Understanding ISO Sensitivity

On Film Speed And Sensor Sensitivity
In film photography, different films have different degrees of light sensitivity that affect their speeds of reaction when exposed to light. For digital cameras, it's also a matter of reacting to light. Digital camera's ISO settings offer the convenience of on-the-fly adjustments whereas film photographers would require a different roll of film in order utilize a different ISO rating.

ISO: Benefits And Tradeoffs
The key benefit of using a higher ISO setting is to enable the sensor to generate an exposure faster or with less light, hence a faster shutter speed or a smaller aperture can be used, resulting in less chances of image blur as well as better chances of taking shots of subjects in motion. Therefore, high ISO levels  are especially useful in situations that usually require slow shutter speeds like night photography, shooting with small apertures ( for greater depth of field ), or macro photography. Likewise, in cases where flash or tripod use is not appropriate or prohibited such as in museums and similar tourist attractions, high ISO levels help to illuminate the scene by capturing more light.
Unfortunately, there are various disadvantages to using high ISO levels, The by-product of signal amplification is "noise" which is represented by the grain, or the dots and speckles that you see in a high ISO image. The higher the ISO level used, the more noise there will be in an image. Thankfully, there are some genres that lend well to a grainy-looking image, such as fine art, black and white, and reportage photography.
Noise is usually most visible in the red and blue channels. It is also most noticeable in shadow areas as well as in sections consisting of a uniform color such as the blue sky. Loss in finer details and poorer color reproduction are two other common problems involved with high ISO.

Cameras with larger sensor sizes, such as DSLR's, are better equipped to limit image noise, as are those with advanced noise reduction technologies. Note though that the latter may tend to overcompensate and result in the loss of detail in some cases. Also, since most digital cameras come with an Auto ISO feature, be wary of such automatic adjustments if you want to manage your image noise levels. Generally, it is still more recommendable to manage your ISO settings manually.
The camera's ISO setting is its sensitivity to light. The higher the ISO, the more sensitive it is. This is measured according to international standards, so ISO100 on one camera will be exactly the same as ISO100 on another. Each ISO setting is double the one before: if you increase the ISO from 100 to 200, you double the camera's sensitivity; and if you increase it from 200 to 400, you double it again. This carries on through the ISO scale. This is deliberate. The ISO settings are designed to double (or halve) the exposure in the same way that the lens aperture settings and shutter speed settings are, and this is why the lens aperture, shutter speed and ISO are often described as the 'exposure triangle'.
For example, if you want to use a faster shutter speed without changing the aperture, you could increase the ISO instead. This relationship between lens aperture, shutter speed and ISO could quickly get complicated, but there are drawbacks to changing the ISO which mean that in practice you tend to change the ISO only when you have to. 

ISO drawbacks:
When you increase the ISO setting, you're not really making it more sensitive to light, you're simply amplifying the light values it's managed to capture. The problem with this is that all digital images have some background noise. Usually, you don't see it because it's faint compared to the light falling on the sensor, but when you increase the ISO, you amplify it, and it shows up as a kind of random speckling. The higher the ISO, the worse the noise.The 'signal-to-noise' ratio is one of the things we measure in our camera tests. Low ISOs offer a high signal-to-noise ratio (lots of signal, not much noise), but higher ISO settings bring a lower signal-to-noise ratio, which means that this random noise is making up a larger part of the picture.
Exposure options:

ISO adjustments are best kept for situations where they're absolutely necessary.  The regular exposure adjustments of lens aperture and shutter speed are the ones to use on a daily basis, and you only need to think about the ISO if the light levels fall to the point where you 'run out of apertures' (you're at the maximum aperture, and are risking camera shake from slow shutter speeds). If your DSLR has a full auto mode, it will make these decisions for you, increasing the ISO where it has to in order to avoid camera shakeOtherwise, you need to keep an eye on the camera settings yourself and increase the ISO if you're shooting in marginal conditions (or use your camera's Auto ISO function). It's often best to take control rather than leaving it to the camera, because there will be situations where you're the only one who can judge what a suitable shutter speed will be. 

This chart shows which shutter speeds and ISO settings to use when taking different pictures. 
TIP: Your ISO setting will also affect the brightness of the picture in conjunction with the quality. Without getting into the complexities of your camera's electronic sensor, suffice to say that the higher the ISO, the brighter the resulting picture. Unfortunately, as ISO increases so does color noise, which really brings down the quality of the photo.

"ISO" actually refers to the International Organization for Standardization ( ISO being derived not from the organization's acronym,but from the Geek work, "isos" which means equal. ) The ISO scale is the organization's ISO 5800:1987 standard that has become the universal scale for measuring the light sensitivity of a film. This scale was based on an earlier American Standard Association (ASA) scale, so the terms ISO and ASA are interchangeable, although ISO is far more widely now. :)


I hope you enjoy reading the 5th part of digital photography basics.
Later then. ^_^
Enjoy mastering yerrr crafts! :)



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