Processing confusion!

[drumsolo]

Ok I really hope some one out there can help me out here. There are way too many conflicting articles on how to post process with levels and curves. I feel like pulling my hair out as some articles contradict each other and suggest doing it the opposite way around. When using levels should I be moving the left hand slider towards the black edge of my data and clip the dead space after each curve stretch? Should I ever move the other 2 sliders?

i would be extremely grateful for a definitive answer if there is one!

 

thanks

stuart

[D4N]

Yes normally levels is used to move the black point and occasionally to clip noise (as in pixels that are darker than the rest of the sky are likely cold pixels).

You would never normally need to move the white point and moving the central slider is a very course way of stretching the data, curves is much better for revealing faint detail without blowing out highlights.

[drumsolo]

Hi thanks for the reply.

should I move the left hand slider all the way to where the histogram starts or somewhere in the middle?

 

thanks

[D4N]

When I use levels in PI it tells you how many pixels are being set to black when you move the black point, the first time I take it up to the edge of the histogram and clip the few pixels that are noise then after that move it to the edge but don't clip any.

[ollypenrice]

Very experienced imagers use very different approaches to levels and curves so don't worry about that. Crop off all edge artefats after stacking.

A first check in Levels is needed to make sure that the black and white points meet the histogram data line. If there is a gap at either end, move the black or white point slider in to meet the data. If you don't do that you won't exploit the full range of brightnesses available to you.

After that you can stretch by moving the mid point slider to the left to apply a pure log stretch or you can shape a curve yourself. I tend to use a simple log stretch for my colour layer but a far more aggressive Curves stretch for Ha and sometimes luminance, followed by specialist Curves shaped for particular parts of the image and layered in selecively. There are also 'bought in' pre-formed curves stretches like DDP which some people use.

The thing to do in Curves is understand what you are doing. Think about what parts of the image lie on what part of the curve. To find out for real in Ps just open Curves, put the cursor an a part of the image and alt click. A point will appear on the curve telling you where your chosen part of the image lies. Roughly speaking this is what you'll find on most images:

This is a very aggressive curve which many imagers would reject but I and many others use them routinely on strong Ha signal. What does it do? Well, looking at the labels on the Curve, it creates huge changes in relative brightness (ie contrast) between the very faint, the faint and the middle bright nebulosity but it will also lift the stars by a lot. On Ha this may not matter but it might in luminosity, so you could put the bend in the curve at a lower point.

After each iteration of the stretch, be it in levels or curves, you need to check the black point. Bring it in a bit by all means but don't get anywhere near the histo peak. Keep the background light till you get right to the end of the processing when you can bring it down safely.

Every image needs a dfferent stretch so I suggest you try several and see which ones you lke best. You'll also get a feel for what kind of stretch has what effect and this is something you need to have clear in your mind.

Olly

[Uplooker]

Thanks Olly for some valuable tips in your reply above