What is ISO?

i-1d345b701ab4f86a1a048f126d485116-iso2.jpgYou've seen the ISO button on your camera. What does it do, and why does it matter?

ISO- an abbreviation for International Organization for Standardization- refers to a standard measure of the sensitivity of film or digital sensors to incoming light. The higher the ISO rating, the more sensitive the sensor and the less light required to form an image. Wikipedia carries a nice summary of the technical details.

Most digital cameras support a range of ISO ratings from 100 to 3200 or even higher. These ratings are calibrated so that a doubling in number indicates a doubling of sensitivity. ISO 400, for example, is twice as sensitive as ISO 200 and can form an image with half the available light. Set the ISO up to 6400 and you can photograph night scenes, concerts, and dinner parties at fast shutter speeds without using a flash. Basically, ISO is a way to control exposure from the sensor side of the equation rather than managing incoming light with shutter speed (=duration of exposure) and aperture (=size of the opening in the lens).

But all you ISO jockeys out there in cameraland should be aware of the bad news. ISO is intertwined in a steep trade-off with image quality. Sensitive sensors record more noise. Consider this photograph of a window, captured with Canon's EOS 7D dSLR.

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A series of 100% crops of the window pane taken at different ISO ratings (and changing the shutter speed to maintain a comparable exposure) reveals the trade-off:

i-d8da9c67de51424c31ea51fa5ec9d394-iso100.jpg

i-4af898ea16780d41e54c51c8a302b201-iso400.jpg

i-1fc111a1edd9192c19e7f9c622db83f3-iso1600.jpg

i-2f5bc40627701dc67f0ad0b07e05fc2a-iso6400.jpg

As you can see, the relatively insensitive ISO 100 produces a smooth, clean, noiseless image. That's because the sensor is recording data by averaging over a greater number of incoming photons, allowing it to form a more accurate picture of the color and intensity of light. The shot at ISO 6400 is taking scant information from relatively few photons and running with it, and the image is grainy and imprecise.

So what ISO setting should you use? As a general rule it is best to shoot with lowest ISO setting possible given the lighting conditions.

I keep my cameras at a default setting of ISO 100 so the images have minimal noise. Only under low-light conditions where the optimal shutter speeds become unacceptably slow do I raise the ISO.

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I've always been confused by ISO on digital cameras, since it seems like its the same sensor no matter what the setting. At first this explanation helped, thinking it might bias the sensor to make it more or less sensitive, but then I got to the "That's because the sensor is recording data by averaging over a greater number of incoming photons, allowing it to form a more accurate picture..." explanation.

Wouldn't the number of photons hitting the sensor be related to the aperture and shutter speed?

"I keep my cameras at a default setting of ISO 100 so the images have minimal noise. Only under low-light conditions where the optimal shutter speeds become unacceptably slow do I raise the ISO."

After this weekend, this is now also my policy. I spent all day Sunday driving around and taking pictures, mostly of birds. I seem to have finally figured out focusing on my new-to-me Pentax K10D, but those nice in-focus pictures are all very noisy, because I'd set the ISO to 1600. Oops. I was wondering why it kept telling me the shutter speed was maxed out...

"Wouldn't the number of photons hitting the sensor be related to the aperture and shutter speed?"

Yes. Thus you can use a smaller aperture and / or a faster shutter speed with higher ISO for a given light condition. Like Alex said: "Set the ISO up to 6400 and you can photograph night scenes, concerts, and dinner parties at fast shutter speeds without using a flash." Note the "at fast shutter speeds".

I really like the "and run with it" explanation here. Thanks!

By TheBrummell (not verified) on 25 May 2010 #permalink

Dave X:

For a digital camera the ISO setting sets the gain of the amplifier that amplifies the signal coming off the chip. In bright light and low ISO, there is little to no amplification of the signal, so there is no noise. (Think of turning the volume control down on your stereo: You here little to no hiss.) At high ISO dim light there is a lot of amplification of the signal which amplifies the noise. (Turn up the volume control of the stereo and you start to hear hiss.)

ISO- an abbreviation for International Organization for Standardization

Actually, "ISO" comes from the Greek for equal. It is not an acronym, although it sure looks like one.

By william e emba (not verified) on 25 May 2010 #permalink

Does 'ISO' have the same meaning as the old 'ASA'?

KeithB:
But even in a perfect camera, there's also the variation in the number of photons that arrive at a senor element during an exposure. This creates a noise-like effect (hence the name "shot noise" which I think is something of a misnomer), and it's more conspicuous when the average number of photons is small.

Thanks for the explanation, Alex, and for the technical details KeithB. I've been meaning to play around with ISO and f settings on my camera, so this might be a good motivation.

Just wondering what you call low light, especially when shooting outdoors with natural light? And I still don't seem to get how flash interacts with ISO.

By James C. Trager (not verified) on 26 May 2010 #permalink

James: Light levels are relative to what you want to shoot. In planning any photo, I usually follow this process:

1. What depth of field do I want? --> adjust aperture
2. Do I want motion blur, or frozen action? --> adjust shutter speed
3. Can I achieve 1 & 2 at ISO 100? --> add flash; or increase ISO

Note the importance of aperture- it's singularly important to the look of a photograph, and all else follows from there.

Flash & ISO are ways to deal with light levels that are too dim for the optimal shutter speed/aperture combination. They also both have undesired qualities. With flash, it's a harsh unidirectional light that takes effort to make look natural, plus it messes with motion blur in a way that may or may not be wanted. With ISO, it's noise. Which of these you choose to adjust depends a great deal on the situation.

Since I usually shoot insects and can exert great control over the flash position and intensity relative to the subject (that's the benefit of macro!), I mostly use flash. But in situations like the roller derby bout I photographed last week in a large indoor arena, my flash setup just didn't have the flexibility and power to do the job, so I had to shoot at ISO 1600-3200.

Lens choice affects this, too. For the roller derby it might be better to forgo the F4.5 zoom for the f/1.8 el cheapo 50 mm.

Keith- Indeed. I shot a lot of the action during the bout using the El Cheapo f2.0 35mm.

My next lens (next year) will be either an "El Cheapo" large-aperture prime like a F2.0 35mm (for miscellaneous low-light situations, like dusk / sunset / indoors), or a Sigma Bigma (50-500mm telephoto zoom) for the birds. Depending on budget constraints, mostly. Will I have $1000 to spend, or merely $300?

By TheBrummell (not verified) on 26 May 2010 #permalink

jdhuey:
Yes, ISO is equivalent to ASA. The mfgs can play some tricks though, so there is some specmanship. So, if you have an exposure calculator from 50 years ago it still works today. In fact some large format film photographers use a digital cameras as light meters.

TheBrummel:
$300? I was thinking along the lines of the $100 Canon f/1.8 50 mm. (It was only about $70 not too long ago which made it a steal.)

Hey Alex,

I have been wondering when looking at your pictures - do you do a lot of digital postprocessing? Denoising programms can reduce the "noise" problem somewhat, finding pixels that are too bright compared to the environment. I'm not sure how big the loss in detail is (I have not done too much experimenting) and if it can be countered by sharpening. I hope that is not too much of an undecent question ;)