if u put a tube b4 a smc da lens u will see just darkness ( max. f value f22 f40 or what ever..)
The point is ' apreture ring '
there is a pin which controls apreture automaticly from body. when u put a extension tube between lens and body, u kill the control mechanism.
manual lenses which have manual apreture control mechanism allows you to control apreture manually thus, u need to use a lens which allows u to control apreture ring
there is another pratical solve which I always use with my smc da 50-200 lens
u can easily make an aluminium foil ball between your fingers and put it next to that control mechanism pin. it will hold it on min. value of f
this pic. will help u to see that pin