BMCC how to calculate Field of View?

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James Moore

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BMCC how to calculate Field of View?

PostSat Aug 12, 2017 3:47 pm

Hi,

I have a BMCC with a metabones speed booster (I believe it is the .58x version) and a Nikon 12mm-24 mm zoom lens mounted on the camera. I shot wide at 12mm. I would like to know what the Field of View (horizontally) expressed in degrees is. I am having trouble wrapping my head around the math.

Thanks in advance for any help offered.

Sincerely,

James
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Lee Gauthier

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Re: BMCC how to calculate Field of View?

PostSat Aug 12, 2017 5:10 pm

To calculate this you need the following measurements:

* Sensor size

* Focal length of lens

To calculate AOV without speed booster, use this (excel) formula:


AOV=DEGREES(2*ATAN(SensorDiag/(2*FocalLength)))

To determine AOV with speed booster:

sb_AOV = AOV/.58

Where .58 is the multiplier of your speed booster.
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James Moore

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Re: BMCC how to calculate Field of View?

PostSat Aug 12, 2017 5:47 pm

Thanks for you help Lee!

With respect to focal length I found:

12 - 24mm
Comparable 35mm Equivalent on DX Format Focal Length: 18 - 36 mm

https://www.bhphotovideo.com/c/product/ ... AF_S.html#!

So, the focal length when shot at 12mm is 12mm?

As to the BMCC sensor size I found:
16.64 x 14.04mm
https://www.dpreview.com/articles/50104 ... ema-camera

I should be able to calculate the answer using the above data. Do those numbers seem right?
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James Moore

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Re: BMCC how to calculate Field of View?

PostSat Aug 12, 2017 6:56 pm

hmmm, I came up with 5.39 degrees which doesn't seem right. Shouldn't it be near 90 degrees My math:

sensor diagonal:
(16.64)(16.64) + (14.04) (14.04) =z*z
276.8896 + 197.1216 =z*z
474.0112 =z*z
21.77= z

AOV=2*(ATAN(21.77/2*12))
AOV=3.126281362

AOV/.58 =5.390140279
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rick.lang

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BMCC how to calculate Field of View?

PostSat Aug 12, 2017 8:12 pm

The active sensor for your purposes on the BMCC is 2400x1350 photosites or 15.6x8.775mm based on the raw image size. The camera reads a larger area, 2432x1368 if memory serves me, but the extra photosites just help in determining 'colour' values at the edges.


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Last edited by rick.lang on Sat Aug 12, 2017 8:18 pm, edited 1 time in total.
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James Moore

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Re: BMCC how to calculate Field of View?

PostSat Aug 12, 2017 8:16 pm

It seems that Excel gave me the weird inverse tangent result. Using a calculator I came up with a number more reasonable. A diagonal angle of view of 145.55 degrees. Probably me not using excel correctly.

A few questions:
Am I using the correct sensor dimensions? I found another website that had quite different (smaller numbers). I can't find any spec information here at blackmagic design - maybe they've discontinued the product but you would think the specs must be around somewhere on this site.

I have a BMCC micro 4/3rds mount. does the crop factor have to be taken into account to calculate the Angel of View?

Is the term "field of view" interchangeable with "angle of view"? I presume it is.

Any idea how I might identify which specific metabones speedbooster I have. I don't see any particular markings identifying it on the booster itself.
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James Moore

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Re: BMCC how to calculate Field of View?

PostSat Aug 12, 2017 8:17 pm

Thanks Rick, I'll try using that number.
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rick.lang

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BMCC how to calculate Field of View?

PostSat Aug 12, 2017 8:32 pm

According to the Angle of View iPhone app, at the wide end the horizontal angle is about 96 degrees and at the long end it is about 58 degrees with a 0.58x focal reducer. The lens with focal reducer acts like a 7-14mm zoom on the BMCC.


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James Moore

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Re: BMCC how to calculate Field of View?

