Help with ISO

Started May 15, 2013 | Discussions
TheVyper06
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Help with ISO
May 15, 2013

I'm sure this has been ask before but I can't find a thread on it, so can someone help explain to me what ISO does?

I haven't really experimented with it much and from what I have researched I still don't understand it well. So if anyone has some insight I would appreciate it.

Deleted1929
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Re: Help with ISO
In reply to TheVyper06, May 15, 2013

To understand ISO you need to understand that an exposure setting is the combination of three parameters : ISO, aperture and shutter speed.

Aperture relates to the physical width of an opening in the lens that controls how much light is let through.  Aperture also controls another aspect of image formation called Depth of Field.

Shutter Speed is easy enough to understand.  The longer the exposure takes the more light is captured.  Shutter speed also affects how much motion ( and shake blur ) affects your shots.  Sometimes you want some motion blur, sometimes you want none,usually there's a balance that's acceptable.

ISO in film days related to the sensitivity of the film to light.  A sensor has only one sensitivity and ISO is an amplification setting that amplifies the values the sensor returns.  It also amplifies the noise ( as the sensor cannot distinguish between noise and valid data ).  Raising ISO means you need to light to capture the scene, but amplifies noise.

To understand the relation between these three things in an exposure setting I'd suggest you read these :

http://www.fredparker.com/ultexp1.htm

http://www.cambridgeincolour.com/tutorials/camera-exposure.htm

And to understand how aperture affects depth of field read this :

http://www.cambridgeincolour.com/tutorials/depth-of-field.htm

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StephenG

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jrtrent
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Re: Help with ISO
In reply to TheVyper06, May 15, 2013

TheVyper06 wrote:

I'm sure this has been ask before but I can't find a thread on it, so can someone help explain to me what ISO does?

I haven't really experimented with it much and from what I have researched I still don't understand it well. So if anyone has some insight I would appreciate it.

Sjgcit has covered what ISO is very well, and given links to some great articles (the one by Fred Parker has been particularly helpful for several of my friends over the years).

What remains is for you to see what effect different ISO settings have on your camera's output quality.  A short Nikon article puts it this way, "Raising ISO sensitivity amplifies the electronic signal, which also amplifies any noise in the signal; as a result, the higher the ISO sensitivity, the more obvious the effects of noise on your photographs."  Sometimes the effect of noise isn't necessarily just more grain, but also a smudging of detail as noise reduction features try to mask the appearance of the noise itself.  Imaging Resource didn't have a full review of the A65, but DPReview said that its image quality was the same as for the A77, and Imaging Resource had this to say about that camera's noise characteristics at different ISO settings:

"The Sony A77's images are quite clean and detailed at ISO 50 through 200, and even ISO 400 looks quite good. Some noise "grain" is noticeable at ISO 800, as well as a small amount of smudging in lower contrast areas, but the camera did a good job of holding on to fine detail here. There's some stronger smudging of fine detail at ISO 1,600, but detail remains good. At ISO 3,200, fine detail suffers from higher luminance noise and more aggressive noise reduction, though some detail remains. Chroma noise starts to become more noticeable at ISO 3,200 as well, but it's not too bad. As you might expect, fine detail disintegrates at ISO 6,400 and especially at 12,800 and 16,000, where chroma noise also becomes much more noticeable and images have a strong stippled effect."  http://www.imaging-resource.com/PRODS/AA77/AA77IMAGING.HTM

The best thing is for you to experiment with your camera and see where you find its image quality deteriorating past the point that you want to live with.  That might differ with your intended output needs.  That is, if you normally make small prints for a photo album, you might find that even a fairly high ISO setting works just great.  For larger prints, or even viewing pictures on computer monitors or televisions, a higher ISO might result in a drop in quality that would annoy you.  As an example, for my Sigma SD14, I found that ISO settings of 100 or 200 looked great for color, and I was willing to go as high as 400 if I knew I'd be converting to black and white.  Sometimes, capturing a picture under dim conditions is important enough to you that you'll put up with higher noise levels just to be able to get any picture at all (most of the time, I'm happier if I just put the camera away until lighting conditions improve).

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Gerry Winterbourne
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Re: Help with ISO
In reply to TheVyper06, May 15, 2013

As already stated, increasing ISO in a digital sensor increases the noise.  The dynamic range (the ability of the sensor to capture detail in both deep shadows and bright highlights) is reduced when shadow detail is obscured by noise.

For high contrast subjects, therefore, it's best always to shoot at the lowest possible ISO consistent with the other parameters of aperture and shutter speed.

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Doug Pardee
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ISO setting does two things
In reply to TheVyper06, May 15, 2013

The ISO setting in a digital camera does two separate things:

  1. Sets the exposure index used by the metering and auto-exposure systems, and
  2. Sets the [apparent] sensitivity of the sensor to light.

These two are basically set to follow each other. A higher ISO setting will cause the auto-exposure to reduce the amount of light that reaches the sensor and will adjust the sensor to be more sensitive such that the reduced exposure results in the same overall image brightness.

You can use Exposure Compensation to provide an offset between the two factors.

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KCook
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Re: Help with ISO
In reply to TheVyper06, May 15, 2013

The principles behind ISO are fairly simple, and have been nicely outlined in prior posts.  And it is a fact that increasing ISO also increases noise.  But many of the explanations swing the noise bat so often that a beginner can come away with the impression that an ISO 10% above rock bottom will cause noise to blow up in his face.  Which can lead to low shutter speeds and blurred photos.  Links to articles that take a more practical approach to the ISO treatment -

http://www.digital-photo-secrets.com/tip/1625/your-camera%E2%80%99s-settings-iso-speed/

http://www.picturecorrect.com/tips/advantages-and-disadvantages-of-various-iso-speeds/

http://neilvn.com/tangents/weddings-working-with-higher-iso-settings/

Kelly Cook

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gwlaw99
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Re: Help with ISO
In reply to TheVyper06, May 15, 2013

TheVyper06 wrote:

I'm sure this has been ask before but I can't find a thread on it, so can someone help explain to me what ISO does?

I haven't really experimented with it much and from what I have researched I still don't understand it well. So if anyone has some insight I would appreciate it.

ISO is the amount of analog gain applied to the data captured by the sensor after exposure.  In other words, the higher the ISO the more brightness.  Think of it as a form of analog post processing.  ISO does not cause noise as much as it reveals noise by making an underexposed image brighter.

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Deleted1929
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Re: Help with ISO
In reply to gwlaw99, May 15, 2013

Analogue gain is not always applied these days.  I think some sensors apply ISO as a digital multiplication ( which is how software does it ).  The difference is that the sensors apply it on the raw sensor data directly, but some cameras do not store the full bit depth in RAWs ( e.g. a sensors captures in 14 bits, but the the RAW may only store 12 bits - still plenty, but not exactly what the sensor counted ).

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StephenG

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HumanTarget
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Re: Help with ISO
In reply to Deleted1929, May 16, 2013

sjgcit wrote:

Analogue gain is not always applied these days.  I think some sensors apply ISO as a digital multiplication ( which is how software does it ).  The difference is that the sensors apply it on the raw sensor data directly, but some cameras do not store the full bit depth in RAWs ( e.g. a sensors captures in 14 bits, but the the RAW may only store 12 bits - still plenty, but not exactly what the sensor counted ).

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StephenG

It can get confusing, since the ISO standard is about mapping exposure index values to tone values, and there are lots of ways manufacturers can do that, and they don't need to specify what they're doing.

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