What is ISO?

[TNH]

I was using my camera today, and I saw a place in my camera's Shooting Menu that said 'ISO sensitivity.'
And you could change the ISO to different numbers. I've heard people talk about ISO, but I have a question.

What is ISO? And what exactly does ISO do, and when do you need to use it? And what happens when you put it on different numbers?!
I have no clue what ISO means or what it is! I'm so embarrassed! I'm just a beginner, so I really don't know that much about camera settings.
I would appreciate any help!! Thanks so much!

[Ett]

ISO Settings in Digital Photography

[RichardTaylor]

You may want to have a look at this multi part tutorial (on DPS) on exposure.


Exposure (1) A balancing act

[inkista]

Originally Posted by TNH

What is ISO?

From a larger point of view, ISO is the governing body for international standards (International Organization for Standardization. A standard allows manufacturers to make compatible equipment with each other and to be sure that they're using the same standard of measurement.

In photography terms, ISO is short for the ISO standards that were developed regarding film speed (most notably ISO 6:1933, which was for B&W negative film sensitivity). It's basically a way of measuring how sensitive a photographic medium is to light. The higher the number, the more sensitive it is.

And what exactly does ISO do, and when do you need to use it?

In film, ISO was governed mostly by the size of the film grain: the bigger the grains, the faster light could be grabbed, but the grainier the image would be. With digital sensors, ISO sensitivity is achieved by controlling the voltage gain across the sensor--kind of like turning up the volume on audio equipment. The higher you crank the sensitivity, the more noise you get.

You generally use higher iso settings when you need to get more light into the exposure and are willing to trade off for the higher noise levels. Low light situations where you need to maintain a fast shutter speed are one type of situation that might require higher iso settings. Lower iso settings would be when you can sacrifice a longer shutter speed for less noise. Landscape shooters tend to use tripods and very low iso settings.

...And what happens when you put it on different numbers?!

As I said, the lower the number, the less noise, but the longer it will take to get a good exposure. And the higher the number, the more noise, but the less time it will take to get a good exposure. If you're in an automated exposure mode, increasing the ISO is liable to cause your shutter speed to get faster and/or your aperture to get smaller. This could help eliminate blur, and/or get more of your scene in focus, but will make the image a little noisier.

Generally, with non-flash photography, you control the exposure with three settings: ISO, aperture, and shutter speed. The ISO is how sensitive the sensor/film is to light. The aperture is how large the opening in the shutter is set when you take the picture (the larger the opening, the more light you get, but the smaller your depth of field), and the shutter speed is how long the shutter remains open during the taking of the picture (the longer it's open, the more light you get, but the more chance you have for motion blur).

A certain amount of light is needed for a good exposure, but different types of images have different priorities when it comes to blur, noise, and depth of field, so you sometimes trade off these settings one against the other, using stops.

A stop is simply a measurement of light in an exposure. 1 stop = 1EV (exposure value) = 1 doubling/halving of the light. So, shifting from iso 200->iso 400 will get you one more stop (+1EV) of light. So will moving from f/4 -> f/2.8, and moving from 1/100s to 1/50s. The tricky one is the aperture, since they're given in f-numbers.

Don't worry. It's a lot of information to absorb all at once, but we all go through it. The best book for breaking this down and feeding it to you in an easily digestible manner is Bryan Peterson's Understanding Exposure, but there are tons of web pages written on the subject of the "exposure triangle". Just take it slow and easy, play around with the settings, and you'll pick it up. Since we're shooting digital, there's really no penalty for getting it wrong as you learn.

[BuddhaPi]

what she said!

[TNH]

Originally Posted by Ett

ISO Settings in Digital Photography

Thanks, but I saw that earlier today and it actually confused me a little. I didn't get all the big words and stuff...If I knew more about cameras, I'm sure that would really help. Thanks so much for trying to help me out, though! I really appreciate it!

Originally Posted by RichardTaylor

You may want to have a look at this multi part tutorial (on DPS) on exposure.
Exposure (1) A balancing act

Thanks, I'll check that out!

[TNH]

Originally Posted by inkista

From a larger point of view, ISO is the governing body for international standards (International Organization for Standardization. A standard allows manufacturers to make compatible equipment with each other and to be sure that they're using the same standard of measurement.

In photography terms, ISO is short for the ISO standards that were developed regarding speed film (most notably ISO 6:1933, which was for B&W negative film sensitivity). It's basically a way of measuring how sensitive a photographic medium is to light. The higher the number, the more sensitive it is.....

Thank you SO much!! You said that in a way that I could understand it! You've been a big help to me!

Originally Posted by BuddhaPi

what she said!

LOL, that's funny.

