I recently bought some vintage manual focus lenses to use with my Fujifilm X-T1. I'm most interested in using the depth of field scales on the manual focus lenses for zone focusing. However, I am wondering if the depth of field scales will work because my camera has a crop sensor. Do the depth of field scale markings only work precisely for full frame sensors because they were designed for 35mm film camera? In other words, will the distances indicated by the depth of field scale be inaccurate because of the smaller sensor size?
Note that depth of field markings never work precisely. The lens focuses somewhere, and it is only this exact spot (plane, really, and sometimes not even a flat plane) that is in focus. Depth of field markings show areas that will appear acceptably in focus. It's worth pointing out that everything within this range is not somehow in focus, with everything outside being out of focus – it's a gradual defocusing, so the depth of field limits are more guidelines than anything strict.
Also whether something appears acceptably in focus depends on other factors, such as degree of enlargement and viewing distance. In theory, an image made by an APS-C sensor requires a greater degree of enlargement than an image made by a full-frame sensor (all other things being equal), and so the apparent defocus effect on the APS-C image is exaggerated. So, yes, the DoF scale is for full-frame, and doesn't technically apply to APS-C. But, the key point is that the DoF scale is not a strict rule in any case, so if I were you, I'd experiment a bit and see what turns out acceptably in focus to you.