Exposure Part 1

Exposure Part 1

Early Cameras were totally manual in nature, you had two things to set, how fast the shutter would operate, and how much light enters the lens.  Even the latest high tech DSLR that has more computing power than an IBM/360, as the ability to make these two settings.  The majority of people don't really think about this, they let the computer to all the thinking, but that can result in a poor image.

Shutter speeds are counted in seconds, fast shutters can exceed 1/2000th of a second, and many can be held open for long periods of time.   The amount of light is expressed as a fraction of the focal length of a lens.  A lens that is 100 mm long, at f/4 will have an opening that is 25mm in size, these fractions are based on a series of numbers, where each number allows double or half the amount of light, the lower the number, the larger the amount of light, these numbers are logarithmic in nature.  The typical values are 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32 although it's technically possible to go smaller then f/32 it would be quite rare, rare is another word for expensive.  In the 1970's and 1980's when 400 ISO (usually expressed as ASA in those days)  film was considered very fast, the standard lens was the 50mm, and most were between f/1.4 and f/2.  Current digital zooms, are quite a bit smaller, with f/4 to f/5.6 being common, then again with ISO values of 3200 and 6400 being common, this isn't a problem.    

Early cameras were all manual, and you would use rules like Sunny 16, to determine exposure.  Then light meters came out, which would measure the light, and provide values to the photographer.  Now, fast shutter speeds can stop motion, such as in sports, the smaller the aperture, the greater the depth of field (range of distance of image that is in focus), the larger the number, the smaller the depth of field.  With the same lens, the shorter the lens, the greater the depth of field, and the longer the lens focal length, the shallower the depth-of-field.   A short lens with a small aperture can have a nearly endless depth of field.

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You can creatively make use of this.  The image below has the ice on the rock in the foreground and the much lake also in focus, it was taken on a bright winter day, with a small aperture.  in this case the exposure was f/11 with a 22mm lens and a shutter speed of 1/200, with an ISO of 100.  I know this, because the original image was taken with a digital camera, that records this.   So the rock that is only 2 metres away and the lake horizon 1km away, are both in focus.  With other items you can use a larger aperture and a longer focal length to throw a messy background out of focus.

Some photographers will intentionally use a slow shutter speed to emphasize movement, this can give a really nice effect to moving water, giving it a dreamy quality.

I try to keep these postings fairly short, as long postings can get dull and boring, More on this next week in Part 2on exposure, when I will go into the mechanics of making it work with the modern camera.

That's all for this week
W