ISO, Aperture and Shutter

 
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ISO, aperture and shutter speed are the basic elements concerning light in photography. Since "Photos" derives from the greek word for "light" it cannot be understated its importance.
We will have a brief introduction here. If you wish more in debt information follow the provided links.

ISO - Film Speed

Another name for ISO is "Film Speed" because previously when using film cameras the films were seperated by its ISO settings. In short this measures the light sensitivity of the Camera sensor. If you use a low ISO number e.g. 100-200, you will need more light for your photograph but your noise level will be lower. When increasing ISO you will be able to take pictures in lower light and with lower shutter speed. However, the price you have to pay is added noise in your photographs.

Same as with your speaker system on your stereo, you can increase the volume to a certain level without hardly noticing reduction of quality. This is particularly true if you have a high quality amplifier. With your ISO from 100-400 you will hardly notice a quality difference regardless of your DSLR. When your ISO level creates too much noise is highly dependent on your camera. If you need high quality in low light and you are shooting handheld, you will unfortunately need to pay up. If, on the other hand, you can use external lighting and/or tripod you will not need the most expensive cameras advanced features.

Today high quality cameras with full frame sensors and not excessive noise in lower light situations (e.g. ISO 1600-3200) costs around US$ 2000. Considering the price of quality lenses you will have to pay a significant amount. That being said, this is much lower price than what you would previously need and considering the life-span of the gear (particularly well kept lenses), photography is not an extremely expensive hobby.


Aperture

Lens speed refers to the maximum aperture of the lens. A fast lense can have a aperture of e.g. 1,4 which is a large aperture and is found in expensive prime lenses. A slower lens might have a aperture of 4,0. With a larger aperture (lower number) you will let more light into the sensor. You will therefore also have a slower shutter speed. Something else that happens is also that you will have a shallower debt of field. This might be desireable or not depending on your picture. Usually it is desireable in a portrait. However, it is also relevant to the distance from your object. In landscapes an f/11 will often get everything in focus. Using the same f-stop in macro will not give you large parts of the object in focus. Consider the following pictures:

F2,8 shot at about 5 feets from the subject

F4 shot at about 5 feets from the subject

F16 shot at about 5 feets from the subject

F2,8 shot at about 1,5 feet from the subject
 
F16 shot at about 1,5 feet from the subject
Notice in particular that when you are close to the subject and shooting with f2,8 the focus is very short indeed and not even the whole tongue of the teddy-bear comes in focus.

Lenses that can shoot with large opening e.g. f/1,8 might not be very sharp at that aperture. Instead lenses have a sweet spot. For example the Canon 50mm 1,8 which is famous for high quality relative to price is often considered to produce the sharpest pictures around F5. Large aperture (low f number) is often not only desired for the extra light, meaning you can have higher shutter speed and lower ISO, but also for the blown out background (Bokeh) which is great for portraits. If you don't need very large aperture you can save a lot of money by buying lenses with smaller aperture.


Shutter Speed

The third element to control whether you will get enough light to create the photograph is - Shutterspeed. The longer your camera sensor is exposed, the more light will enter. If you want to take of a city during nighttime and want those light-stripes provided by the cars, you might want to have an exposure of 20 seconds. On the other hand, if you wish to take picture of a water drop you might want an exposure of 1/4000 seconds. Too slow shutter speed will make a picture all blurry, and this you cannot change in Photoshop. Knowing shutter speed can help you create the motion blurr illustrating motion.


For more in debt explanation of the three concepts, visit one of the links below.
Another very good instruction if you want more in debt explanation visit these guides Iso Aperture Shutterspeed or Iso Aperture Shutterspeed.