Camera ISO is a system for measuring the sensitivity of a film or digital camera. The lower the number, the less sensitive to light it is.
So if your camera is set to "200", it's twice as sensitive to light as if you used "100" ISO. This means that with your 200 ISO setting, you can use a shutter speed twice as long without worrying about blur caused by hand movement, compared to using 100 ISO.
For example: If at 100 ISO, 1/60th second shutter speed is sufficient without any worry of motion blur, at 200ISO this doubles to 1/30th second, which would allow more ambient light into the picture and produce more natural photos in low light condition.
What ISO settings to use?
Here are some considerations:
- If you can't really control the lighting, such as when taking landscape photos at night, set the ISO to a high number such as 1600 or 3200.
If you use a tripod/monopod or just don't care too much about camera shake, then you should take away that grainy look and sharpness of higher ISO shots by increasing the shutter speed and decreasing your aperture, so it all balances out in the long run and doesn't come out looking blurry. - If you can control the lighting but want to capture more ambient light while still maintaining good image quality, try using lower ISO values like 100-400 (depending on how dark it is).
- For action shots that require a high shutter speed, you need to increase ISO. Depending on your camera mode/settings, 100 can be as low as 400-800.
- If the scene is really dark and you want to capture more detail in shadows or suppress image noise, try using a higher ISO value like 800 or 1600. However increasing it too much means you will end up with grainier images - so keep this in mind if you're going for quality! As a general rule of thumb:
ISO lower numbers = less sensitive to light/less digital noise/more detail in shadows but requires longer exposures
ISO higher numbers = more sensitive to light/more digital noise/shorter exposures but less shadow
As an example, at ISO 800 your eye would need twice the amount of light to see an object in the same way that it would at ISO 400. This means that in order to see or photograph something with clarity you need more ambient light than before, this is when you should increase your camera's iso, but be aware of image grain (noise).
If there isn't much ambient light and you want a faster shutter speed, for example, if photographing sports, decrease aperture size by switching to lower f-stop numbers (i.e. f/2.8) this decreases the depth of field (the area in focus), making objects blurry behind your subject when they are far away - meaning all attention will be on your main subject!
Higher and lower ISO
Higher ISO speeds allow the use of faster shutter speeds, therefore freezing action and allowing more light into the camera sensor. This can be essential in photographing sports or wildlife that moves quickly.
However, if there is too much ambient light (too bright) or not enough light (not bright enough), then using high ISOs will make no difference because you need a longer shutter speed to compensate for the lack of available light, which is why it's important to know your equipment.
The higher the ISO number the more 'noise' you tend to get in photos shot with higher sensitivities (so 800ISO would show significantly more noise than 100ISO). Software tricks can help reduce some noise, but generally pixel size decreases as ISO rises so images become noisier at higher ISOs.
Lower ISO speeds reduce the sensitivity of the sensor, resulting in less 'noise'. This makes them useful for more artistic shots where you want to achieve certain effects rather than just taking a photo.
For example, if you are shooting portraits and want to use shallow depth of field or blur backgrounds, then using a lower ISO allows you to do so because it reduces the light coming into your camera allowing longer exposure times (slower shutter speeds).
So... basically what iso does is help obtain proper exposure by adjusting either how sensitive the film/sensor is to incoming light or adjusting how much time there is while that same amount of light hits your sensor.
How to reduce noise?
The first thing you can do is to use a tripod. This prevents camera shake, therefore getting clearer photos by allowing longer exposure times.
Another way to reduce noise in digital photos is to take RAW images instead of JPEGs. The reason for this is that when creating JPEGs, cameras automatically apply certain 'post processing' filters which introduce their own type of noise, so if you take the raw images straight out of your camera and then convert them into whatever format you want without applying these post-processing effects, it will have less noise overall.
If taking pictures using higher ISO settings/shorter shutter speeds regularly, try using a lens with an image stabilization feature (the best kind being optical image stabilizer), or use binoculars so become more stable.
You can also reduce the amount of noise after taking a picture by going into Photoshop and applying 'despeckling' or 'denoise' filters. The key here is knowing how much to apply these filters without causing any other unwanted side effects such as introducing more blurriness, reducing sharpness, etc...
