When talking about exposure, it is important to understand the concept of a stop. Changing the shutter speed by one stop means to either halve or double the amount of time the shutter is open. For instance if a particular shot was taken at 1/200 of a second, the same image at 1/100 of a second would receive twice as much light.
For aperture, things are a little more complicated. The aperture scale in full stops goes f2, f2.8, f4, f5.6, f8, f11, f16. Usually the aperture selection in a DSLR can be changed in 1/3 increments. This means between each full stop there are two other selectable stops, like f4.5 and f5 between f4 and f5.6.
ISO can be doubled of halved to allow either twice or half as much light for the image exposure. The scale usually starts at 100 and continues to double all the way up to 1600 and beyond. (With each new camera model ISO capabilities keep getting higher and better. Some cameras go as high as 108000 ISO). 400 ISO allows twice as much light as 200, while 100 is half 200, etc. Remember, the higher the ISO number, the more image noise or grain will show up in the image.
Photographers need to be aware of shutter speed, aperture and ISO so that they can get their desired result. Understanding how each of these factors work is especially important for photographers using manual mode, but should still be understood by all photographers. If a photographer wants to double the shutter speed (for example, to better capture movement) they need to choose whether to open the aperture diameter by one stop or double the ISO to keep the same exposure. Each has it’s consequences. Opening aperture will make a shallower depth of field, while doubling the ISO will increase image noise.
- 1/200 of a sec. shutter speed exposes the image to half the light that 1/100 of a sec. does.
- at 1/100 of a sec. a fast moving object will look more blurred than at 1/200 of a sec.
-f8 exposes the image to half the light that f5.6 does.
-f8 will have more depth of field (more in focus) than f5.6
- ISO 1600 is 4 stops faster than ISO 100
- ISO 1600 can take the same exposure
In the Understanding Exposure article I used the example of photographing our love birds, Mr. and Mrs. Waffles. Their cage is on the window sill, so they are backlit when it is bright outside. I figured out how I wanted to expose the picture; I used +1EV because of the backlight. But the problem was that the picture I took at f4, 1/50th of a sec. ISO 200 had a slow shutter speed which made the birds blurry because of their fast movements.
f4, 1/50th, ISO 200
Keeping the same exposure, lets try a few things. First I opened the aperture 2 stops from f4 to f2. In order to keep the same exposure, I need to change either the ISO or the shutter speed 2 stops. I left the ISO alone and bumped the shutter speed up 2 stops to 1/200th of a sec.
f2, 1/200th, ISO 200
At f2, its difficult to keep Mrs. Waffles head in focus with such a shallow depth of field. (Note: when photographing animal and people, its usually best to focus on the eyes.) So lets bring the aperture back to f4, leave the 1/200th shutter speed and bring the ISO to 800.
f4, 1/200th. ISO 800
The picture turned out pretty well, the depth of field is nice, the shutter speed is fast enough to stop the motion of the active bird, but the ISO is a little high for my camera (Canon Rebel XSi). It’s fine for a regular size print, but if I want to blow it up, ISO 400 might be better. Let’s try ISO 400, f4, 1/100th of a sec.
f4, 1/100th, ISO 400
Here’s the final picture. I brightened their color a little, but didn’t adjust much else.
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