While researching both the Blackmagic Pocket Cinema Camera and the Panasonic Lumix DMC-GH4 for my own personal use, I came across many reviews stating that it’s “okay” to have a Super 16 or a Micro Four-Thirds sensor because as long as you use a Speedbooster, you’ll be able to increase your sensor size. Many also state that this changes the crop factor of your sensor – even the Metabones website contains a comparison of crop factors.
I suppose this “decrease in crop factor” is an easy way to explain the fact that there is more in frame when using a Speedbooster than without. However, because “crop factor” is a property of the sensor (not the lens), this designation is erroneous.
Make no mistake – a Speedbooster does increase the field of view that the sensor sees. But it does so by optically modifying the field of view of the lens – not the sensor! Nothing changes in the hardware of the camera – you are still compressing the image onto the same sensor size, resulting in less detail (like when you down-scale an image in Photoshop).
Another way to think about this is that just because you’re looking through a telescope – your eye doesn’t change size!
The reason the field of view is greater with a Speedbooster is because it needs to be (in order for the adapter to “boost” the speed of the lens). Because the claim to fame of Speedboosters is that they lower your lens’ f-stop by as much as 1.6 stops, effectively upping the “quality” of your lens collection. But in order to get that extra light, the adapter has to widen the field of view. More in frame = more light in frame = more light in the sensor.
Don’t get me wrong: a Speedbooster is essential if you are shooting film with anything less than a Super 35mm sensor (though APS-C may still cut it for web). But it’s not a magic potion. When choosing between a full-frame camera like the Canon 5DMkIII or the Sony A7s, consider the extra cost and hassle that a Speedbooster will bring.
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