Metabones Speedbooster questions

[Manuel Rodrigues]

Hi There,

 

I have two questions on the Metabones speedbooster which I own. I can't find a manual on the metabones site so ill try my luck here.

 

First question:

according to this graphic:

 

BMCC Matrix S-1.png

 

The crop factor is 1.75 when using the BMD pocket cam with the Speedbooster and full frame lenses. As far as I know this means that ilI have to multiply the focal length of my lenses with 1.75 to get the actual focal length behaviour on my camera. So a 53 mm lens becomes a 61 mm lens. This is what happens in my experience. But Metabones states this in the same article:

 

"For example, a 50mm f/1.2 becomes a 32mm f/0.80, and Sigma’s new 18-35mm f/1.8 will become an 11.5-22.5mm f/1.1."

 

I find this confusing. They say that the focal length will become shorter and longer in the same article!?

 

Link to the article:

http://www.metabones.com/article/of/metabones-nikon-g-to-bmcc-bmpcc-speed-boosters-available-starting-today

 

Next question:

 

In the same article they say:

 

"Nikon mount versions of both new Speed Boosters feature an improved version of Metabones’ unique manual G aperture adjustment mechanism, which has the same angular throw distance as earlier Nikon Ai/AiS lenses. This improved version features a linearized f-stop adjustment as a function of aperture ring adjustment angle. The mechanism is clickless in order to maximize its usefulness to cinematographers."

 

What is this exactly and how does it work?

 

Again, ive tried to find a manual but it is not available on the metabones site.

 

Hope you can help!

 

Thanks!

 

Manuel

Edited March 27, 2014 by Manuel Rodrigues

[Manuel Rodrigues]

53mm should be 35mm off course...

[Mark Dunn]

It's an optical adapter which reduces the focal length and focal ratio so the crop factor isn't directly comparable. There's a reducing lens in it.

The aperture explanation seems clear enough to me. It has equal spacing between stops and doesn't have detents so you can pull stop or set intermediate stops.

[Dom Jaeger]

I find this confusing. They say that the focal length will become shorter and longer in the same article!?

 

You're confusing crop factor with changing the focal length, which is not uncommon really.

 

Crop factor is a term that has crept into the filmmaking lexicon thanks to DSLR proponents, but it seems to cause more confusion than clarity. It uses full frame 35mm still photography as a standard by which to compare the angle of view a lens might give on another format. It doesn't mean the focal length changes. It simply means the view you might see on a full frame 35mm still camera will be different when the same focal length lens is put on a 35mm cinema camera or a BlackMagic or whatever the next camera of the month may be, because the sensor sizes are different.

 

The sensor is literally cropping the image circle of the lens. But the focal length is unchanged.

 

Unless you can only think in terms of a full frame camera's angle of view with a given focal length it's a waste of time to talk about crop factors really. You should just get used to how a particular focal length looks on your BlackMagic. The only use for crop factors I can think of is deciding what range of focal lengths you might need when you first buy the camera.

 

With your SpeedBooster however, you are getting an actual change in the focal length of the lens you attach. A 50mm becomes a 32mm, not because of crop factor, but because the optics in the SpeedBooster are condensing the image circle of the lens. This means the image circle that used to cover a 35mm full frame (and the angle of view contained in that image) now covers a smaller frame. It effectively makes the lens a wider focal length with a smaller image circle. Since the same amount of light is being condensed into a smaller circle, the added benefit is a brighter image, effectively "speeding up" the lens.