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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?

 
Ralph ­ III
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Dec 23, 2011 12:57 |  #1

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?

In the firm era, individual film was manufactured with different features (black/white/color) and in case of ISO different sensitivities. There is a physical or processual difference.

However, digital camera use a "sensor" that in effect simply converts light into a digital image. In what way could the sensor change when setting different ISO's?

I guess the question is WHAT is happening with ISO on a digital camera and why can't Aperture just suffice with these computerized camera's?

Ralph


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Dec 23, 2011 13:01 |  #2

The iso adjusts the sensor's sensitivity to light


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Ralph ­ III
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Dec 23, 2011 13:21 |  #3

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

Ok, I undestand the ISO adjusts the sensitivity as noted, but how does it do such on a digital sensor? How does your sensor change, as it is not like film?

Also, why is such necessary on a computerized camera? The camera can read your Aperture/Shutter speed settings and make adjustments accordingly, it would seem? Whereas the necessity/benefits of manufacturing different types of film is understandable.

Thanks


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altitude604
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Dec 23, 2011 13:38 |  #4

http://www.cambridgein​colour.com/tutorials/c​amera-sensors.htm (external link)

This is a great link with a ton of information on how the sensors in our DSLRs work. :)


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Dec 23, 2011 13:52 |  #5

Ralph,

Just as using high ISO film results in grainy photos, high ISO digital photography may result in digital noise ("grain").

So, just like film, the photographer should use the lowest ISO that provides an appropriate exposure for the circumstances and should raise the ISO only as needed in situations with less light.

Dwain


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Dec 23, 2011 14:26 |  #6

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

Strictly speaking that's not actually true. Increasing the ISO on a digital camera is like turning up the volume of your stereo; the signal from the sensor is 'amplified' according to your ISO setting, which is why grain or noise on images gets worse as you ramp the ISO up.


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Dec 23, 2011 14:27 |  #7

I look at it as cheating the the sensor. At ISO 100 the sensor gets all the light it needs to produce the best quality. At 12,800 the sensor is not as happy with the light as ISO 100 because we are cheating or robbing it of light. It will still record some light but won't look as good as ISO 100.

Cheating may have not been the best word to use but it was the only non technical term I could think of.


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Ralph ­ III
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Dec 23, 2011 15:36 |  #8

DwainRowe wrote in post #13590765 (external link)
Ralph,

Just as using high ISO film results in grainy photos, high ISO digital photography may result in digital noise ("grain").

So, just like film, the photographer should use the lowest ISO that provides an appropriate exposure for the circumstances and should raise the ISO only as needed in situations with less light.

Dwain

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?

Film was manufactured with different sensitivities to light, in order to assist in getting proper exposure. You could select different film to accomplish your goal but that isn't the case with digital camera's.

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.

In contiplating; maybe ISO is simply a useful tool on digital because Aperture affects both light and depth of field. Whereas, ISO can be used to simply affect light.

My other question still stands though. How is this accomplished with digital? Sorarse gave a good example "turning volume up on stereo" but how exactly does this work on digital sensor????


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kjonnnn
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Dec 23, 2011 15:43 |  #9

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?

Film was manufactured with different sensitivities to light, in order to assist in getting proper exposure. You could select different film to accomplish your goal but that isn't the case with digital camera's.

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 contiplating; maybe ISO is simply a useful tool on digital because Aperture affects both light and depth of field. Whereas, ISO can be used to simply affect light.

My other question still stands though. How is this accomplished with digital? Sorarse gave a good example "turning volume up on stereo" but how exactly does this work on digital sensor????

You should probably rephrase your question and do a Google search. "How does a digital camera sensor work?" would probably put you on a better path than your original question, as you're asking something technically and electronically specific. We don't necessary need to know HOW the sensor works to use our cameras, no more than we need to know how ABS brakes work in order to use them.




  
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Curtis ­ N
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Dec 23, 2011 16:02 |  #10

Ralph III wrote in post #13590577 (external link)
why can't Aperture just suffice with these computerized camera's?

