### Author Topic: Calculate lens crop on Raw  (Read 2946 times)

#### theartofweb

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##### Calculate lens crop on Raw
« on: March 29, 2015, 12:34:49 AM »
Hi,

I have a 40mm f/2.8 on the 5D2 I'd love to figure out a way to calculate the crop factor when I am shooting at 2.35:1 at different resolution: 1280x544, 1536x654 and 1792x762. I understand the higher the resolution the wider the field of view. Yet, I need to calculate how wide is it on the 40mm lens (45mm, 50mm?).

Thanks

#### Walter Schulz

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##### Re: Calculate lens crop on Raw
« Reply #1 on: March 29, 2015, 01:28:27 AM »
Shooting in crop mode:

40 mm x crop factor
Crop factor = Sensors full pixel count (long side) / cropped pixel count (long side)

#### theartofweb

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##### Re: Calculate lens crop on Raw
« Reply #2 on: March 29, 2015, 01:47:55 AM »
Thanks. I think on the 5d2 that should be 5616 / the long side eg 1280. Yet, I am not shooting in crop mode just normal mode. Does that mean I have do something like 5616 / 3 and then divede that by 1280?

I have noticed on the 5d2 the only resolution that covers the liveview is 1792x(...) should U use this number as a reference?

#### reddeercity

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##### Re: Calculate lens crop on Raw
« Reply #3 on: March 29, 2015, 03:25:07 AM »
I have a 40mm f/2.8 on the 5D2 I'd love to figure out a way to calculate the crop factor when I am shooting at 2.35:1 at different resolution:
1280x544, 1536x654 and 1792x762.
Thanks
If I understand the Question , Take Maximum Width In 1:1 which is 1856 or on Older nightly builds 1880 Divide by the new width = Crop factor
It seems to work for me e.g. 1856/1280=1.45 Crop from full frame 1:1mode
So to get the Equivalent focal length Multiply by the crop factor : 40mm @ 1856 = to 1.45*28mm =  40.6mm
I'm quite should there is a way to Calculate this in a formula but Math is not my Strength
If you shoot a scene @ 40mm at 1856*xxxx  and then drop down to 1280*xxxx  That's a Crop factor of 1.45, you need  28mm to get the
Same  Field of View.
1:1 width/new width=Crop Factor
Focal Length/Crop Factor=the Equivalent Length.
From my example:
Using 40mm lens
1856/1280=1.45
40mm/1.45=27.586mm round up 28mm For the same field of View
So , 1280=1.45  1536=1.208 1792=1.035

If you do some web searches you will find a great wreath of information about this topic.

I hope I understand what you are asking .
You want get the Same Field of View  At different frame resolution ,Right ?

#### theartofweb

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##### Re: Calculate lens crop on Raw
« Reply #4 on: March 29, 2015, 09:36:39 AM »
I am sorry, I was wrong in mentioning the 40mm focal lenght

My goal is to be able to have a 21-31mm zoom by using only a 21mm prime on the 5d2.

Aspect ration is 2.35:1 So, based on your calculation:

at 1856 (x1,00) the equivalent lenght stays the same (21mm),
at 1536 (x1.21) the equivalent lenght is ~25mm,
at 1280 (x1.45) the equivalent lenght is ~31mm.

I will then upscale to a fixed resolution (1920) in DaVinci Resolve.

Should I need longer focal lenght then 31mm I will change lens and apply the same math

Thanks