To give you a better idea of how this all plays out in real life, look at Figures 4.3–4.7 and note the various focal lengths used to create each image. Photographed from the same position, the only difference between each shot is the focal length.

focal length中文

The numbers on a point-and-shoot lens mean the same thing, but the scale is radically different, so don’t panic if your lens indicates a focal length range of 6–22.5mm (the equivalent of roughly 28–105mm).

Lenses with larger maximum apertures (generally 2.8 or larger) are often referred to as being “fast” because the larger apertures allow more light to reach the camera sensor, letting you shoot with faster shutter speeds in low-light situations where you might otherwise need a tripod. (Remember, larger apertures are actually smaller numbers; thus, f/2.8 is a larger aperture than f/8.)

Aperture

These days, film has been replaced by digital sensors, and as luck would have it, they’re not all the same size. There’s a lot of math and science involved in the full explanation, but essentially, the discrepancies in sensor size are responsible for what we now refer to as “crop factor.”

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To determine your camera body’s crop factor, you’ll have to read some of the techno babble in your user guide or jump online and look it up. Crop factor is typically listed among all the other tech specs related to your camera and usually has a value of something like 1.3, 1.5, or 1.6.

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For the sake of example, let’s say your camera’s sensor has a crop factor of 1.5, and you’re curious how a 50mm lens would behave on it. To figure it out, take 50 (the focal length of the lens in question), multiply it by 1.5 (your camera’s crop factor), and you get 75. Thus, on your particular camera, a standard 50mm lens would behave like a 75mm lens.

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Depending on where you are within the zoom? What in the world does that mean? For example, if you’re zoomed all the way out wide to 24mm, you could achieve a maximum aperture of f/3.5. But once you zoom in, you lose the ability to open up your aperture all the way to f/3.5 as before and can then only open to f/5.6.

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Focal length

A range of numbers, expressed with a dash such as 70–200mm, indicates a zoom lens, capable of different focal lengths—in this case ranging from 70mm to 200mm.

Appearing on your lens right next to the focal length, you will find a numerical expression representing the maximum (largest) aperture opening that particular lens is capable of, usually expressed as “1:” followed by the maximum aperture. For example, a lens described as a 24mm 1:2.8 would be a fixed lens with a focal length of 24mm, whose largest aperture setting is f/2.8. The lens is still capable of smaller apertures (like f/11), but the largest it would be capable of is f/2.8.

The bottom line? Depending on your camera body (see the “Crop factor” sidebar), you may be able to get the close-up shots you’ve always dreamed of without having to pay for a super telephoto lens. In some cases, your 200mm lens might behave like a 300mm lens. Now that’s some serious bang for your buck!

It’s worth pointing out that some lenses are made specifically for cameras with smaller-sized sensors and have focal lengths that have already been converted. When in doubt, read the specifications or ask a knowledgeable salesperson.

Back in the glory days of 35mm film, it didn’t matter what camera body you used; as long as you were shooting 35mm film, the negatives were all the same size.

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A single number, such as 24mm, represents what’s known as a prime or fixed lens, meaning that it’s not capable of zooming. Such a lens is designed and optimized for only a single focal length—in this case, 24mm.

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The first number, or set of numbers, you see on your lens refers to its “focal length,” measured in millimeters (mm). Roughly translated, focal length relates to how close up or far away objects will appear when viewed through the lens. Essentially, the bigger the number, the more up close subjects will appear, and the smaller the number, the farther away things will appear.

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What is focal length oflens

In Chapter 1, we talked about aperture and the role it plays in creating an exposure (keeping in mind that aperture is a function of the lens itself, not the camera body). Similar to the pupil of your eye, the aperture can dilate or constrict, letting in varying amounts of light and affecting the depth of field.

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The result? Everything appears closer when shot on a camera body with a sensor that’s smaller than traditional 35mm film. Thus, a 50mm lens attached to a camera with a full-size sensor (referred to as full-frame) will behave like a regular 50mm lens. But the same lens on a camera with a smaller sensor (sometimes referred to as a cropped sensor) will be more like a 75mm lens.

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focal length是什么

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If you’ve spent time shopping for lenses, you may have noticed that a lot of zoom lenses feature not a single maximum aperture but, rather, an aperture range. If you have a lens that says 24–150mm 1:3.5–5.6, it means the lens is a zoom lens with focal lengths ranging from 24–150mm, whose maximum aperture varies depending on where you are within the zoom.

When you look closely at a lens, there’s a fair amount of numbers printed on either the front of the lens itself (Figure 4.1) or somewhere around the rim (Figure 4.2)—and though it may seem like a bunch of mumbo jumbo, it’s actually pretty important info and can tell you a lot about the lens and what it’s capable of.

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Confused? For a visual explanation, take a look at Figure 4.8 to see how your lens views the world. The large box displays the image as it would be captured on a full-frame sensor, and the smaller box shows how the scene would be captured on a smaller-sized sensor. Same scene, same lens—different effective focal length.

Depending on the camera body you have, you may have experienced or heard people talk about something known as “focal length crop factor,” “focal length multiplier,” or even just plain ol’ “conversion factor.” While this may, at first, seem confusing, it’s really quite simple and can sometimes be advantageous.

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Of course, if your camera has a smaller sensor and you like to shoot at a lot of wider angles, the opposite would also be true. The 24mm lens you loved at the camera shop might behave like a 36mm lens on your camera body, meaning you’d need an ultra-wide lens to get a standard wide-angle shot.

Lenses with focal ranges of 35mm or smaller are generally considered “wide,” and lenses with focal lengths of 85mm or more are often referred to as “telephoto.” A lens around 50mm is roughly close to the way we see things with our eyes and is generally considered neither wide nor telephoto. It is often referred to as “normal” or “standard.”

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