Astro Landscape - Should I Stretch Before I Stitch or Stitch Before I Stretch?

[Hugh Brown]

Hi Guys

I am stacking together ten positions of 10 images each for the sky (so about 100 images total) which then translates into ten stacked images for the sky.

I’ve discovered an issue with a Canon RF 15mm - 35mm f2.8 L Series lens model I have been using for astro landscape when shot at f2.8 and 13 seconds and an ISO of 6400.

Significant vignetting in the corners equal to as much as four to five stops. 

So my question - the images are coming out dark (which could be purely a function of the dark skies where I live)…

Should I stretch my images - ie, lighten exposure by as much as 2.0 stops - before I stitch in PT Gui?  Or should I stitch before I stretch?

The aim obviously being to optimise output - defined by the best stitch possible and the best ultimate result re noise reduction.

[PTGui Support]

Hi Hugh,

I don't think the stitching quality will be different (whether you
brighten the images before or after stitching). You can also increase
the brightness in PTGui (see my previous reply).

Kind regards,

Joost Nieuwenhuijse
www.ptgui.com

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[Hugh Brown]

There was a reason for this question.

I plugged AI with the same question before raising the query here and it came back with the following points (summarised by me) which made sense:

Exposure adjustments are to be done AFTER stitching rather than before because.

1.     Even when shooting in Manual Mode, cameras often have slight exposure variations (±0.3 stops) between frames due to metering shifts, temperature changes, or lens flare as you pan.

If you brighten BEFORE stitching: You permanently bake these slight differences into the files. When PTGui blends them, it has to work much harder to hide the transitions. If the differences are too large (which they will be after a +2.0 stop lift), PTGui cannot fully hide them, resulting in visible vertical bands or "paneling" where each of the 10 frames looks slightly different in brightness. 

2.     The files before exposure adjustments are "flat" and dark. This preserves the maximum highlight and shadow information for PTGui’s blending engine to analyze.

Once the panorama is created, you have a single 32-bit or 16-bit file with a unified histogram. Applying the +2.0 stop lift here affects the entire image uniformly, ensuring the sky gradient remains smooth from left to right.