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Old 18-04-2018, 07:14 AM
Jay-qu
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Milkyway rising over the desert

Last year I took the plunge and picked up a Canon 6Dii and Sigma 20mm f1.4. Now that the milkyway is coming back into view, here is my first serious attempt with the new kit. I did a series of shots at different ISO/exposure/aperture settings to see what worked best with this setup, so I'm including some commentary for others that might be interested.

My goal with this kit was to keep things simple, I'm planning to do a bit of backpacking, so trying to economise on size and weight, which means no tracking mounts/gizmos (though I might explore the polarie if I can handle the extra 650g). I also wanted to keep the budget within reason, the 5DIV is amazing, but more $ and more weight/size. You might ask why I didn't go mirrorless, alas I'm already invested in the canon ecosystem. Hopefully they'll bring out a competitive FF mirrorless when I can afford one

OK, first off, this lens is a beast which will guzzle down photons BUT it does have some pretty unappealing coma at low f.. nevermind, I can shoot at f1.8 where it's much less of a problem AND still be letting in 2.4x the light than more common f2.8 lenses (wide open)

Now for exposure, I tried 20s - on the conservative side of the '500 rule', but there was just too much movement in the stars. Maybe I'm pixel peeping too much (26MP gives you a bunch of detail) or maybe my standards are too high coming from DSO's imaging.. so I settled on 10 seconds.

Finally, ISO. The 6Dii got a real rough ride in the review cycle, I tried to ignore it and bought the camera the fit my budget and requirements. Sure it is missing some features and DR for a 2017 camera, but this thing performs up there with the best at ISO 3200-6400. And the usefulness of the flippy screen cannot be understated, that's a feature I will use everytime I take this out at night. The large aperture negated the need to shadow push, so I was comfortable using the images shot at ISO6400 (though there is some low frequency colour noise that crept into the image).

One final thing, compositing and stacking widefield shots is really not my cup of tea. I just find the processing a bit tiresome (this is my break from DSOs) and hard to make it look natural. With that in mind, this final image is simply a two shot panorama, each at f1.8/10s/ISO6400, processed in lightroom (making use of the selection brush to process the sky and foreground seperately).

In summary, I'm quite happy with this setup and look forward to seeing what I can get out of it.

Thanks for looking
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Old 18-04-2018, 06:19 PM
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FlashDrive (Poppy)
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Very Nice ...
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Old 19-04-2018, 04:11 PM
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I like that a lot. A few points though. The border of the sky and land is very harsh and takes my attention. That must have been the selection. Next time I would suggest feathering the selection. In the meantime I would run the blur tool over the border just to soften it a bit and get rid of that slight white halo.

ISO6400 runs the risk of blowing out the stars and they end up all white. It may be safer to expose at ISO3200 and boost in post.

But its a great composition.

Greg.
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Old 20-04-2018, 01:33 AM
Jay-qu
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Thanks for the tips Greg
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Old 20-04-2018, 05:58 AM
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Excellent capture, with a beautiful foreground, like it very much well done indeed.

Leon
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Old 21-04-2018, 04:10 PM
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Hi Jay Qu,

That’s a very nice image!
For pixel peeper spot stars/large prints, for your 6dii I get 5s at 20mm 1.4 and 6s at 20mm 1.8. That’s at 0 declination.

Nice setup! Happy imaging

Hemi
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Old 22-04-2018, 07:19 AM
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A great composition indeed. The foreground is captured nicely.
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Old 22-04-2018, 03:32 PM
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OzEclipse (Joe Cali)
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Hi Jay Qu,
Very nice composition. Love the image.

With respect to your comment about exposure times, trails and pixel peeping...

These rules or formulae like the 700, 600, 500 rule are based on a set of starting assumptions and have been kicking around since the days of film.

In the old days of 35mm film very fast films used for tripod astrophotography had very course grains and so a bigger movement could pass off as no drift. It was based on someone looking at a 8x10inch enlargement at normal reading distance perceiving trails as almost stationary.

The eye can resolve about 1 arc minute so it can in fact resolve approximately 200 microns on a print which scales back to 25 microns on the film on an 8x enlargement.

Using the number 700 corresponds to a trail length of about 50 microns on the 8x10 print, 600 to about 42 microns and 500 to about 35 microns. These in turn represent about 9, 7, and 5 pixels on a modern DSLR. But today, we mostly look at screen based images and images that are highly reduced from full size.


A few years ago, I've derived a formula, modernized for DSLR's, that takes, pixel size, and final viewing size into account.

max exposure time(s) = [14*N*P] / [FL*cos(d)]

where
N..........Number of pixels of drift
to work this out load any full frame image from your camera and scale it to the size you want to view on screen. Look at the scale percentage. Number of pixels drift = 100 / percentage

P..........Pixel size (microns) Most DSLR's are between 3 and 6.5 microns. You can look it up in your manual or just split the difference and make it 6.5

FL.........Focal length(mm)

cos(d)...cos of the declination. Use the declination of the stars in the field that are closest to the celestial equator in the camera field

Example : Using a 14mm lens, 5 pixels drift, on an APS-C sensor with 5.4µm pixels pointed at the celestial equator can take a 27 sec exposure. Pointed at the SCP the lens will see 45 degrees each side of the pole extends the time to 38 seconds.

cheers

Joe
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