What's ISO and Depth Of Field (DOF)?

Hi guys! Earlier you read about shutter speed and aperture. In automatic camera modes, these factors are automatically calculated and and set for every shot depending on the light on the subject. Other than the shutter speed and aperture, there are other factors that do affect how the image looks. One of these factors is the Depth of Field or D.O.F in short.

Depth Of Field means how much is in focus from near to far. That is, when the lens has focused on a certain subject, the areas closer to the lens and areas farther from the lens are out of focus. This is because, the focal point or point of focus lies at the focal plane of the subject and at other places far and near the lens, the image will be blurred due to loss of focus. Imagine that there are a number of laser beams attached to the circumference of the lens. All these beams converge at a certain point depending on the focus. Now, the point where all the beams converge, is the point of focus and will be the sharpest in the image. Now, the beams converge and then diverge after meeting. Hence, the places where the beams are divergent are the places where the image will be blurred or out of focus.
If its confusing, then just forget it. Lets just see how it can be controlled. For a shallow DOF, (that is, only the areas that are meant to be in focus are sharp and other parts of the image are blurred), the aperture must be large. That is, if the aperture is say f1.8, the DOF will be much shallower than when the aperture is smaller say f8.0. In general, for Portraits and people shots, the DOF may be shallow (aperture may be bigger) and for Landscape pictures, usually everything should be in sharp focus whether the objects are near the lens or far, and hence the DOF must be deep (that is the aperture must be smaller).

ISO is a standard number denoting a camera's sensitivity to light. The higher the number, the more sensitive to light the camera will be. The more sensitive it is, the more noise it gathers and hence, high ISO images will have noise (grainy). Most new DSLRs have excellent high ISO performance and the images are often very usable even at ISO 1600 and sometimes ISO3200.
ISO is generally pronounced by saying each letter separately. It is the same as ASA which used to be the way sensitivities where numbered back in the 1980's.
OK, how do we use it? Simple, just set the lowest ISO possible to get the shutter speed and aperture you want. If you want a smaller aperture and a faster shutter speed, then increase the ISO. At low light, there is not enough light to shoot a picture at small apertures or faster shutter speeds. Just increase the ISO until you can use a fast enough shutter speed and/or a smaller aperture. Higher the ISO, more the noise. This is less in newer cameras because of how technology just gets better. Also, on pocket cameras, the sensors are not as large as the sensors in DSLRs. Hence, the pixels are smaller and are less sensitive to light. This is why the low light shots taken with your P&S camera is much noisier. The highest ISO possible in P&S are generally ISO 800. New (and more expensive) DSLRs can pump up the ISOs to even 12800 and are still usable.
That's about it. Just use the lowest ISO at which you can comfortably shoot a picture.

Remember, all these technical stuff has nothing to do with taking good pictures. They only help you to understand what to do if you want to take control. Enjoy taking pictures the way you like. There is no thumb rule here!