[eeyore]

although that said i have a non digital tamron 90 and that is fine

very cheap makes such as cosina / vivitar etc (actually made in the same factory) use cheaper glass to keep the price down and this leads to a greater degree of chromatic abberation , and distortion it has little to do with being optomised for digital as they will give mediocre results on film also

and to my mind build quality is an issue - if it is cheaply made the focussing, aperture adjustment etc will not be as smooth or accurate - and also the lens will not stand up to life in the field , and is likely to die an early death. it is definitely better to buy a well made lens even if it costs three times as much as it will undoubtedly last more than three times as long and will take better pictures too.