Exposure Part III : Time

Today we are gonna talk about exposure time, which is controlled using the shutter. In the stone ages of photography exposure times were very long. So cameras didn’t need a shutter, just uncover the lens for seconds, minutes or even hours and when done cover it up. However, now a days due to high sensitivity exposure time is shorten to thousandths of a second. So now we are dealing with ultra faster shutters in comparison.
Shutters are made up of thin leaves of metal which are connected to a complex mechanism of gears and springs which make them open and close the path of light through the lens.

ShutterJust like intensity, shutter speed are designed to allow the time to be changed by a factor of two. That is each change in the exposure will change the exposure time by either doubling or halving it. Modern shutters timings are controlled electronically, this makes the mechanism smaller and lighter as lot of early timing gears are eliminated in favor of a very small computer chip. The shutter speed includes full second exposure times of 2,4,8, and some times longer. I find it very little need for the digital enthusiast to go in depth on the common sequences of the shutter speeds, So I will skip it.

Electronic shutters can operate at intermediate speeds, but usually only when the camera is operating in an “automatic” mode. They also have less moving parts, hence less vulnerable. On the negative side these shutters use batteries, which can fail at unexpected times. So it is good practice to carry spare batteries always.
Now to put some of what we have learnt together. In the beginning in part I, I did mention about equivalent exposures. Any number of combination of light intensity and time which will result in the same exposure. The scale goes something like below.

Time Scale

Remember the two scale runs in opposite directions, each of the above combination of f/number and shutter speed provides exactly the same amount of exposure. That is, an exposure of 1 second at f/2 will have the same darkening effect as an exposure of 1/250 second at f/2 and so will all the others in between.
All SLR cameras comes with modes of A, S & M. The A mode will allow the change of aperture and the camera will calculate the required shutter speed. Similarly the S mode will allow to change the shutter speed while the camera will change the aperture size automatically for us. While in manual mode we have total control of changing both the aperture and shutter speed. Most of the other modes are programmed modes which I will be deal in a later stage.
So you want to play around in the manual mode to get a correct exposure? Lets say you are in a dim room, 1 second at f/32 probably won’t be enough and the image will be underexposed. But perhaps an exposure of 1 second at f/5.6 would be enough. All you need to do is slide the two scales past each other until the f/5.6 is opposite the 1 second mark. Then all other shutter speed and f/number combinations will give the equivalent exposures. This is called an “equivalent exposure calculator”. Well, this only comes in handy when you want to play around in the manual mode. So here it is, shutter speeds and f/stops coordinating to give us a variety of choices. So what are the best known, more or less proven, exposure values for photography? Come back soon for the daylight exposure table.

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