The Basics of Aperture and Balancing Exposure The reason that the higher end lens with the constant max aperture is far more expensive is because the lens manufacturer has to figure out how to maintain the same max aperture without actually having to increase the diaphragm diameter. Typically, it is the high-end zoom lenses like the Canon EF 24-70mm f/2.8L USM Standard Zoom Lens that have a constant max aperture, while the cheaper zoom lenses like the kit lens that comes with a DSLR like Canon EF-S 18-55mm f/3.5-5.6 IS II SLR Lens have a variable max aperture. Zoom lenses come in two varieties, variable maximum aperture and constant (or “fixed”) maximum aperture. Fixed Aperture Prime Lensīecause they can gather light quickly, prime lenses with large apertures (f/1.8 or wider) are considered “fast lenses.” On the other hand, a “slow lens” might have a maximum aperture of f/4. Some lenses, like the Voigtlander Nokton 25mm or the Leica Noctilux 50mm have a maximum aperture of f/0.95! Now that is blazing fast! Of course, you pay the high price for these lenses, with the Leica Noctilux retailing for just under $13,000. This is what is known as the lens speed or the maximum aperture available for that lens. You may have noticed that lenses are typically labeled with an f-stop number. Here is the same diagram with the 1/2 stops and 1/3 stops added. These fractional f-stops gives you more control over how much light you allow in. In between the full f-stops are 1/2 stops and 1/3 stops. How Is Aperture Measured? Getting to Know Other Stops of Light So here is a diagram that shows you the list of full f-stops from f/1 to f/22: So starting back at f/4, to go one full stop up, you divide by 1.41 to get f/2.8. Now, when you want to double the amount of light coming in, or go one full stop up, you divide by 1.41. So unlike the pizza slices where a 1/8 pizza slice is half the size of a ¼ pizza slice, an f/8 aperture lets in a quarter of the light compared to an f/4 aperture. If you cut that light in half again, you get f/8. So, going from f/4, if you want to cut the light coming in by half or go one full stop down, you multiple f/4 by 1.41, which gets you f/5.6. What Is a Full Stop of Light?ĭoubling or halving the light is called one full stop. In order to double or half the amount of light coming in, multiply by a factor of √2 (approximately 1.41). With f-stops, the halving/doubling relationship doesn’t quite work in the same way. Going back to the slice of pie analogy, we all know that a 1/8 slice of pizza is half as big as a ¼ slice of pizza, and a 1/16 slice is going to be half as big as a 1/8 slice of pizza. Got that? Ok, because the basics of aperture may get a little more confusing from here. How Is Aperture Measured? Introducing the Factor of √2 In other words, a large or open aperture has a low f-stop number, while a small or closed aperture has a high f-stop number. So an aperture opening with a lower value like f/4 is a larger than an aperture opening with a higher value like f/8. A quarter (1/4) slice of pie is larger than an eighth (1/8) slice of a pie. A way to visualize this is to think of slices of a pizza pie. This means that just like a fraction, the larger the number is, the smaller the diameter opening. The main thing that you have to remember about f-stops is that they represent inverse values. A Pizza Analogy for Understanding F-Stops Unlike shutter speed, for instance, which is measured in absolute value of time, an f-stop is relative to the length of the lens and the diameter of the aperture opening itself. Now, for a 100mm lens, you only need an aperture diameter opening of 25mm to get to f/4. This is because aperture is measured in what is known as f-numbers or f-stops, which is the ratio of the lens’ focal length divided by the effective aperture diameter. In other words, if you take a 200mm lens and divide it by a 50mm aperture diameter opening, you end up with an f-stop of 4, or f/4. Understanding how is aperture measured can be confusing at first. Some modern lenses, and the majority of vintage lenses, have an aperture ring on the lens that allows you to control the diaphragm directly on the lens. Most modern lenses allow you to control the size of the aperture opening from the camera itself. The wider the aperture opening, the more light pours into the sensor. The wider you open the faucet, the more water pours out. Just think of this opening like the valve handle of a water faucet. Alongside shutter speed, aperture limits the amount of light that can hit the sensor at any given time. Moreover, a set of overlapping metal blades, called a diaphragm, determines the size of the aperture’s opening. The simplest explanation for understanding the basics of aperture is this: aperture refers to the opening inside the lens in which light can shine through. The Basics of Aperture Guide: How Is Aperture Measured?
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