GH2 Variable Aspect Ratio

Started Apr 23, 2012 | Discussions thread
Steen Bay Veteran Member • Posts: 6,974
Re: Corrected: GH2 Image Sensor Diagonal and Horizontal Crop Factors

Detail Man wrote:

Steen Bay wrote:

Detail Man wrote:

Shouldn't it also be taken into account that the effective diagonal of the FF sensor is reduced when the native 3:2 FF image is cropped to 4:3 and 16:9? I'd say that both the diagonal and horizontal crop factor is 2.0x when shooting 3:2 images with the GH2, and less than 2.0x when shooting 4:3 and 16:9 images, because the multi-aspect ratio GH2 sensor uses the full image circle with all three aspect ratios, whereas the FF sensor only uses the full image circle when shooting 3:2 images.

All Diagonal Crop Factors :
4:3 - Crop Factor = 2.069320
3:2 - Crop Factor = 2.087004
16:9 - Crop Factor = 2.087553

So, none of them are exactly 28mm, all of them are a little bit "narrower" than 28mm (in terms of the Field of View), and the 4:3 aspect-ratio mode is the "widest" of them all (by a miniscule amount, anyway), having the smallest Crop Factor.

All Horizontal Crop Factors (corrected):
4:3 - Crop Factor = 2.586650
3:2 - Crop Factor = 2.508267
16:9 - Crop Factor = 2.395354

The 16:9 aspect-ratio mode is clearly the "widest" of them all, having the smallest Crop Factor.

So 14mm is not 14mm. It is 14mm multiplied by the Crop Factors listed above.

Do the math ... I used a 3.63 Micron pixel-pitch and the JPG sizes from the GH2 Op. Instructions.

OK. If cropping a 3:2 FF image to 16:9, then the effective sensor diagonal is reduced from 43.267mm to 41.3mm, and the effective crop factor between FF and GH2 (with a 21.63mm image circle) is therefore 1.91 when shooting 16:9 images. For 4:3 images the effective crop factor is 1.85 (40.0mm / 21.63mm), and when shooting 3:2 images the crop factor is 2.0 (assuming that the GH2 uses the full 21.63mm image circle with all three aspect ratios).

35mm equivalent focal length has nothing whatsoever to do will cropping a 3:2 aspect-ratio full-frame of physical dimensions 36mm width, 24mm height, and 43.267mm diagonal. Nothing ...

It is derived by comparing any given image-sensor's (aspect-ratio mode relevant) active-area physical width, height, or diagonal dimensions with that of 35mm full-frame size (listed above).


FOV = (2) x ( Arctangent ( ( S ) / ( ( 2 ) x ( L ) ) ) )


FOV is Field of View;
S is the frame-size (either width, height, or diagonal);
L is the Focal Length.

Re-arranging the identity stated above yields:

EFL = ( S ) / ( ( 2 ) x ( Tangent ( ( FOV ) / ( 2) ) ) )

where EFL is the 35mm Equivalent Focal Length; and
S is the width, height, or dagonal dimension of 35mm full-frame.

Calculate the physical dimensions of the (aspect-ratio relevant) active area for any image-sensor in order calculate these things, and then compare those particular dimensions with the physical dimensions of 35mm full-frame. That is all that there is to it. Nothing more; nothing less ...

The active area of a FF sensor when shooting images with the three different aspect ratios is :

4:3 - 32x24mm - diagonal = 40.0mm and crop factor = 1.85 (40mm/21.63mm = 1.85)
3:2 - 36x24mm - diagonal = 43.3mm and crop factor = 2.00
16:9 - 36x 20.25mm - diagonal = 41.3mm and crop factor = 1.91

Anyway, that's the comparison I find most relevant, if talking different aspect ratios and FF vs. GH2.

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