micro 4/3 field of view - math question

[Tim Gray]

I'm building out my GH2 lens collection.  One gap is wide and fast.  I use the Canon 16-35 2.8 on a ff for night sky shots - needing fast to keep exposures under 30 seconds, and liking the fov at 16mm.  Images are typically portrait in orientation.

My questions is - if I take a 20mm Panasonic lens on the GH2 (40mm ff equivalent fov), set the aspect ratio to 16x9, shoot landscape and do a vertical panorama stack to a 3x2 or 4x3 aspect ratio, what 35mm ff focal length would give me the equivalent field of view?  Would I get close to 16mm?

Thanks.

[Christoph C. Feldhaim]

If I'm not mistaken the different aspect ratios in the GH2 fully exploit the image circle, since the sensor is bigger than just what is covered at 4:3.
So I'd guess the different aspects share the image diagonal and thus the rule of Pythagoras  (a^2+b^2=c^2) is your friend.

[bg2b]

You want the ratio of (horiztonal/vertical/diagonal) sensor size to focal length to be the same.  On your FF, shooting vertically the horizontal angle of view is determined by the ratio 24/16 (sensor size along the short dimension divided by lens focal length).  For a GH2 shooting horizontally in 16x9 mode, the sensor width is about 19.5mm.  19.5/x = 24/16 means x = 13mm.

For a dedicated lens, that means one of the 7-14mm or 9-18mm wide angles, or if you can live with 14mm, then any of the zillion choices.  If you want a somewhat fast lens, closest probably be the Panasonic 14mm pancake.  One question is whether you're sure you can stack reasonably if you're doing 30 second exposures--are there any issues due to star rotation?

[Tim Gray]

Ah! Perfect, thanks. As for star rotation - certainly an issue, but at 30 seconds, the light points don't smear much and I'm not planning to necessarily be astronomically correct so even if I just do a slight gradient blend at the edges I should be ok.  Alternatively I could always slide the upper image slightly - it should still line up accurately - at least for the stars - any foreground in the second shot would still be an issue.

The problem is that the 7-14 etc. is too slow to give me a decent exposure at 30 seconds (the limit to avoid trails). The 14 mm 2.5 is probably the best option based on your math.

Here's an example that I'd like to be able to replicate: