Mastering Exposure – The Final Stop

 Having the skills necessary to adjust shutter speed, aperture, and/or ISO without changing the overall exposure is extremely helpful when out in the field. For example, if one is working on taking a photograph of a subject at night, they could increase their ISO to the max and open their aperture as far as possible. This will enable them to use the shortest shutter speed as possible. Once they get the correct exposure (sometimes the best method to confirm the correct exposure is to take a test photograph), they can bring the ISO back down for better quality and shrink the aperture to give them more depth of field. While this will lengthen the shutter speed, they will already know that their exposure is definitely correct. If the photographer wants to check the focus also, they might only increase the ISO and leave their aperture at whatever setting they need to get the desired focus and depth of field. This can be a life-saver when they are taking an hour or longer exposure, and they want to be positive that their photograph is properly focused and exposed. It is never fun waiting for an hour or more to get a photograph and then seeing the result as a dark and poorly focused photograph. The exposure of a photograph is often measured in stops.  The correct exposure for a photograph is measured at EV (exposure value) 0.00; if the photograph is overexposed by one stop, the exposure would be written as EV 1.00. There are three ways to change the exposure of a photograph: shutter speed, aperture, and ISO. To increase the light in the photograph by one stop, an individual can double the shutter speed, widen the aperture by a stop, or double the ISO. Exposure Stop Example While the example above uses shutter speed, one could get EV 0.00 in the above example by changing the ISO to 200 and shortening the shutter speed to 1/16 (normally rounded to 1/15) of a second. This is because the ISO has been changed to let in one more stop of light, but the shutter speed has been shortened to let in one stop less of light. This cancels out and gives the same exposure as the above example at EV 0.00. At the bottom of this page is a chart. While some photographers adjust their exposure in ½ stops, it is most common to use 1/3 stops, as the chart below. Using the chart, convert the exposure 1/250, f/4.0, ISO 100 to f/8.00 without adjusting the ISO - the answer for this practice question is is at the bottom of the page. Tips: 1. On most cameras, the adjustment increments are in thirds. Thus, to change the aperture by a third, the dial only needs to be moved one click. 2. I'd suggest you click on the chart below, and print the PDF - it will come in handy when you are in the field.
Exposure Chart

Click on the chart for a larger range of values

  Answer: It can be seen that an aperture f/8 is a decrease of 6 third stops (or two whole stops) from f/4. Since ISO is ignored in this example, the shutter speed will need to be increased to compensate for the smaller aperture which is letting in less light. Again using the chart, we see increasing two whole stops of shutter speed takes the shutter time to 1/60 of a second.

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