KKR and Hellman & Friedman have also been linked with bids according to Reuters, which cited people familiar with the matter.
They said several bidders have already moved past the first round and were meeting with Carestream management.
Onex picked up Carestream through a $2.35bn deal in 2007, which one of the Reuters sourced said valued the company at five times EBITDA at the time.
They said Onex was hoping to make eight times EBITDA from the sale, but added the buyout bidders were hoping for a deal as low as six times.
Carlyle, TPG Capital, Blackstone and Singapore sovereign investor Temasek have formed a buyout group according to the Wall Street Journal, which said KKR was considering raising a rival consortium of its own.
Strategic buyers Danaher Corp and Thermo Fisher Scientific are also believed to be considering bids for the California-based company, which is currently valued at $12.5bn including debt.
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