How does ISO affect digital photos?
ISO is the same on digital as it is a film (film ISO 200 = digital ISO 200), except that with a digital sensor, there isn't anything that you can change to make the sensor more or less sensitive. So what happens in digital is that if you increase your shutter speed and/or aperture (aperture priority mode) at a given ISO (100 for example), your photos will start to get noisier and noisier.
ISO and post-processing
It is important to note that when you use lower ISO settings and slower shutter speeds, you will likely need to correct the white balance of your photos in photoshop because light will be 'warmer(more orange) due to longer exposure time.
Another thing to remember is if using slow shutter speed and taking pictures of moving objects such as people walking, cars driving, or water flowing: you can get motion blur which will require editing in Photoshop because it isn't possible to freeze the movement effectively while hand holding a camera. Hence why tripods are essential for these kinds of shots.
So why does each film have a different ISO?
You might be wondering this now that we've covered what ISO means and how it works, but what's the difference between ISO100 film and ISO400 film? The truth is, they are not really different at all. Both ISO100 and 400 films have exactly the same "speed". What makes them different is their sensitivity to light.
If you know nothing about how film works, basically there are two 'components' in every piece of film: the silver halide crystals which turn black when exposed to light, and other chemicals that enable these crystals to react easily with light.
This combination sets the overall sensitivity of the film, called its speed. Some films also have certain layers that can reduce or eliminate any color flaring if using very fast shutter speeds because it acts as a neutral filter. The only difference then between 100 and 400 films is that the ISO400 one has a more sensitive layer, so it will react to the same amount of light sooner than an ISO100 film.
As for digital ISO, all you can do is adjust how sensitive your camera's light sensor is. It doesn't change the sensitivity of anything else in your camera or on your lens, so there are no 'layers' like with film which lower or increase color flaring, etc...
What makes them different than (eg: 200 vs 400) is that they control the final output; their maximum rating. All cameras have a rating based on bits that define how many decimal places will be used when recording data (and therefore determine how precise/fine-grained they will be).
What is auto ISO?
Auto ISO is a mode found on most DSLRs. It automatically determines the exposure setting combination needed to get the picture you want, regardless of the scene being shot. To accomplish this it changes both shutter speed and aperture (aperture priority) at a rate determined by how much light your camera's light meter says is necessary based on its reading of all available light sources in the frame.
It does this by increasing or decreasing the ISO setting being used until it gets an exposure that corresponds to what you have indicated to be your desired exposure settings using S1/S2 buttons. If using P mode, then it will simply change accordingly without you needing to press any other button first; depending on which type of shooting mode you've set your camera to.
Detail of camera settings in Auto ISO mode, showing shift value (1/3 stop) and shift direction(+/-).
The real issue with Auto ISO is that if you are using P mode, then it will increase or decrease the shutter speed and aperture until it gets an exposure that corresponds to what you have indicated as your desired exposure settings.
This means that if you set S1 for 1/15th at f5.6, it may take longer than 1/100th at f8 to get the correct exposure so the picture might be blurry due to either camera shake or movement becoming blurred - unless you've mounted your camera on a tripod or other stable surface first.
If you don't mind this happening just once or twice in a while, then you really don't need to keep this setting on the 'off' position. The issue arises if you want to be able to adjust your exposure settings without having to worry about how much light is available because auto ISO will change all of them for you instead - using whatever combination it thinks best for that situation.
Of course, Auto ISO has its uses, but not so much for general photography. It's best used when shooting action scenes whether they are sports or moving objects like cars and trains; or people moving quickly between several different light sources or where there is erratic lighting., making it hard for even pro photographers to get the shot without a camera shake (eg: at night).
Conclusion
The only real difference between the '100' and '400' films is that they have different sensitivities to light, not anything else. The same goes for digital ISO settings - except it has an effect on camera sensor sensitivity instead of having a delayed reaction like film does.
Now you might ask yourself why there's a difference in how sensitive negative color or black and white films are to light when simply changing their speeds doesn't change this fact?
Because as we mentioned earlier, it isn't so much about how sensitive they are but rather what will happen to them when exposed to light; ie: longer exposure times let more of those silver halide crystals react with the light because they aren't as easily affected by temperature as the developer chemicals are.