Crop Factor (Not Full Frame related)

[Martin Facci]

Hi

I don't know if this is the exact forum for this topic but i hope it is

So... I have to shoot with a GH5 and i was looking around about crop-factor information and i hit something that never was pretty clear for me.

I understand that the crop factor it's found dividing the largest side of the sensor between the 2 sensor sizes. For example, the claimed " x 2.0 " crop factor of the GH5 is calculated dividing the FF sensor size (36mm) between the GH5 17,3mm sensor. So, 36/17,3 = 2,08

So every manufacturer calculates their crop factor from a 36mm full frame sensor.

RED (Dragon) and ARRI cameras does not have a 36mm full frame sensors, they have 30,7mm and 28,17mm respectively.

Does this mean that if i shoot with a GH5 (17,3mm sensor) and i calculated the crop with, for example, the ARRI sensor (28,17) my crop factor relative to the ARRI sensor would be 17,3 / 28,17 = 1,77 and with a Speedbooster that is 0.64 should i get a "crop" of x 1.13 relative with i would get with an ARRI sensor?

Summing up... If i put a 35mm (Meaning an 35mm lens not a 35mm focal lenght) lens on a GH5 with a x0.64 speedboster whoud i get a field of view very close to what an ARRI sensor would see? Just beeing crop by x1.13?


I'm not taking in count the fact that the GH5 has diferrent sensor crops depending on the shoooting mode. Just wondering if that "math" is correct.


Hope you can help me...

Thank you!!

 

[Björn Benckert]

Better use this and compare instead of converting with a crop factor in between.

https://www.pointsinfocus.com/tools/depth-of-field-and-equivalent-lens-calculator/#{"c":[{"f":13,"av":"8","fl":35,"d":3048,"cm":"0"}],"m":0}

 

[Aaron Lochert]

That's exactly right. You can calculate a crop factor between any two formats. Divide your "master" format by your comparison format and you'll have your answer.

RED Dragon width: 30.7mm
Arri Open Gate width: 28.17mm
Arri 16:9 width: 23.76mm
GH5 width: 17.3mm

30.7/17.3 = 1.77x crop factor from Dragon to GH5
28.17/17.3 = 1.62x crop factor from Alexa Open Gate to GH5
23.76/17.3 = 1.37x crop factor from Alexa 16:9 to GH5

So yeah, you then can take any of those numbers and multiply them by your Speedbooster's 0.64x reduction and get your final effective crop factor.

1.77 x 0.64 = 1.13x effective crop factor from Dragon to GH5
1.62 x 0.64 = 1.04x effective crop factor from Alexa Open Gate to GH5
1.37 x 0.64 = 0.88x effective crop factor from Alexa 16:9 to GH5

So your 35mm + Speedbooster on GH5, a RED Dragon with a 40mm lens, an Alexa with a 36mm lens in Open Gate, and an Alexa with a 31mm lens in 16:9 mode will all be visually similar as far as field of view is concerned.

Of course, I did it in multiple steps, but you could plug all of that into one equation:

(Sensor Width 1 / Sensor Width 2) * Speedbooster factor * focal length

(30.7/17.3)*0.64*35 = ~40mm Dragon
(28.17/17.3)*0.64*35 = ~36mm Alexa Open Gate
(23.76/17.3)*0.64*35 = ~31mm Alexa 16:9

Does the GH5 actually change the width of the recording area when shooting different modes? I thought I heard somewhere that 4K mode didn't have that extra bit of cropping going anymore, like the GH4 did. I'm pretty unfamiliar with what prosumer cameras are doing these days to be totally honest.

Edit: Just now noticed you said 35mm format lens not 35mm focal length so I didn't need to dive in as far, but I'll leave my explanation there as an example.

 

[Martin Facci]

I not sure about the part of the crop on the GH5, i do think that you are right and there are no differences in crop between the modes in the new models but i have to look into it.

Thank for the fast replys!

 

[David Peterson]

Yes, the difference between a Micro Four Thirds camera with a focal reducer and the classic S35 film size is basically nothing really, they're almost the same.