There was a period during the 50's when Kishore Kumar wasnt considered the singing equivalent of a Rafi or a Mukesh or even a Hemant.
Raja Venkateswar tells us it would be an interesting exercise to understand how and why the great music directors of that age, Naushad, Madan Mohan and the rest ignored the fresh and funny Kishore totally. Naushad for example never recorded a song with Kishore in the fifties when he was arguably at his peak.
Despite all the transformation that O.P. Nayyar and Shanker Jaikishen brought to the Hindi Film music scene they utilized Kishore only for the movies where he was the actor and even in such cases, used Rafi to sing for an on screen Kishore for various reasons (SJ in Sharat, OP in Ragini). Take for example this song from Film Shararat - "Ajab hai dastan teri ye zindagi.
On the other hand, SD even used Kishore for Karan Devan for example in Bahar - Vyjayanthimalas first film -
Bahar 1951, Qasoor Appka -


