Professional digital SLR cameras, hereinafter referred to as CZK, have a full-frame matrix, which is the size of a standard 35mm film. Entry-level CZKs have a smaller matrix.
They are called cropped CZK including here https://www.artlook.us/service/gay-wedding-photographers-nyc/. The crop factor shows the ratio of the CCD sensor to the standard full-frame sensor. For example, entry-level Nikons, such as the Nikon d3100, have a crop factor, that is, a matrix, that is 1.5 times smaller than a full-frame matrix.
This means that the FR of the lens for such a CZK must be multiplied by 1.5. For example, an 18mm FR on a Nikon d3100 will have the same field of view as a 27mm FR at full frame. And FR 50 mm will be like 75 mm in full frame.
In order for the viewing angle on a cropped CZK to be the same as on a full-frame one, you need to multiply the FR of the lens by the crop factor of the CZK.
There are lenses specifically for cropped matrices; Nikon has such lenses marked DX. But the FR on such lenses still correspond to the full frame. Remember, lenses for cropped CZKs cannot be used in full frame, lenses for full frame can be used in crop.
