One of my readers Peter Loughran has been sending me some very interesting facts about crop factors…
Regarding the question of crop factors on the GH4.
Having used and tested these things on the the GH3 for over a year (and currently waiting for my GH4 Body to ship!) I’ve made some observations and calculations which may (if they prove correct when we get the actual camera) offer some clarification. Sorry this is a bit dry but there’s a lot of misinformation and speculation around so I thought I’d best justify my thoughts by showing the calculations used (please feel free to offer corrections if I’ve got anything wrong).
Because the GH3 (unlike the GH2 which has an over-sized constant crop factor ‘Multi Format’ sensor) actually crops its 4:3 format sensor (2x crop factor applies ONLY @ 4:3) down vertically to give 3:2 & 16:9 formats, the crop factor at 16:9 is actually ~2.18 (based on 43.27mm diagonal of 35mm still frame format of 36x24mm).
Full Frame Film/DSLR format = 36x24mm = 43.27mm diagonal (image circle diameter)
GH3 sensor (@4:3 full sensor) = 17.3×13.0mm = 21.64mm diagonal
Crop Factor = 43.27/21.64 = 2
GH3 sensor (@3:2 crop) = 17.3×11.53mm = 20.79mm diagonal
Crop Factor = 43.27/20.79 = 2.08
GH3 sensor (@16:9 crop) = 17.3×9.73mm = 19.85mm diagonal
Crop Factor = 43.27/19.85 = 2.18
(GH3 video EXIF data shows this scale factor as rounded to 2.2 but shows 35mm equivalent focal lengths correctly as multiplied by 2.18)
GH3 sensor when using the ‘Extra Tele Conversion’ 100% pixel for pixel 1920×1080 16:9 video crop mode = 7.21×4.06mm = 8.28mm diagonal
Crop Factor = 43.27/8.28 = ~5.22
(GH3 video EXIF data shows this scale factor as rounded to 5.2 but shows 35mm equivalent focal lengths correctly as multiplied by 5.22)
The GH4s new sensor appears to be same size & resolution as that in the GH3 so crop factors seem to be the same when shooting at Full HD.
Panasonics online specs. sheet for the GH4 shows exactly the same range of magnifications when using the ‘Extra Tele Conversion’ mode as those listed for the GH3:
“Extra Tele Conversion: Motion picture: 2.4x (FHD), 3.6x (HD), 4.8x (VGA)”
(Remember those magnifications are in addition to the crop factor of the sensor and selected format).
No official crop factor is so far listed, as far as I can see, for the C4K & QFHD video modes so whether the crop factor is different from that in Full HD (where it appears to use the full sensor width) remains to be confirmed.
If the GH4 is using a 100% pixel for pixel crop to obtain the 4K & QFHD images from the sensor the crop factors would be something like this:
Image sensor size 17.3 x 13.0 mm (in 4:3 aspect ratio)
Full horizontal sensor resolution is 4608 pixels over 17.3mm
C4K = 4096×2160 would imply a sensor crop of ~15.38×8.14mm = 17.4mm diagonal = Crop factor of ~2.5
4K = 3840×2160 would imply a sensor crop of ~14.42×8.11mm = 16.54mm diagonal = Crop Factor of ~2.6
Some ‘sources’ are claiming that the GH4s 4K crop factor is ~2.3 but seem to be working on the false assumption that the 16:9 (full sensor width) crop is 2x (which ONLY applies at the full 4:3 sensor aspect ratio) whereas the actual calculated 16:9 (full sensor width) crop factor is 2.18 as shown above.
Add their approximately 0.3 extra crop factor to my calculated (and EXIF confirmed) figure of 2.18 and you come to a very similar figures for the possible (100% pixel for pixel) crop factors (as calculated above) of ~2.5 to 2.6.
If that proves to be true I anticipate a lot of wailing and gnashing of teeth from those folks who long for bigger and bigger sensors.
Personally, I’d be delighted if the crop factor is a touch higher than in 1080p mode as it would give my longer lenses even more ‘pulling power’ in 4K (when shooting nature with telephoto and macro lenses) whilst still having the possible equivalent of ~18mm wide-angle coverage (more than enough for most purposes) by use of the Panasonic or Olympus 7-14mm lenses.
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