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I'm confused after reading the question and the first reply. The asker asks,

I'm becoming more interested in landscape photography and want to add to my Nikon DSLR a wide angle lens, but I dont really understand what this means. According to Wikipedia (not the most reliable source I know!) common wide angle focal lengths are 35, 28, 24, 21, 18 and 14mm.

So this does this mean that the kit lens that comes with the Nikon (18-55) is considered to be wide angle at the shorter focal length?

Also, what is the difference between ultra-wide and normal wide angle? And would an ultrawide be useful for landscape photography?

The first reply says,

On a crop sensor body (DX) that lens has a focal length of 27-82mm

The question doesn't mention about the type of camera body (DX/FX) or lens (DX/FX). To my knowledge, for the explanation to be true, it has be an 18-55 FX lens mounted on a DX camera body. Can't then only the crop factor (1.5X) kick in? But, 18-55 FX lens does not exist in Nikon lineup.

The way I understand crop-factor is that a 12mm lens mounted on an FX body will provide the same field of view of an 18mm lens mounted on a DX body, provided the distance between camera and subject remains unchanged. Going by the logic, the reverse should also be true, i.e. an 18-55 DX lens on a DX body should be equivalent to 12-35 FX lens on a FX body.

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  • Sometimes it helps to draw simple ray diagrams so you can see the actual image size as well as the percent of the image area covered for each focal length lens. So far as "wide" vs. "ultra-wide" labels go, these are not locked to exact focal lengths and 35-mm film - equivalent focal lengths. – Carl Witthoft Jun 15 '16 at 11:35
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    "The way I understand crop-factor is that a 12mm lens mounted on an FX body will provide the same field of view of an 18mm lens mounted on a DX body," It's the other way round. 12mm on FX is very wide, so wide there are barely a few lenses that offer such a focal length. – D. Jurcau Jun 15 '16 at 12:06
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    @CarlWitthoft Good advice. I actually drew such a diagram in my answer to What is “angle of view” in photography? – mattdm Jun 15 '16 at 18:24
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The way I understand crop-factor is that a 12mm lens mounted on an FX body will provide the same field of view of an 18mm lens mounted on a DX body, provided the distance between camera and subject remains unchanged.

You have it backwards.

What crop factor means is that an 18mm lens on DX yields the same field of view as 27mm (18mmx1.5) on full frame.

A focal length is a physical property of the lens, and that doesn't change just because you change the size of the sensor you put behind it. What you need to do is to think of using a smaller sensor as cropping.

From the dpreview review of the Canon 5D:

crop/full frame demonstration

The smaller format with the same focal length yields a smaller frame. To get an equivalent FoV to full frame, you need a wider/shorter lens. So, to equate a smaller format to a larger one, you multiply by the crop factor. Equating a larger format to a smaller one is when you divide (e.g., DX equivalence to a 24 on FX would be 24mm/1.5 → 16mm).

See also:

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for the explanation to be true, it has be an 18-55 FX lens mounted on a DX camera body. Can't then only the crop factor (1.5X) kick in?

No. The focal length of a lens is the same no matter what size sensor is in the camera body. But a smaller sensor sees only the middle part of the image that the lens projects, so the field of view as seen by the sensor is smaller than that seen on a full frame camera. So, even though the focal length ƒ is the same, the lens gives the same field of view that a longer lens would on a full frame camera.

The reason that Nikon lenses have a FX or DX designation is similar to the difference between EF and EF-S lenses in Canon's system. Lenses made specifically for use with APS-C sized sensors don't need to project an image that can cover a full frame sensor, which means that the optics can be smaller, lighter, and less expensive. You can put a FX (or EF) lens on a DX (or EF-S) body with no problem -- the large image circle will still cover the sensor just fine. But if you put a DX lens on a FX body, the image won't cover the entire sensor. For that reason, Nikon full frame bodies have a DX mode that uses just the center of the sensor. Again, because the sensor (or the part that's being used) is smaller in DX mode, you have to factor in the crop factor to figure out what field of view you'll get with a given lens, but the focal length remains the same.

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Lenses are described by their focal-length because this is a physical property. It does not change with sensor size.

What people really care about though is the angle-of-view. The angle-of-view provided by an 18-55mm lens on a DX sensor is the same as the angle-of-view of a hypothetical 27-82mm lens on a full-frame sensor. This is why it is said to be equivalent, because the resulting angle-of-view is the same.

The 35mm format - now known as full-frame - has become the de-facto standard even for fixed-lens cameras. This is why you see small ultra-zooms with lenses described as 24-240mm but if you read the labeling on the lens itself, you will often see that the actual focal-range is something like a 4-40mm (made up numbers for easy math). Since people are assumed to know what angle-of-view a 24mm or 240mm lens on a full-frame produces, 24-240mm conveys more information than 4-40mm.

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  • "The angle-of-view provided by an 18-55mm lens on a DX sensor is the same as the angle-of-view of a hypothetical 27-82mm lens on FX sensor" - probably would be better – Holmes.Sherlock Jun 19 '16 at 1:20

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