Originally Posted by Daisy Mae
Cheapness; end of story!
Using any old film lens on a DSLR, with or without adaptor, (they are not always necessary) will almost inevitably mean;
manual aperture (via ring on lens) which doesn't communicate to camera body, therefore;
exposure meter doesn't know what's going on (shoot and review histogram and blinkies to get correct exposure)Plus their image quality isn't as good as current lenses, particularly on things like the lens coatings to control flare. Then there is the possibility it may be damaged in some way; scratched, fungus, stiff or loose controls, etc.
aperture not recorded in EXIF data for future reference
Reversing an old lens - i.e. using a filter to filter adaptor ring is a technique for extreme macro shooting, not general photography.
Hope that helps, ask further if anything there isn't covered sufficiently.