[Doug Pardee]

The camera's ISO setting does two different things simultaneously:

1. It adjusts the sensitivity of the sensor.
2. It adjusts the Exposure Index used by the camera's built-in metering system, and therefore, if you're using Automatic Exposure, the exposure settings: aperture and/or shutter speed.

The idea is that the second part adjusts the metering system to allow for the first part. Simple enough.

The Exposure Compensation (EC) control slips the two settings apart by a selected amount. To be consistent with film cameras, the sensor sensitivity always stays at whatever ISO setting you've selected, and the Exposure Index is tweaked. So, if you've selected ISO 800 with a +1Ev Exposure Compensation, the sensor is set for ISO 800 and the metering is set for ISO 400. We just tend not to think of that way, but rather as "ISO 800+1Ev EC", because that's the way the camera displays it.

Canon's Highlight Tone Priority (HTP) system adds a new wrinkle. When HTP is activated, the sensor sensitivity is set to 1/2 the requested ISO (1 Ev slower), but the metering is left at the requested ISO. Notice that this is the opposite of EC, because here the metering tracks the ISO control and the sensor sensitivity is set to a different value. To remind you of that, Canon uses small "o" characters instead of zeros: ISO 8oo instead of ISO 800.

This HTP combination results in effectively a -1Ev exposure compensation, but the in-camera JPEG conversion is aware of that and automatically brightens the image by 1 Ev, with special handling of the highlight areas to try to maintain detail there. Canon's DPP software also recognizes HTP and handles it similarly. Many third-party Raw converters don't recognize HTP, and give dark images.

(To be picky: the sensitivity shift for HTP is not guaranteed to be exactly 1Ev, but it's somewhere around there. Canon's view is that all we photographers need to know is that whatever the actual offset might happen to be, the in-camera JPEG engine and DPP know what it is, so they can produce properly brightened JPEGs.)

[Jim Bryant]

Originally Posted by Doug Pardee

The camera's ISO setting does two different things simultaneously:

1. It adjusts the sensitivity of the sensor.
2. It adjusts the Exposure Index used by the camera's built-in metering system, and therefore, if you're using Automatic Exposure, the exposure settings: aperture and/or shutter speed.

The idea is that the second part adjusts the metering system to allow for the first part. Simple enough.

The Exposure Compensation (EC) control slips the two settings apart by a selected amount. To be consistent with film cameras, the sensor sensitivity always stays at whatever ISO setting you've selected, and the Exposure Index is tweaked. So, if you've selected ISO 800 with a +1Ev Exposure Compensation, the sensor is set for ISO 800 and the metering is set for ISO 400. We just tend not to think of that way, but rather as "ISO 800+1Ev EC", because that's the way the camera displays it.

Canon's Highlight Tone Priority (HTP) system adds a new wrinkle. When HTP is activated, the sensor sensitivity is set to 1/2 the requested ISO (1 Ev slower), but the metering is left at the requested ISO. Notice that this is the opposite of EC, because here the metering tracks the ISO control and the sensor sensitivity is set to a different value. To remind you of that, Canon uses small "o" characters instead of zeros: ISO 8oo instead of ISO 800.

This HTP combination results in effectively a -1Ev exposure compensation, but the in-camera JPEG conversion is aware of that and automatically brightens the image by 1 Ev, with special handling of the highlight areas to try to maintain detail there. Canon's DPP software also recognizes HTP and handles it similarly. Many third-party Raw converters don't recognize HTP, and give dark images.

(To be picky: the sensitivity shift for HTP is not guaranteed to be exactly 1Ev, but it's somewhere around there. Canon's view is that all we photographers need to know is that whatever the actual offset might happen to be, the in-camera JPEG engine and DPP know what it is, so they can produce properly brightened JPEGs.)

True and good answer.

[SwissJon]

Best thing to do is switch your camera from the lowest ISO, take a photo, then put it on the highest ISO and take a picture (Don't worry, your camera won't break). Look what happens to the aperture and speed. Then take a close look at the photos you've taken and examine the quality. The settings between lowest and highest will be somewhere in between.

The ISO increase degrades the quality of the picture but makes the pixels more sensitive.. Increasing the sensitivity allows you to take faster pictures (say 1/200s instead of 1/20s so you can shoot faster moving subjects) or allows you to use a smaller f/stop (Say f/16 instead of f/5.6, so you have an increased depth of field and can get more things in focus.) However the consequence of these two (a smaller aperture hole or a faster shutter speed) is that less light gets to the sensor, hence the degraded picture quality. The more expensive cameras are much better at coping with high ISO's the Nikon D3s for example has a maximum ISO of 102,800, where as a D60 has a maximum of 1,600 (The D3s costs about 100x more too! )

Don't be embarrassed about being a beginner.. We were all beginners once, ask all the qustions you want to ask, but don't forget, there's a search function at the top too.. sometimes questions get asked lots of times. (I don't think this one has)