Luminosity vs. LAB

[DanK]

I was curious about the interchange a few days ago about LAB vs.luminosity blending. I just tried them both with an S-shaped curve, and they seem the same, as I expected. Unfortunately, the two non-RGB contrast adjustments aren't exactly the same because my deactivated luminosity curve disappeared when I converted to LAB, and I didn't manage to exactly replicate it.

Here is the original image, with a standard, normal-blend-mode (RGB) curves adjustment added. Look at the green on either side of the vertical tree reflection in the middle as a reference.

1




Here's the luminosity blend mode. The curve is otherwise identical. Note that the greens I mentioned are a bit less saturated.

2




Now, here's the LAB mode curve. The contrast itself is a bit different, for the reason I mentioned, but the greens are again a bit less saturated. I think they are the same as 2.

3



The reason this matters, IMHO, is that using luminosity blending is a much easier, if you don't also want to work on the A and B channels

[Manfred M]

Thanks for posting these Dan.

Your experience with Luminosity blending mode is a bit different than mine and I have no idea why. When I use a curves adjustment layer for dodging and burning, I find I get a change in saturation as well, so I often have to follow with a saturation adjustment layer to tone down the saturation. I normally work in ProPhoto RGB and wonder if that might be the reason.

I just did a major edit using the Lab colour space and only resorted to a saturation adjustment once, and that was to knock a bit more blue out of the sky. I exported into the Lab colour space from ACR (having done the global colour adjustments in the raw convertor). The rest of the adjustments were all done on the "L" channel as I had no need to touch either the a or b channels.

The workflow was faster than with the RGB colour space (less steps) and the two images are not identical (both the crop and the edits are a bit different). Both images were converted to sRGB before posting.


1. Edits in Lab colour space





2. Edits in ProPhoto RGB colour space

[pschlute]

Manfred a question to you on your use of a curves adjustment layer to do dodging and burning. I am used to using layers is PS and use the dodge and burn tools.

Do you use the curves adjustment layer to give a general change to the picture and then dodge/ burn on top of that ?

[Donald]

Do you use the curves adjustment layer to give a general change to the picture and then dodge/ burn on top of that ?

I'll let Manfred explain what he means, but for my part, the Curves Adjustment (with a Layer Mask) is the D or B tool. The Photoshop D & B tool never comes into the equation.

There are many ways to D & B, but I think all good courses will show the D & B tool to be the least effective of all those and recommend never using it. I now use Luminosity Blending (the Blend-If function) to dodge and burn, which is a refinement using the Curves Adjustment layer.

The best online course on D & B that I have found is at - https://phlearn.com/tutorial/master-...urn-photoshop/ (fee to pay). It walks you through all ways of D & B.

[Manfred M]

Peter - here is a screen shot of the method I was trying with the image using the Lab colour space:



I use many (usually from 5 - 10 curves adjustment layers (when the lighting is "good") right through pushing 100 curves adjustment layers when the lighting is not so good (for instance this shot taken mid-afternoon with hard light). I make selections, so covering large areas and others fairly small ones) to get the areas that need adjustment. I pull the mid-point up or down until I get the effect I am looking for. Most of my adjustments are made with the quick selection tool, although there are times I will paint on the selection with a soft white or black brush. I will sometimes give the selection a Gaussian blur to soften the selection and even more rarely I will use the expand and feather (Modify Selection) options.

I don't use the Blend If tools that Donald mentions. While I use that tool for sharpening, I find it is not what I am looking for in dodging and burning.

The main problem with the dodge and burn tools in the Tools palette is that they are difficult to control and slow to use. They also leave artifacts (saturation or colour issues (add gray)) when used.

[DanK]

Interesting.

I work in ProPhoto also.

In your edit in the RGB space, did you blend all layers involving any changes in tonality using luminosity blending mode? That's what you would have to do to make them comparable.

Thanks for posting these Dan.

Your experience with Luminosity blending mode is a bit different than mine and I have no idea why. When I use a curves adjustment layer for dodging and burning, I find I get a change in saturation as well, so I often have to follow with a saturation adjustment layer to tone down the saturation. I normally work in ProPhoto RGB and wonder if that might be the reason.

I just did a major edit using the Lab colour space and only resorted to a saturation adjustment once, and that was to knock a bit more blue out of the sky. I exported into the Lab colour space from ACR (having done the global colour adjustments in the raw convertor). The rest of the adjustments were all done on the "L" channel as I had no need to touch either the a or b channels.

The workflow was faster than with the RGB colour space (less steps) and the two images are not identical (both the crop and the edits are a bit different). Both images were converted to sRGB before posting.


1. Edits in Lab colour space





2. Edits in ProPhoto RGB colour space

[Manfred M]

Interesting.

I work in ProPhoto also.

In your edit in the RGB space, did you blend all layers involving any changes in tonality using luminosity blending mode? That's what you would have to do to make them comparable.

Yes Dan, Luminosity blend mode PLUS and additional adjustment layer where I worked solely on the saturation. Effectively I used a single adjustment layer in Lab versus two in ProPhoto. At a high level I used 80 layers in the ProPhoto edit and less than 40 in the Lab edit.

[pschlute]

Thanks for the info folks. Gosh the "digital darkroom" is a never ending quest for ideas and techniques. I shall try the different methods proposed for D+B and see how I get on.

[Manfred M]

Thanks for the info folks. Gosh the "digital darkroom" is a never ending quest for ideas and techniques. I shall try the different methods proposed for D+B and see how I get on.

That it is...

The instructor for the printing class I am attending right now has a very strange technique for dodging and burning. He does not use adjustment layers at all and only uses a single layer for all his work.

