Histogram Question

leon · #1 25-10-2007, 09:48 PM

Hi All, seeing I'm asking all these questions tonight, i would also like to ask this one.

Often it is mentioned, when processing that the Histogram should not be clipped, now I understand this to mean, please correct me if I'm wrong, that one should try and keep the Histogram as far left as possible and at a very steep spike as possible.

Is this correct, if not could some one tell me what this means, as i have heard it so many times and am not exactly sure what it means.

However i do know that it is important in the final results of a processed image.

Thank You in advance,

leon

[1ponders] (Paul) · #2 25-10-2007, 10:45 PM

Not quite Leon. Yes your aim is to raise the black point (the left triangle in photoshop) so that it remains just to the left of the rise in the histogram. But it also means that when you stretch the data using curves for example you don't clip at the other end and have yours stars as pure white. This is seen as the histogram line starting to bunch up at the far right hand end.

Overall your histogram should end up spread more evenly across the range of the histogram.

The first histogram is what I'm imagining you are talking about (you can see it starting to clip on the white end) and the second is along the lines of what you might want to try to achieve if you were doing a widefield of the milky way. Its a bit of an extreme example of a histo stretch. The third one is of a widefield with the ED80 of M31.

Hope these help.

dugnsuz (Doug) · #3 25-10-2007, 11:09 PM

Don't know if this is what you're getting at leon - but it can't hurt!!

http://www.itchysastro.net/avoiding_...m_clipping.htm

Cheers
Doug

iceman (Mike) · #4 26-10-2007, 05:57 AM

Tony was kind enough to lend that article to IceInSpace too, Doug

Avoid Histogram Clipping in ImagesPlus

Leon - Paul explained it well enough. But to add to it, if you raise the black point, you darken your background which is often what you want. But if you raise it too far, you'll start removing the real detail (the faint stuff) as well. Similarly from the other end of the histogram, raising the white point will bring the levels up and make faint objects brighter - but you might start clipping already bright objects and making them pure white.

Ideally your histogram should spread across the whole range - that you'd call a broad dynamic range. If your histogram is a small peak that you squash the black and white levels in-between, you're reducing your dynamic range.

leon · #5 26-10-2007, 07:36 AM

Gee you guys are champions, I will have a good look at that when i have a little more time tonight,

Leon

leon · #6 26-10-2007, 08:00 AM

Well just couldn't wait and had a good read of the links provided, yep I've got it chaps, and get the picture, thanks again.

leon

Alchemy (Clive) · #7 26-10-2007, 08:00 AM

As a luddite on processing myself , i went out and bought Photoshop astronomy by r scott ireland. it seems to cover all sorts of stuff and has tutorials, ive done the tuts on levels and part of curves so far: and boy what a difference! just about to start "Applying Curves to a selection- introduction to Masking and Layer Masking". theres a huge amount of info to absorb but its probably the best 80 or so dollars ive spent recently.

leon · #8 26-10-2007, 08:49 AM

Hey, that sounds like a great book where might i be able to pick this up.

leon

[1ponders] (Paul) · #9 26-10-2007, 09:01 AM

It is the astronomy photoshop bible that is for sure.

http://www.bintel.com.au/Books_O_P.html

though you can get it direct from Willmann-Bell in the states. just don't look at all their other astronomy books

g__day (Matthew) · #10 26-10-2007, 09:45 PM

I'd also recommend the Zone System for Astro photography by Rod Wodaski, especially if you're using a not too old Photoshop.

leon · #11 27-10-2007, 09:44 AM

Thanks for that i will look into that one as well.

Leon

pvelez (Pete) · #12 27-10-2007, 09:40 PM

Quote:

Originally Posted by dugnsuz

Don't know if this is what you're getting at leon - but it can't hurt!!

http://www.itchysastro.net/avoiding_...m_clipping.htm

Cheers
Doug

Doug

you are a legend.

After reading this article, I had another go at processing some shots out took out West near Gulargambone last month. The first is the original - 2 x 360 sec at ISO 800. The second after reprocessing.

Have a look at the difference!

I didn't apply flats to the redone shot so the dust and other gunk on the sensor of thecamera are apparent. That said, the detail in the redone photo is amazing.

Now I will get cracking on the others in the PC - there might be some real gems lurking in there.

Pete