Thread: Metabones speedboosters and the pocket 4k

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  1. #1 Metabones speedboosters and the pocket 4k 
    Hey folks! Okay, got a question.

    I own the Pocket 4k with the generic Metabones Ultra .71x speedbooster. I use the Sigma 18-35 on it.
    Now, I know, as I'm sure a lot of you here know, at 18mm there is slight vignetting. I think what A LOT of people miss is that this camera is NOT a micro 4/3rds. It even says so on the packaging of the box. It is a FULL 4/3rds camera sensor. I'm wondering if that is why there is slight vignetting (in the 4K DCI format) at 18mm. It's gone by 20-21mm, which is just fine.

    Okay. Now for the question.

    Does the DEDICATED Metabones BLACKMAGIC POCKET 4K Speedbooster eliminate that vignette at 18mm, giving the shooter the full range of the focal lengths on that lens? I know one benefit is that the speedbooster rear element comes up closer to the sensor compared to the generic 4/3rd speedbooster, and thus creates a slightly sharper image.

    But what about the vignetting? Is that gone? Any help would be greatly appreciated! Thank you!
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  2. #2  
    Senior Member Samuel H's Avatar
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    Micro four thirds and full four thirds sensors are exactly the same size: the "micro" means that there's no mirror and the focal flange distance is smaller.

    Now, leaving language behind... the p4k has a 18.96x10.00 mm sensor, whereas standard m43 is 17.3x13 mm. In terms of horizontal, which is what *I* care about when I calculate equivalent focal length and DoF (because I'll crop the taller image formats anyway), that's a 1.90x vs 2.08x, which kind of implies that the p4k has a much-bigger-than-four-thirds sensor. But in terms of the diagonal, which is what matters for vignetting, it's 21.44 mm vs 21.64 mm, so, the p4k sensor actually requires a smaller image circle than the m43 standard.

    In terms of crop factors, those diagonals imply 2.02x and 2.00x. The Sigma 18-35 must be designed for the biggest APS-C sensor commonly available, which I think is 1.52x. Apply your 0.71x speedbooster to that, and you get an image circle with 2.14 crop factor. So yes, if it's designed to just-about cover APS-C sensors, it can vignette badly on the p4k, and even slightly more in a standard m43 sensor.

    That 0.71x reducing factor is exactly one stop of light, which is convenient for exposure calculations, and it works great if you're using FF lenses on APS-C sensors, but here it's just not ideal.
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  3. #3  
    Senior Member jambredz's Avatar
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    I can use my PL mount tokina 11-20mm which is made for APSC/super 35mm, plus my metabones (pocket 4k version) x71 Speedbooster with no vignetting at 13mm.
    Darren Scott
    Freelance Director/Director of Photography


    https://vimeo.com/jambredzvisions/videos
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