PostSat Aug 12, 2017 8:44 pm

using the formula from above which is the same as the wiki formula I'm getting rather different numbers:

AOV=2*(ATAN(15.6/2*12))
AOV=2*(ATAN.65)
AOV=2*(33.02)
AOV=66.04

speedbooster .64x
66.04/.64 = 103.19

speedbooster .68x
66.04/.58 = 113.86

p.s. I'm just working with the wide angle at 12mm for now
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Re: BMCC how to calculate Field of View?

PostSat Aug 12, 2017 10:58 pm

pCAM app results:
BMCC with 12-24mm: H=66.75° V=40.61° and H=36.46° V=20.96°
With 0.58 speedbooster 6.96-13.92mm: H=97.28° V=65.07° and H=59.18° V=35.38°
Sensor size: 15.810 x 8.88 mm
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James Moore

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Re: BMCC how to calculate Field of View?

PostSun Aug 13, 2017 12:23 am

My head spins.

I was always under the impression that the metabones adaptor increased the zoom of the lens but everything here has it making it wider.
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Lee Gauthier

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Re: BMCC how to calculate Field of View?

PostSun Aug 13, 2017 9:19 pm

James Moore wrote:My head spins.

I was always under the impression that the metabones adaptor increased the zoom of the lens but everything here has it making it wider.


No, it makes it wider and brighter, which is a pretty neat trick.
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Tristan Pemberton

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Re: BMCC how to calculate Field of View?

PostMon Aug 14, 2017 2:14 am

James Moore wrote:Hi,

I have a BMCC with a metabones speed booster (I believe it is the .58x version) and a Nikon 12mm-24 mm zoom lens mounted on the camera.

The speedbooster with 0.58x reduction is for the Pocket Camera only. If you were to attach it to the BMCC it would likely collide with the IR filter/sensor, and cause damage.

If you have the purpose built speedbooster for BMCC it has a 0.64x reduction.
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Tristan Pemberton

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Re: BMCC how to calculate Field of View?

PostMon Aug 14, 2017 2:18 am

Lee Gauthier wrote:
James Moore wrote:My head spins.

I was always under the impression that the metabones adaptor increased the zoom of the lens but everything here has it making it wider.


No, it makes it wider and brighter, which is a pretty neat trick.

As Lee already mentioned, it doesn't magnify (increase effective focal length), it reduces (decrease effective focal length).
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rick.lang

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Re: BMCC how to calculate Field of View?

PostMon Aug 14, 2017 3:24 am

Robert's numbers are precise with the advertised sensor size. Close to the numbers I gave as approximate values based on the slightly smaller sensor size determining your actual image.


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James Moore

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Re: BMCC how to calculate Field of View?

PostMon Aug 14, 2017 9:32 am

I have the BMCC version of speedbooster so. 64x

Roberts numbers are for. 58x. What would the be for the .64? His AOV numbers don't appear to be derived from z/.58
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Robert Niessner

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Re: BMCC how to calculate Field of View?

PostMon Aug 14, 2017 10:46 am

BMCC with 12-24mm: H=66.75° V=40.61° and H=36.46° V=20.96°
With 0.64 speedbooster 7.68-15.36mm: H=91.65° V=60.07° and H=54.47° V=32.25°
Sensor size: 15.810 x 8.88 mm
Robert Niessner
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James Moore

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Re: BMCC how to calculate Field of View?

PostMon Aug 14, 2017 8:54 pm

Thank you very much for that Robert! The numbers seem to work out and the math makes sense to me.

A key factor appears to be to calculate the focal length of the lens BEFORE calculating the angle of view. Having the correct figure for the speedbooster and Sensor size is also handy ;)

12mm x .64 = 7.68

AOV=DEGREES(2*ATAN(SensorHoriz/(2*FocalLength)))

AOV= 2 x ATAN(15.81/2 x 7.68)
AOV = 2 x ATAN(15.81/15.36)
AOV = 2 x ATAN(1.029296875)
AOV = 2 x 45.827
AOV = 91.65

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