焦距转换率是摄影中的一个概念，它在不同底片格式中相同视角对应的不同焦距的创建对应关系。

一个简单的例子，135底片相机的标准镜头是 50 mm 镜头，而120底片规格的标准镜头则为 80 mm ，亦即这两只镜头在各自底片格式上的视角是近似的。当需要在不同底片格式上互换镜头使用的时候——如果可能——焦距转换率可以为摄影师提供一个换算计算依据，了解在另外一种底片格式上某一特定焦距的镜头会有在某一底片格式（常用135底片格式为基准）上的等效焦距视角。

焦距转换率可能也被称作焦距转换系数，焦距折算系数等。

In digital photography, the crop factor of an image sensor format is related to the ratio of the dimensions of a camera's imaging area compared to a reference format; most often, this term is applied to digital cameras, relative to 35 mm film format as a reference. In the case of digital cameras, the imaging device would be a digital sensor. The most commonly used definition of crop factor is the ratio of a 35 mm frame's diagonal (43.3 mm) to the diagonal of the image sensor in question; that is, CF=diag_35mm / diag_sensor. Given the same 3:2 aspect ratio as 35mm's 36mm x 24mm area, this is equivalent to the ratio of heights or ratio of widths; the ratio of sensor areas is the square of the crop factor.