Luminance Required?

[BlueAstra]

Quick question, probably obvious answer, but you know when you start thinking about something you go down too many dead ends - If you don't bin your RGB lights (ie shoot at 1x1), do you still need a luminance light?

[ollypenrice]

Duplicate, sorry. Please remove. Olly

[ollypenrice]

Most people, I think, would say 'Yes,' but a notable exception would probably be Peter Shah. I hope he comes in on this one.

I find 'RGB only' very soft and indecisive-looking. An image leaps into life once luminance is added, at least following my routines.

However, you might find that removing luminance from stars will let them keep more colour and clusters are better without it if no nebulosity is involved.

I don't find you need as much colour as luminance. I find that on a serious image which is getting lots of time, 90 minutes per channel colour can take as much luminance as I have ever managed to get, ie 6.5 hours on M31.

How you add it affects things as well. You can convert the RGB to Lab colour mode, split channels and simply recombine them with the luminance image replacing the Lab mode's interpolated Lightness channel. If your balance of L to RGB is right, this is great. However, you might want to partially apply the L in which case Layers in blend mode Luminosity is the way.

Lastly, if you are only in posession of RGB you can process the luminosity separately by extracting the Lightness channel in Lab colour and processing that differently from the RGB. I would go for sharpening in that channel and process the colour for low noise, doing any smoothing in those channels

Also in Lab colour, pushing the a and b channels for contrast is a great low noise way of intensifying colour. (Credit to MartinB for that excellent tip.)

Olly

[BlueAstra]

Thanks for that, not as obvious as I thought! I'm just starting my first LRGB imaging sequence with my FW, so I'll try the result with and without Luminance to see the difference. I was originally going to save time by taking RGB unbinned with no L thinking that would be the same as L(RGB @ 2x2). Lots of things to think about with this LRGB lark! (Hmmm, should I use Ha as L? ... another story I think)

[roundycat]

Just for the record, people shoot Luminance to go with binned colour. The theory being that binned colour will be low resolution but the human eye is not too bothered about that. What does help a picture is if there is an un-binned L component that significantly sharpens the image.

What this is supposed to mean in practice is that you shoot colour quickly (as it is binned) and then shoot hours of L and blend the three channels properly (?).

I find that the loss of resolution due to binning colour is not acceptable. We do not have clear enough skies (think; up a mountain) in this country to do that.

Olly, your comment about not needing as much colour as L. Many people, including me, find that unless the RGB and L are in about equal amounts the L washes out the colour. Do you not find this a problem? Do you shoot the same exposure length for your L or just add more frames? What I mean is do your L exposures go deeper? If so you should be finding that you have more low level detail in your L and it is that that washes out the colour.

Dennis