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An easy example is when shooting architecture or landscapes. For those, you often might want to be able to fit quite a wide perspective, or large angle of view, in the frame.

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The practical angle of view of a lens varies based on the camera’s sensor size, so I’ve included some of the most common sensor sizes.

By having a reference point you can compare lenses in an apples-to-apples kind of way. That can be very handy when choosing the right lens for the job.

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Here’s a calculator for the angles of view from lenses on digital camera sensors, as well as angle of view charts for common focal lengths. And I’ve also included an explanation of why the diagonal angle of view matters more than you might think.

Yes, this angle-of-view calculator works for zoom and prime lenses. You can enter any single, specific focal length along the zoom range. For instance, for a 24-70mm zoom lens, you can enter 55mm to see the angle of view at that point in the zoom range.

This is for full-frame digital cameras with a sensor that measures 36mm x 24mm, which is the convention for a full-frame sensor. It also applies to SLR cameras using 35mm film.

Think of a marching band that travels across a smooth concrete parking lot onto a rough and muddy grassy playing field. The band members can take larger steps and move faster on the concrete than on the grass. Yet, being band members, they all remain in step, taking steps at the same pace whether on concrete or grass. This causes the band to change its direction as it moves from the faster concrete to the slower grass.

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The same general principles apply for film, but instead of using the size of the sensor you’d use the size of the exposed film plane.

Knowing the angle of view gives you a baseline reference for how wide a perspective you can fit in the frame. In a practical sense, I find it useful for choosing which lens is right for the job.

Q: What is the speed of light in a glass whose index of refraction is 1.60? A: The index of refraction of a medium is n = c/v. We can multiply both sides by v and divide both sides by n to get v = c/n Now we can use numbers to calculate v, the speed of light in the glass, v = ( 3.0 x 108 m/s ) / ( 1.6 ) v = 1.88 x 108 m/s Reflection Total Internal Reflection Back to "Reflection and Refraction" Page (C) 2003, Doug Davis; all rights reserved

Light behaves in much the same way as it moves from air, where it travels very fast, into another medium like glass or plastic or water, where it travel more slowly.

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When you see that a lens has so-and-so degrees angle of view, it’s actually telling you it’s from one corner to the diagonally opposite corner. It’s not telling you the angle of view from side to side.

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Reflection Total Internal Reflection Back to "Reflection and Refraction" Page (C) 2003, Doug Davis; all rights reserved

This bending of light is known as refraction. It is due to a difference in the speed of light in the two media. Instead of describing the speed of light directly, we usually talk about the index of refraction and label that as "n". The index of refraction is the ratio of the speed of light in vacuum, c, to the speed of light in a particular medium, v. That means

A larger angle of view corresponds to a wider lens, meaning you can fit more perspective within the frame. Conversely, a smaller angle of view gives you a narrow perspective, as you might get with a telephoto lens.

For 35mm film, that’s going to be functionally the same as using the full-frame option in the calculator above. I haven’t added other film sizes, but I might try to put together a separate calculator for that.

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The data here is calculated. For more specific data that applies to a particular model of lens, check the manufacturer’s spec sheets for that lens. Sometimes there might be, say, a half degree of variation, but in general, these rounded numbers should get you very close. I’ve compared them with a number of lens spec sheets and found the data here to be quite accurate.

The reason I’ve singled Canon out as a special case here is that Canon’s APS-C sensors are just a smidgeon smaller than most other APS-C sensors.

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That’s because it’s the spec that lens manufacturers include in their spec sheets. So when you see that a lens has so-and-so degrees angle of view, it’s actually telling you it’s from one corner to the diagonally opposite corner. It’s not telling you the angle of view from side to side. In that respect, it’s a bit like the size spec that TV manufacturers use (i.e., a 55-inch TV is 55 inches from one corner to the diagonal corner, not from one side to the other side).

A: The index of refraction for water is 1.33 and the index of refraction of glass is about 1.55 so the index of refraction of water is less than the index of refraction of glass. This is the same situation we had in the example oflight passing from air into water. So light will be bent toward the normal.

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Olympus and Panasonic Lumix are the most prominent manufacturers using this sensor format. In practice, there can be very slight variations in the dimensions of the sensors from model to model.

When you’re taking photos, the angles of view horizontally or laterally across the frame or vertically up and down the frame are probably the ones you’re most focused on. But it’s actually the diagonal measurement that comes up more often.

Q: If light travels from water into glass, will it be bent toward or away from the normal? A: The index of refraction for water is 1.33 and the index of refraction of glass is about 1.55 so the index of refraction of water is less than the index of refraction of glass. This is the same situation we had in the example oflight passing from air into water. So light will be bent toward the normal. Q: What is the speed of light in a glass whose index of refraction is 1.60? A: The index of refraction of a medium is n = c/v. We can multiply both sides by v and divide both sides by n to get v = c/n Now we can use numbers to calculate v, the speed of light in the glass, v = ( 3.0 x 108 m/s ) / ( 1.6 ) v = 1.88 x 108 m/s Reflection Total Internal Reflection Back to "Reflection and Refraction" Page (C) 2003, Doug Davis; all rights reserved

It’s also a fixed reference point if you’re using the same or similar focal lengths but on different-sized sensors. For instance, a 20mm lens will give you a wider angle of view on a full-frame camera than on a Micro Four Thirds camera.

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These charts offer an alternative way of visualizing the calculated data above. It might be useful if you’re approaching it from the other direction and trying to find a lens that has a particular visual coverage that you’re after.