A "nifty-50"mm lens on an APS-C (1.5x crop factor) camera will have a field of view more akin to a 75mm lens on a full-frame camera. The 35mm focal length lens on an APS-C camera is needed to get the traditional 50mm “normal” field of view of the classic 50mm lens. The aperture value of a FF lens does not change; the smaller sensor just doesn't make use of all the light (crops it away). And the smaller sensor is not using the lens's full image circle, resulting in a cropped field of view. All of these effects are roughly equivalent to the crop factor i.e. a 50% crop (area) equates to 1 stop more DoF, 1 So, as other are saying, a telecompressor is specifically for use on full frame lenses and APS-C camera body. You don't need the speed increase with a full frame camera, just use the lens natively (with a Metabones, or equivalent Canon EF to Sony E adapter) since the larger sensor offers this same "speed" boost by being able to shoot at higher I've got a question regarding focal length conversion of lens to/from APS-C sensors: Full frame lens to APS-C: 1.5(ish) X focal length I get that since we're taking a larger area, and cropping it down by (theoretically) making the focal length of the lens longer, to bring down to the smaller sensor size of APS-C cameras Equivalence helps compare different lenses and cameras by using 35mm 'Full Frame' as a reference point. This doesn't mean that Full Frame is the best, or even the optimal format, it's just a reasonably well understood point of comparison. However, it's not just focal lengths that can be thought of in equivalent terms. The 'crop sensor' crops the angle of view, compared to 'full frame'. A 35mm lens on an APS-C camera gives a similar angle of view to a 50mm lens on a full frame camera (a useful general purpose lens). A 50mm lens on an APS-C camera gives a similar angle of view to an 80mm lens on a full frame camera (useful for portraits). Nikon Z50. A little gem of a camera with some powerful features. Sensor: APS-C CMOS Megapixels: 20.88 Lens mount: Nikon ZAF System: Phase detection with 209 AF points, Eye AF and Subject Tracking Viewfinder: 2.36million-dot electronic viewfinderScreen: Tilting 3.2-inch 1,040,000-dot touchscreen, Max video Resolution: 4K at 30fps and Full-HD at 120fps Dedicated fisheye conversion lens for the FE 28 mm F2 provides 16 mm focal length with fisheye perspective and a full 180° angle of view (diagonally). The converter has been specifically designed for the FE 28 mm F2, and therefore offers expressive fisheye perspective with excellent image quality from infinity down to the closest focusing distance, without compromising the basic image quality GCce7X.