Aperture affects depth of field. Sometimes you want a lot (to keep objects at different distances in focus) and sometimes you want a little (to blur the background and separate your subject). So you manage depth of field by adjusting the aperture.

Similarly, shutter speed affects motion blur. Sometimes you want to eliminate it, sometimes you want to include it (to show movement).

So, the ability to adjust ISO lets you manage those things. Here's the procedure I recommend:
1) Select the shutter speed necessary to eliminate visible motion blur
2) Select the aperture that gives you the depth of field you want
3) Select the ISO that gives you correct exposure.

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.


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Dec 23, 2011 16:04 |  #11

Ralph III wrote in post #13590656 (external link)
Ok, I undestand the ISO adjusts the sensitivity as noted, but how does it do such on a digital sensor? How does your sensor change, as it is not like film?

My understanding of HOW it works is that the ISO boosts the analog amplifier by a factor of 2 for each full ISO stop increase. It does this before the photon counts are digitized/written to the RAW image file. (It also boosts the noise, which leads to a reduction in the output signal/noise ratio, so while an image taken with higher ISO will always be brighter than the same image taken with a lower ISO multiplier, it will contain disproportionately more noise)

Hope this is right...but that's my (very limited) understanding of HOW it does it.


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Dec 23, 2011 16:08 |  #12

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

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. Unfortunately, usually when you change the gain, you change the signal to noise ratio. At a low ISO setting, the signal is greater than noise (hopefully) in this ratio. At a higher ISO setting, the amount of noise in ratio to signal is increased, thereby giving you more noise (digital grain) in your image.

If you were to set your ISO on automatic (as some EOS cameras provide), the ISO will self adjust to help you meet the aperture/shutter speed/ISO triad as you describe above when you day, "...slow shutter setting and wide open Aperture, is a request for more light?" But then you have ceded that control to your camera and risk introduction of unwanted noise.

As I understand it, the sensor has a "native" ISO setting at which it has its best signal to noise ratio. This native ISO is not adjustable... it is what it is. When you move from that value, the data quality decreases due to the greater noise in your signal to noise ratio.

This is an over-simplistic explanation and therefore must contain inaccuracies.

Perhaps if you are more clear with your questions we can provide better help for you. A few of us have tried to explain this to you given the questions that you have asked.

If you desire more specific electrical engineering based explanations, perhaps someone will come along. Otherwise, I would suggest performing an internet search.

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Allan.L
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Dec 23, 2011 16:13 |  #13

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?

Film was manufactured with different sensitivities to light, in order to assist in getting proper exposure. You could select different film to accomplish your goal but that isn't the case with digital camera's.

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.

In contiplating; maybe ISO is simply a useful tool on digital because Aperture affects both light and depth of field. Whereas, ISO can be used to simply affect light.

My other question still stands though. How is this accomplished with digital? Sorarse gave a good example "turning volume up on stereo" but how exactly does this work on digital sensor????

Are you not reading what others are posting????

It makes NO sense to only use aperture and shutter speed to get the correct exposure... photography would then be VERY limited. IE only shooting sports during very good light...


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Dec 23, 2011 16:30 |  #14

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?


Your question contains the answer.

There are three things that effect exposure, and between them is a standard unit of measure to make setting exposure easier.
ISO is a standard of measure for sensitivity to light of a photographic medium, and like shutter speed and aperture (which all have different units of measure) increments in units are done in equivalents of "stops" of light.


So you can adjust shutter speed by 1 stop in one direction and adjust ISO by 1 stop in the other direction and still have the same exposure.


In theory you could have a continuously sliding scale of sensor sensitivity, that could slide up and down to adjust for different light levels, but it would make setting and changing exposure very difficult, both for you if shooting manual and the automated processes in the camera, which are all designed to work all 3 elements together.


By converting the signal gain back to the old film equivalent it also provides continuity in the process as well as consistency.


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Dec 23, 2011 16:36 |  #15

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.

We are surrounded with such things, that live on from legacy systems just because they're familiar.

Do you shoot auto-ISO?


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Why is ISO on Digital Cameras?
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