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Back focus issues (B4 lens on Canon dslr)

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James Wallace
Back focus issues (B4 lens on Canon dslr)
on Aug 10, 2017 at 11:19:03 am


I'm playing around with Magic Lantern Raw on the Canon 50d and achieving some good results so bought a cheap old Fujinon B4 7-50mm f2.8 zoom lens to experiment with recording full 1080p raw in crop mode whilst still achieving a reasonably wide frame (crop mode on the 50d gives a whopping 4x crop factor!!). It's a pretty beat up old thing, but I didn't want to fork out too much before knowing the basic concept was sound.

I'm using a simple non optical B4 to Eos adapter to mount the lens. Trouble is I'm having real issues setting backfocus on this camera/lens combo, not matter what I try the difference between sharp at 7mm and sharp at 50mm is nearly the full range of the backfocus adjuster and nothing I try will reconcile the two.

I have experimented with mounting B4 lenses on Micro 4/3 cameras in the past with similar adapters and have been able to set the backfocus with relative ease.

Has anyone else experimented with B4 Broadcast lenses on Canon DSLR's? Did you have similar issues?

I'm thinking there's a possibility that the lens is a duffer. The backfocus mechanism seems sound, but maybe an internal element is out??

But what I'm thinking is more likely is that the flange focal distance on Canon DSLRs is simply too large for this type of lens designed for much smaller mirrorless sensors.

I'd appreciate any thoughts/experiences you can relate.

Cheers :)

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