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While I'm satisfied (despite the fringe) with shoots from a partially visible moon with a cheap 500 mm lens:

enter image description here

1/100 ISO 100 f/?

I cannot understand why the image quality is extremely diminished when doing the same with an almost fully visible moon:

enter image description here

1/80 ISO 100 f/?

I suppose that fully visible is much brighter and when I try to compensate the situation by changing the aperture these effect show up.

Is there an obvious explanation or do I just mess up the focus every time?

EDIT: I repeated the process with an APS-C instead of full-format camera, now one of the images looks better (showing more details):

enter image description here 1/250 ISO 100 (APS-C)

I'm not sure whether the crop factor allows me to focus more accurate or this is due to the 50% magnification. At least it shows more details.

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    \$\begingroup\$ Does photo.stackexchange.com/a/83792/47295 answer your question? \$\endgroup\$
    – Peter Taylor
    Nov 22, 2018 at 20:14
  • \$\begingroup\$ @PeterTaylor not completly I already tried bracketing, and 5 ago minutes repeated the process changing from full-format to aps-c which looks bit better. I will add one of these images later to refine the question, thank you for pointing me to this post. \$\endgroup\$
    – stacker
    Nov 22, 2018 at 20:45
  • \$\begingroup\$ Upper photos look slightly out of focus. Or is this the difference between 1/80 and 1/250 shutter speed? Unlikely. \$\endgroup\$
    – IMil
    Nov 23, 2018 at 1:39
  • \$\begingroup\$ IMO the second image is sharper than the first, even if it has poorer contrast. \$\endgroup\$
    – Darren Ringer
    Nov 23, 2018 at 14:24
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    \$\begingroup\$ Next time you are out experimenting, take at least ten exposures for every different setting and variable you are comparing. You'll see that with the moon, and many other astronomical subjects, there can be significant shot-to-shot differences from one frame to the next using the same exact settings. This is due to the effect of the Earth's atmosphere. \$\endgroup\$
    – Michael C
    Nov 24, 2018 at 11:51

2 Answers 2

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What makes the difference on partially and fully visible moon?

In a word: shadows.

I cannot understand why the IQ is extremely diminished when doing the same with an almost fully visible moon.

The second image does appear to suffer from lower sharpness and overall quality. However, even if the technical image quality factors were equal, most importantly, a full moon will appear flat and uninteresting, as compared to a gibbous (i.e., 3/4-ish) moon.

We see a full moon that is lit directly "overhead". The mountain and crater rims do not cast any shadows that give texture and depth to the moon's surface. Thus, we only get tonal information from the albedo (reflectivity) of the local regolith in parts of the image.

However, with a partial moon, the surface is more side-lit, thus casting shadows. These shadows provide vital depth clues to our eyes, and greater tonal variations. Even if the technical image quality is the same (i.e., same accurate focusing, correct exposure, no motion blur, etc.), a partial moon's greater tonal and texture variations will make for a more apparently higher-quality moon image.

Your images provide for excellent comparison. Looking at Tycho Crater (the large impact crater in the south-southeast view, with large whitish ejecta streaks emanating from it): Notice in the gibbous phase image, Tycho crater has a distinct rim and bowl, and is surrounded by lots of smaller impact craters. Whereas, in the full moon image, the shape of Tycho crater is apparent, but it has no depth, no sense of being an obvious bowl. The smaller impact craters immediately surrounding it are nearly invisible, and the general area surrounding Tycho is just a smudge of middle gray.

The third image has substantially higher quality than the second image. However, even with its increased clarity and dynamic range, notice that the area immediately surrounding Tycho crater still doesn't exhibit much depth. The crater rim has more definition than the second image, but to my eye, doesn't exhibit nearly the same dimensionality and character as the first image does.

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  • \$\begingroup\$ Thank you for this detailed answer +1, could you include the 3rd image (aps-c which is more pronounced ) in your answer. \$\endgroup\$
    – stacker
    Nov 22, 2018 at 21:16
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    \$\begingroup\$ @stacker Looks like a combo of factors. The second image is not focused as well as the third and the third image also appears to have been captured with a different tone curve and has more contrast. The latter is a function of the camera's rendering settings. \$\endgroup\$
    – doug
    Nov 22, 2018 at 21:52
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The full moon is illuminated by the sun which is directly overhead, i.e. noon. Mountains on the moon's surface, at noon local time, cast no shadows. When the moon appears to be partially illuminated, gibbous, 1st.or last quarter, 1/2 moon, crescent etc. This local time is such that the sun casts shadows. A view of the moon from earth is enhanced by shadows as they heighten the illusion of depth.

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  • \$\begingroup\$ [Solar] noon is when the sun is at its highest point of the day, not necessarily directly overhead. Indeed it's very rarely directly overhead any given point. \$\endgroup\$
    – Lightness Races in Orbit
    Nov 23, 2018 at 18:05
  • \$\begingroup\$ @LightnessRacesinOrbit Indeed. As a matter of fact, the sun can only possibly be directly overhead in latitudes between the tropics of Cancer and Capricorn. This is called Lāhainā noon. For the two tropics, Lāhainā noon occurs exactly once per year. For latitudes strictly between the two tropics, it occurs twice per year. \$\endgroup\$
    – scottbb ♦
    Nov 23, 2018 at 21:06
  • \$\begingroup\$ @scottbb That's right. \$\endgroup\$
    – Lightness Races in Orbit
    Nov 23, 2018 at 21:26
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    \$\begingroup\$ For all intent and purpose - A full moon is seen with reduced contrast because mountains on the moon cast little or no shadow when the moon is seen from the earth as a full moon. \$\endgroup\$
    – Alan Marcus
    Nov 24, 2018 at 1:01

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