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10-08-2014, 10:12 PM   #1
PiDicus Rex's Avatar

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Focal Reducer / Speed Boosters ?

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Anybody had any experience with one of these?

Focal Reducer Speed Booster Adapter Pentax K Mount PK Lens to Micro 4 3 GX7 M43 | eBay


Quenox Focal Reducer Speed Booster Adapter Pentax K - Micro Four Thirds buy from online shop

I have the Fotga K-to-M4/3 unit that is just a mount adapter, which I had to shim to get the FFD correct on, now curious to see if the units that compress a FF image circle down to super35 or M4/3 (16mm) are useful. The price difference for the Roxen unit vs a Metabones product is massive, and that's going by the EF units, as Metabones don't make a K-mount version.

10-09-2014, 04:00 AM   #2
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There's been a few threads in the non-Pentax partof the forum about Lens Turbo/ Speedboosters etc on NEX but not M4/3 - eg:

and a few other threads.

They seem to work OK, but some Pentax lenses are quirky, as are some of the Lens Turbos.

I'm waiting for someone to make a FF PK to APS-C PK Speed Booster/ Lens Turbo, to turn stuff like my old 135 f2.5 into a 85 f1.2
10-09-2014, 08:05 AM   #3
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QuoteOriginally posted by rawr Quote
FF PK to APS-C PK Speed Booster/ Lens Turbo
I don't think that would work - the glass element would have to be inside the mirrorbox because of the shared Flange Focal Distance - it could be done, but only for a K-01
10-23-2014, 08:22 AM   #4
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QuoteOriginally posted by PiDicus Rex Quote
it could be done, but only for a K-01
All the current focal reducers reduce the original flange distance (i.e. the Lens Turbo and Metabones Speed Booster is shorter than the "dumb" adaptor).
Furthermore, even if one is to design the focal reducer so that all the elements sit where the K-01's mirror would have been, how does one attach this adaptor without relying on the camera's mount and thus also extending the flange distance?

10-24-2014, 04:53 AM   #5
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All those adapters are for going between mount types - K to M4/3, or K to E-mount - they all go from a mount with a long FFD to one with a shorter FFD.
The one theorised above is K to K, so the optics would need to maintain the FFD.
Which puts the optics inside the Mirror Box,...
It 'could' be done the same way as for rear-mounted filters, which screw in to the back of the lens, but it's still going to extend the lens inside the body of the camera, where all the 'dumb' and 'speedbooster' types keep the optics outside the bodies.
10-24-2014, 07:57 AM - 2 Likes   #6
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Just a simple reminder. these adaptors, and also the IN Glass mount adaptors for SLRs in general, use simple optical elements in order to correct the infinity focus point of the lens..

In the case of most SLR adaptors, the throat of the lens mount is so close between all makers, that aa relatively thick adaptor is needed just to mount the lens, leading to the necessity of a magnifying element to correct for infinity focus. Canon, with its very short regestry distance and very big lens throat, is the exception where it allows for adaptors with no optical correction to retain infinity focus.

For the Micro 4/3 market, regestry distances are quite short compared to SLRS, and simple mechanical mounts can be made with out optical correction, however, the crop factor of the M4/3 format makes almost every lens a tele, which is not good for a lot of users, so they shorten the mechanical spacer and put an optical element to correct for this, but it shortens the focal length also.

For a lens where you wish to extend the mounting further than the regestry distance AND also reduce the focal length, you need a special retrofocus group, invented by Angénieux to achieve this function. this is how modern wide angle lenses are made, because clearly with a 46mm regestry distance, any lens shorter focal length than this can't be mounted to a pentax SLR.

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