From: "Christoph Bosshard" <chboss@hispeed.ch>
Date: November 28, 2007 11:12:09 AM MST
To: Astro_IIDC@yahoogroups.com
Subject: Re: Mars Fri Nov 23
Excellent Duane, thank you for this effort.
I use the Photoshop curve tool for normal photos to adjust white and
black point and the grey balance. I never went into more details...
I will give it a try for the posterization of the dark part of Mars.
regards
Chris
I use Photoshop's curves. Curves are actually pretty simple, but
very powerful.
The shortcut on a Mac is Apple-M, PC is Cntrl-M I think. To remember
that, just remember
how great curves are and think, "mmmmm... Cuuurves!"
When you open curves, you'll see a grid. On the left is the output
(how dark a given pixel
will appear) and on the bottom is the current density (how dark it
is now). From bottom to
top is dark to light, and from left to right is dark to light
(however, this can be reversed).
If you click around on your image while in curves, you'll see a
circle on the curves grid that
represents the pixel you are over.
Now for the meat of it...
There is a diagonal line (lower left to upper right) which
cross-references the current
shade with the output shade. Black represent black and so on. If you
were to click on this
part of the line and raise it up, Black would now be represented by
a lighter shade. Moving
the line moves several shades at a time, so all of these shades are
represented and
altered.
The diagonal is a 1:1 ratio in density to output. But in any place
where you make the line
more vertical, or should I say increase the slope, you are
stretching the detail. Any place
where the line become more horizontal, it flattens and loses contrast.
The main goal is to locate the area you want to bring out detail and
pull the line more vertical in that region.
For somebody new to curves, I have 4 simple ways to start using it:
UP) If your detail is in the dark shades, grab the middle of the
curve and pull it up a bit
(detail pops out on the low end but is suppressed on the high end).
It has an overall affect
of lightening the image.
DOWN) If your detail is in the lighter shades, grab the middle and
pull it down (the
opposite of above).
S) If your detail is in the center, click once on the middle of the
line (it will lock it in place
with a black dot), then pull the line from a quarter of a way from
the dark end, down. You
may want to pull the lighter quarter of the line up. This makes an S
shaped curve (detail
pops out in the middle grays but is suppressed at the extremes).
Reverse S) If your detail is toward both extremes but not in the
middle (rarely happens),
put the lock in the middle and push the darker quarter slightly up
and the lighter quarter
slightly down (detail pops out at the extremes but the middle is
suppressed).
Now for the posterization issue:
Click in the area where the shade changes due to the sphere of the
planet. This will show
you the region on the line where you should make it more vertical.
It won't take too much
—just season it to taste. Be careful you aren't flatting any
important data by keeping an
eye on the important detail in the image.
I hope I wrote that clear enough, it should be a good primer for
anybody who wants begin
to understand curves. There is much more to curves, especially as a
color balance tool. I
hope I get time to add some of this to my webpage someday.
Please let me know if that was helpful. I know it's tough without
graphical aid.
Duane
--- In Astro_IIDC@yahoogroups.com, "Christoph Bosshard" <chboss@> wrote:
Hi Duane
Thanks for your comment.
Can you point me to some detailed sources regarding stretching the
curves? Would you do this in IIDC or in Photoshop?
Hopefully there will be more good material to work with in the comming
weeks. I am still doing some changes to my equipment at the moment to
be well prepared. :-)
I will certainly post the results to get some more tips on how to
improve. I think it is a great way of learning.
regards
Chris
Hi Chris,
Thanks for the feedback. I think that left right alignment is
actually a bit of bubbling (disk
size due to atmospheric refraction) but reprocessing may allow me to
get a better pick on
the photos that are accepted. I rather loath picking them by hand,
but it is another option.
Nice shot too. I always figure that if I can tell which side of Mars
is showing that it was
worth while night. It looks to me like you have the Mare Serinum
side.
There's a little bit of posterization on your image that can be
avoided using curves to stretch the area in the histogram that
contains Mars' data before doing too much
sharpening or other processing.
Keep shooting and sharing. I find it really helpful to see what
others are doing.
Tonight, major wind and jetstream issues here. Time to get some
sleep :)
Duane