Bad GFX 100 Single Shot Astro

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Greg7579
Greg7579 Veteran Member • Posts: 8,111
Bad GFX 100 Single Shot Astro

So there I was, 3 nights ago in the best Dark Skies in the USA and Canada - the high desert of Big Bend National Park in Texas on the border with Mexico.  I had the GFX 100 and the 23 F4 prime.  Big Bend has the best Dark Skies in America or Canada.

I knew from many times shooting the Fuji X system with the Rokinon 12mm F2 at the Milky Way in Big Bend that I needed less than about 22 seconds to avoid star trails on a single shot, non-star-stacking shot with no tracking mounts and no blending in Photoshop of foregrounds shot at dusk.

So I used the Photopills App and learned that a quarter moon was up until midnight and at midnight the Milky Way was going to be straight up overhead and really bright - not a cloud in the sky.  The "Spot Stars" page said that with the GFX 100 and the GF 23mm at F4, it was a 10.56 second exposure for NPF and 28 seconds for the 500 Rule for very minimal star trails.  But I could not figure out on the app how to manipulate the ISO.  It is not on the screen and not an option for entry.  That is baffling because ISO is the most important setting, so I don't know what ISO the app was using for these exposure times.

But I knew before shooting that I was going to shoot 20 seconds and ISO 1600.  I also knew that the 23 was way to slow for Astro at F4.  F4 is just not enough for single-shot Astro.  F2 is what you need, and that is what I shot many times with Fuji X and the Rokinon 12mm and got some really nice Milky Way single-shots at Big Bend.

I set the GFX 100 up on the tripod and used the remote Fuji trigger.  I set it on manual at ISO 1600 (and some 2000), F4 at 20 seconds and IBIS off.

I focused using manual Focus Peaking Blue High at max magnification (by pushing the rear dial) and I think I had exact focus (but maybe I missed it because it was difficult to do).

The Rokinon 12 mm F2 that I have previously used a lot with the Fuji XT-3 and XH-1, has a focusing scale and I knew the exact spot where infinity focus was on the scale.  So I could set it there and not fumble around in the dark manually focusing on the stars.

Here are my observations:

1.  I was disappointed in the results.

2.  The 23mm F4 is too slow for single-shot non-star-trail Astro.

3.  There is too much noise at ISO over 1600 with the GFX 100 for Astro.

4.  At ISO 2000, F4 and 20 seconds, the star trails are just starting.  At 25 seconds there are trails.  20 seconds is the best setting for the GFX 100 and 23mm F2 for single-shot Astro.

5.  At F4, ISO 2000, 20 seconds, when you open the file in LR it is dark black.  You have to lift another 2.5 stops to get anything.  That is not what happened at those settings with Fuji X gear and the Rok 12 at F2.  The F4 23mm at 20 seconds and ISO 1600 is still way underexposed.  So you either push ISO to 6400 or bump it in post.  I guess I could have shot at Base ISO and then pushed it 6 stops in post!  Same thing right?  Base ISO plus 6 stops pushed in post is the same thing as using ISO 6400 at capture.

6.  It was dark out there.  Pitch black dark.  The foreground hill you see was not even visible to the naked eye.  It has a lot of noise here because it was basically pushed 7 stops (ISO and push in post).

I was disappointed in these shots.

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