![]() Therefore, the f-number = focal length (mm)/ aperture diameter (mm). Unfortunately, the f-number naming convention doesn’t make it obvious that f/4 is twice as big as f/5.6 in the same way that ISO 400 is clearly an even multiple of 200 3 The amount of light let through a lens is dependent on two things: the focal length of the lens and the diameter of the aperture opening. The new Canon EOS1DX can be shot with an ISO as high as 102400, but the range above is very common on digital SLRs.Īs with shutter-speed scale above, each step to the right in this sequence cuts the amount of light getting to the sensor in half. ![]() One of my favorite black and white films was Kodak Technical Pan film, which I usually shot at ISO 12, two stops below ISO 50. However, many old films were rated at lower ISOs, and many digital cameras can shoot at higher ISOs. Each time the ISO number doubles, the sensor’s sensitivity doubles, meaning it requires only half as much light to produce the same exposure. The ISO scale is also pretty simple to understand. ![]() Of course, 8, 1/15th and 1/125th fudge the numbers slightly so that we can continue with an easy to remember sequence of numbers. Matthew Gore | Light And Matter 30 – 15 – 8 – 4 – 2 – 1 – 1/2 – 1/4 – 1/8 – 1/15 – 1/30 – 1/60 – 1/125 – 1/250 – 1/500 – 1/1000 – 1/2000 – 1/4000įor each step to the right in this scale, the shutter stays open half as long as the previous setting, which means that half as much light gets to the sensor (if everything else stays the same). Shutter SpeedĪs I just mentioned, the shutter-speed scale on your camera is built on a scale that starts at 30 seconds and is cut in half until it reaches 1/8000th of a second. To do so, you’ll need an understanding of the three exposure settings. With this in mind, it should start becoming clear how you can adjust any of the three settings but maintain the same exposure you simply have to make sure that if you remove a stop from one of the three settings, you must add a stop to one of the other two, or conversely, if you add a stop to one setting, you must subtract it somewhere else. This would mean that the photographer cut the amount of light getting to the sensor in half by doing one of three things: 1) reducing the shutter duration by half, 2) reducing the sensitivity of the sensor in half, or 3) reducing the size of the lens aperture by half. For example, a photographer might say “I underexposed by one stop to maintain highlight detail”. When used in this general sense, the term is usually shortened to “stop” rather than “f-stop”. More commonly though, and more importantly for this article, an f-stop is a unit of measurement for exposure: a change of 1 f-stop = a doubling or halving of the amount of light getting to the sensor, or in the case of ISO, the sensitivity of the sensor to light 1 This is more properly called a “step”, when you’re not talking about aperture, but most professional photographers still use the term “stop” instead. You might hear someone ask a photographer, “What f-stop are you using?”, and the answer would be a simple f-number, like “I’m at f/8”. In the more specific instance, an f-stop refers to a particular aperture setting, or “f-number”. The term “f-stop” is frequently used in two different ways, so let me take a moment to clarify. In fact, this is the unifying framework of exposure: every setting works in doubles or halves. The same is true of aperture settings: changing from one (full) setting to the next lets in twice as much or half as much light, depending on which way you turn the dial. ![]() At the same time, an exposure of 1/30th of a second lets in twice as much light as 1/60th. A sensor at ISO 200 requires exactly twice as much light for a correct exposure as one set to 400 (ie, 400 is twice as sensitive to light as 200). What do both of these series have in common? Each setting is double or half of the setting next to it on either side (or at least, very close). Second, if you had an old camera with a shutter speed dial, the common series of shutter speeds would include 1/30, 1/60, 1/125, 1/250, and 1/500 of a second. First, a common range of ISO settings is 100, 200, 400, 800, and 1600. Doubles and Halvesīefore we do, though, notice a couple of things that we’ve seen already. Now that you’re armed with a general understanding, it’s time to get down to the details that make the information useful in real world applications. In The Three Basics of Photography, I explained how shutter speed, aperture, and ISO relate to each other using the Bucket Analogy. Matthew Gore | Light And Matter Bride preparing for wedding: August, 2011 (1/60th f/2.8, ISO400)
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