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Relation between ISO, shutter speed, aperture and light in photography

Photography is a word having Greek roots, which basically means "drawing with light". When I started doing digital photography a few years ago, this did not make sense to me at all. How can you make a picture just using light? Only light matters? My pictures were blue, black, red and washed out all the time, but I didn't feel like giving up. It took me a fair amount of time to understand controls such as shutter speed, aperture and ISO which was the outcome of non-stop reading and a lot of mistakes. Coming back to the concept of light, it started to make sense when I attended a film photography workshop couple of years ago. The dark room with very dim or near to zero red lights was a whole new point of interest for me. My partner and I made a pinhole camera out of Pumpkin. 

The workshop made me understand how important light is when taking pictures, and the rules apply to both film and digital photography. Basically, the value of Shutter speed and Aperture directly affect the amount of light that enters the camera. ISO determines how sensitive your sensor is to that light entering. These three factors determine the overall exposure of your DSLR photos. This light gets transformed into an image by sensor circuitry. A short and crisp way of explaining the light variation whenever one of these values is changed on a DSLR is shown below.
                                    Effect of ISO, Aperture and shutter speed on exposure

The tricky part here to understand here is how to vary each of the above three things simultaneously. A couple of quick examples:

1) Sunset landscape shot: Slower shutter speed for more light, high f-stop for more details (depth of field). Yes, high f-stop would minimize the light, but slower shutter speed would cover up for it.

2) Bike racer or other moving objects: Faster shutter speed to freeze action. F-stop as low as possible to make the background out of focus. If the image is blown out(too white), then the only solution is to increase f-stop so as to control the incoming light.

Quick tip: When to vary ISO? Well, depends on the situation. Example: A dancer inside a low light room. If the lowest feasible shutter speed (to freeze action) and lowest f-stop are not able to provide properly exposed shot (hard to see dancer's face), then ISO becomes a lifesaver. This concept of learning when to vary what comes with practice and time. 

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