My brief articles on exposure used an 'f-number' when discussing relative aperture and lens speed. The f-number is a very important notation when thinking about exposure in photography. As you may recall, an aperture with a smaller f-number admits more light over a given period of time than one with a larger f-number. Here are the most common full-stop f-numbers, called f stops:
f/1.4 f/2 f/2.8 f/4 f/5.6 f/8 f/11 f/16 f/22 f/32
Each of these represents a doubling, or halving, of light exposure from the adjacent number. It is worthwhile to memorize this short sequence of numbers. Here are a few practical examples. Each of these represents a change of one f stop:
f/1.4 admits twice as much light as f/2
f/11 admits twice as much light as f/16
f/4 admits half as much light as f/2.8
f/2.8 admits four times as much light as f/5.6, because f/2.8 admits twice the light as f/4 and f/4 admits twice the light as f/5.6, and 2 times 2 equals 4, or an increase of two f stops. So you see, the f-number, or f stop, is a convenient system of notation in spite of its weird appearance. Stay with me now as this is important!
The concepts of aperture, shutter speed and ISO are combined for exposure as follows.
Consider a shutter speed of 1 second (shutter open for 1 second). It admits half as much light as a speed of 2 seconds. 1/120 second admits twice the light as 1/240 sec. So a halving or doubling of shutter speed equals a change of one f stop!
ISO follows a similar pattern: ISO 100 is half as light sensitive as ISO 200: 1 f stop decrease. ISO 400 is four times (2 X 100 = 200; 2 x 200 = 400) as sensitive as ISO 100, for a 2 f stop increase.
Aha! In spite of the non-linear appearance of the f-number sequence, we see that f-number (aperture), shutter speed and ISO are closely related through this doubling or halving of exposure.
Lets say you are considering a shot at ISO 100 at 1/60 sec. but you are concerned about hand-held camera shake. If you go from ISO 100 to ISO 200 and simultaneously increase the shutter speed from 1/60 to 1/120 second...presto! Your camera sensor's light sensitivity is doubled and your shutter-open time is halved, leaving your exposure essentially unchanged. And yet your camera shake is reduced (unless you move your camera about wildly, of course!)
Another example: You want to use ISO 100 to obtain a shot with the lowest sensor noise. At the same time, you want a fast shutter speed. An initial setting might be 1/500 second at f/11, but you could change that to 1/1000 second at f/8 and achieve the same exposure with a faster shutter speed.
It takes a while to get the hang of the relationship between f-number, shutter speed and ISO. Practice, and you will soon be tossing these numbers about like a pro!
M is for Manual exposure...do it!
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