He uses the rectangular marquee tool to make a rough selection and uses the tools under the Image tab in Photoshop. He makes a selection with the marquee tool and then uses a curves adjustment to get the luminosity to where he wants it. He then plays with the saturation to get that property to where he wants it. Finally he switches to the History Brush and sets it as soft as it goes and paints out the areas that were over selected. He absolutely flies when he gets going and thinks nothing of doing a couple of hundred small tweaks that way on an image that way.

He's been printing at a world class level for well over 30 years (obviously much of it in the wet darkroom); he was Karsh's printer in the last decade of his career.

[pschlute]

he was Karsh's printer in the last decade of his career.

.....wow !!!

The image of Karsh's portrait of Pablo Casals from behind has stayed with me for decades.

[Rufus]

...Finally he switches to the History Brush and sets it as soft as it goes and paints out the areas that were over selected. He absolutely flies when he gets going and thinks nothing of doing a couple of hundred small tweaks that way on an image that way...

The history brush is another tool I had not investigated , but I have now! It's another small step along the way.

[Manfred M]

The history brush is another tool I had not investigated , but I have now! It's another small step along the way.

It's a tool I used a lot about 10 years ago. but once I really started using layer masks and adjustment layers, it became something that was really superfluous as I could achieve exactly the same results by painting on an adjustment layer without having to worry about the limits of the history brush (you lose all of your history when you close the file).

[Manfred M]

.....wow !!!

This is a short blurb on Michael, taken from the SPAO website: http://spao.ca/faculty

"MICHAEL TARDIOLI: DIRECTOR OF EDUCATION

Michael Tardioli co-founded The School of the Photographic Arts: Ottawa in 2005 to create an intensive photographic experience for those seeking to hone their artistic skills.

Tardioli began his career in photography as an analogue printmaker, most notably collaborating to produce prints for artists Yousuf and Malak Karsh. Shortly thereafter he became a founding partner in the highly successful portrait studio, Yanishevsky Tardioli. Tardioli developed new techniques in silver archival print-making, leading to commissions from the Honourable Mitchell Sharp, Margaret Trudeau, and Roberta Bondar.

Embracing the emergence of digital photography, Tardioli achieved a calibre of printmaking with digital tools that has normally only been associated with traditional techniques. Bridging this gap allowed him to attract such significant clients as the Canada Council for the Arts, the National Gallery of Canada, the Canadian Museum of Nature, and garnered him the Canadian Applied Arts Award.

Tardioli’s work can be found in the collections of the National Library and Archives, the National Arts Centre, and the Children’s Hospital of Eastern Ontario. The Children’s Hospital collection was published as a book entitled Hope.

As a teacher, Tardioli has mentored a generation of students, many of whom have developed into acclaimed artists and successful commercial photographers.

Tardioli’s students have gone on to receive international recognition and prestigious awards; their work has been acquired and commissioned by institutions around the world."

I'm quite lucky to live in a city where there are several highly regarded schools to study photography.


The technique that Michael developed was a bleaching process used on a silver print in which he painted on the bleach. His inspiration was Irving Penn, who used a similar but cruder technique to much acclaim.

[Rufus]

...once I really started using layer masks and adjustment layers, [the history brush] became something that was really superfluous as I could achieve exactly the same results by painting on an adjustment layer without having to worry about the limits of the history brush (you lose all of your history when you close the file).

This is along the lines "caveat usor" or user beware as there can be unintended consequences to using the tool. Thanks for highlightng this before I find out the hard way.

[DanK]

Another, simpler test, with only a single edit to make the effects clearer.

First, here is the original, very low contrast image taken on a foggy day:

1.



Step one: I sent the image (raw, working in the native Melissa prophoto space) from Lightroom into Photoshop, where ProPhoto is my working color space, and increased contrast with a levels adjustment, setting the black and white points at 30 and 180, using the normal blend mode. I made no other adjustments. I used a levels tool because I thought (incorrectly, it seems) that this would allow me to make the same contrast adjustment with luminosity blending and LAB mode. turns out not to be quite true, but I'll get to that. Here is the normal blend, with the increase in saturation apparent:

2.




Step 3: I simply changed the blend mode to luminosity. the increased saturation vanishes, but the image has more contrast than the original.

3.




Finally, I deleted the adjustment layer, converted the base layer to LAB, and added the same levels adjustment, set to lightness:

4.



Like #3, #4 shows increased contrast without much of an increase in saturation. However, #4 is considerably lighter, and in fact, it has some blown highlights. It appears that #3 might have a bit more saturation than #4, but I had a hunch that much of this was just an artifact of the difference in brightness.

Finally, I fiddled quickly with the brightness of the LAB version, doing a modest reduction with a brightness layer and a very small darkening of the highlights with the shadows/highlights tool. This very quick edit makes the two look even more similar. Here is a screen shot, with the edited LAB version on the left and luminosity blending on the right.

5.



They look very close to me, and I think with more careful tonal edits, I could get them closer.

My takeaway from this is that if one wants a contrast boost without an increase in saturation, either approach works, but one has to be careful about changes in overall brightness with the LAB levels adjustment. For me, the choice will boil down to convenience; I think the most convenient approach will depend on the image.

[Manfred M]

Thanks for the update Dan.

I ran a somewhat similar experiment but used the Sampler tool to get readings at the same poin. With ProPhoto RGB colour space with an adjustment curve using the Normal blending mode each colour channel changed by a different amount when I changed the mid-point. When using the Luminosity blending mode, each channel changed by the same amount. When working in the Lab colour (and getting a RGB reading), each channel changed by the same amount, as expected.

Working in the Lab colour space, there appears to be one more advantage; it is the widest colour space available, so any visible colour can be represented. As I use ACR I can select the Lab colour space as my default when I transfer a file to Photoshop. Lightroom does not support Lab (or CMYK) natively, so there has to be a conversion, so there could be some data loss.