
For film ISO was more than simply a number that denoted the film's sensitivity, because the film itself did not have just one sensitivity. Develop it longer or at a higher temperature or in different developer chemicals and the film would be more "sensitive", the negative would be denser. Moreover, every negative had to be printed by shining a light through it. Changing the amount of time of the exposure would also change the final output print.

The ISO standard said that if an 18% grey card was photographed and "x" amount of light struck the film (the exposure) and the film was developed in a standard way (according to the maker's recommendation), the negative would have a certain density (d). That negative when printed in a standard way would make an 18% grey print. In other words, the ISO was dependent on a whole standardized process that produced a defined outcome. If exposure x produced density d, the film was called 100. If exposure 1/2x produced d, it was 200, etc.

In similar fashion the ISO setting on a digital camera is based on an entire process. Voltage from the sensor is amplified and then encoded digitally in the A/D converter to make a RAW file which is then processed and manipulated to make a jpg. If the camera was pointed at a 13% card (the digital standard) you should get a 13% jpg.

Anybody who has shot RAW and used a RAW conversion application knows that the converter always has an Exposure slider. Move the slider to the + side and the whole image gets brighter, same effect as raising ISO in the camera. So if it can be done digitally, at the stage of going from RAW to jpg, who needs that analog amplifier? The answer is that electronic amplification is always better than brightening by digital manipulation - better quality and less noise. In fact, in the early days of digital photography all "ISO" change was done digitally and when the engineers finally figured out how to incorporate a variable gain amplifier into the chip it was a huge step forward.

However, if digital brightening (digitally raising the operative ISO) causes more noise, digital darkening reduces noise and is the cornerstone of ETTR shooting.


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bcd01
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Dec 24, 2011 07:44 |  #24

Ralph III wrote in post #13592891 (external link)
My question was basically two part:
1) EXACTLY HOW IS ISO CONTROLLED ON A DIGITAL CAMERA BECAUSE THERE IS NO APPARENT PHYSICAL CHANGE A SENSOR CAN UNDERGO; WHERAS FILM UNDERWENT A PROCESSUAL DIFFERENCE. THEREFORE YOU COULD PURCHASE FILM WITH DIFFERENT ISO RATINGS.

2) WITH THAT IN MIND IS "ISO" EVEN NECESSARY ON A DIGITAL CAMERA, GIVEN WE ALREADY HAVE APERTURE AND SHUTTER SPEED, AND THE CAMERA IS CAPABLE OF ALGORITHMS TO ACCOMPLISH WHAT WE DESIRE.

I think you will find that the ISO adjustment helps keep the sensor within its dynamic range. The sensor has only an 8 bits per pixel and teh ISO is used to get the sensor within the linear range for the light level.


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jetcode
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Dec 24, 2011 09:53 |  #25

rgs- wrote in post #13590584 (external link)
The iso adjusts the sensor's sensitivity to light

From a users perspective that statement is true but I am fairly confident the ISO sets the gain value of an amplifier following sensor signal output. It is the amplification of the signal that produces the effect of an increase in sensitivity. In silicon devices the noise floor has many characteristics and one is called popcorn noise, there are some white noise elements, and other artifacts. By increasing the sensitivity this noise is amplified as well and filtered through extensive digital signal processing.




  
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Geonerd
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Dec 24, 2011 10:50 |  #26

Ralph III wrote in post #13591141 (external link)
Dwain, thanks but I understand all that as we photogs always state, "ISO affects the sensitivity to light....". But that is a generic answer that offers no explanation!

A) I want to know exactly HOW a digital camera's "sensor" can/is changed with different ISO settings?

The sensor itself cannot be adjusted. The photon -> electric charge conversion efficiency is what it is. As explained, you can turn up the amplification circuits, causing the camera to effectively shift the low signal levels up onto the standard response curve. Also, at higher ISO settings, the camera's firmware/hardware kicks in and begins to smooth the high frequency noise. (If the manufacturers wanted to get fancy, they could do dark-frame subtraction instead. This would avoid the loss of detail that many cameras suffer when shooting at high ISO.)


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tzalman
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Dec 24, 2011 11:26 |  #27

bcd01 wrote in post #13593546 (external link)
I think you will find that the ISO adjustment helps keep the sensor within its dynamic range. The sensor has only an 8 bits per pixel and teh ISO is used to get the sensor within the linear range for the light level.

The sensor doesn't have any bits per pixel, it is an analog devise, not digital. When the analog data is converted to digital data it is initially encoded in 14 bits per pixel depth and later in the stream when a jpg file is created it is 8 bits per channel per pixel which is 24 bits per pixel.

The dynamic range is the range from sensel sauration down to the point where noise obscures image detail. Thus the DR is not dependent on the sensor alone but also on the noise generated by the camera and as the ISO increases the DR decreases.


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tonylong
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Dec 24, 2011 19:35 |  #28

It was mentioned above that the electronic amplification provided by the in-camera ISO setting is superior to doing this digitally, such as in post-processing, and that's an important point -- having the ISO amplification is not just a "convenience" but we have seen that it actually provides better quality.

The reason is that the "noise problem" is there at any ISO and the low signal-to-noise ratio that leads to noticeable noise in high ISO images is because of the low level of light that has been collected and boosted. So, if you collect a low level of light at ISO 100 and boost it to match, say, an ISO 1600 image, the results will be noisy, and, with at least Canon cameras it has been demonstrated that higher "base" ISOs will produce cleaner images than boosting a lower ISO signal digitally.

You can find this out easily for yourself: with your camera in Manual and Raw, set your ISO to 1600 and then expose a scene "properly" with the aperture and shutter speed, so that you don't blow highlights or clip shadows, but everything is range in the histogram.

Then, without changing your aperture or shutter speed, so that you are taking in the exact same amount of light, turn your ISO down to ISO 100 (four stops) and take the exact same shot.

Now obviously the ISO 100 shot will be severely "underexposed", but still the same amount of light has been collected, so the test will be to see how the quality compares -- if you push the underexposed ISO image up by four stops in your Raw processor, will the quality be as good as the ISO 1600 shot, or maybe better (since we've always heard that "High ISO causes noise")?

Try it if you don't know the answer!


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tzalman
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Dec 25, 2011 09:07 |  #29

It was mentioned above that the electronic amplification provided by the in-camera ISO setting is superior to doing this digitally, such as in post-processing, and that's an important point -- having the ISO amplification is not just a "convenience" but we have seen that it actually provides better quality.

One reason that this is particularly true with Canon cameras is that the variable gain amplifier is nowadays integrated into the sensor chip, which shortens the distance traveled by the analog signal and reduces the noise in it. Thus when it is amplified there is relatively little noise to be amplified along with it. The camera-generated read noise that gets added later is fairly constant and largely independent of the amplifier. So if we call the analog noise A and the read noise B, when the ISO is set to 400 we have 4A+B which is smaller than 4(A+B) which is what we would get from a digital boost. [Disclaimer: the real world isn't so simple or so linear. Thus in the 5D2 there is only a tiny difference between ISOs 100 and 200, a very small increase in noise going to 400, a small increase at 800, a bigger increase at 1600 and when you set 3200 the image isn't very different from 1600 pushed one stop in the converter.]


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Why is ISO on Digital Cameras?
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