Inspiring

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Many adjustment layers cause problems or not?

Forum|Forum|3 years ago
January 18, 2023
3 replies
1333 views

I recently heard from one retoucher that it's better to use as little adjustment layers as possible, because if we apply many corrections, it leads to banding, color distortion and "damages" pixels 🙂 is that true?

Does it make any difference if we use a Curves adj. for contrast, another one for color correction, the third one for changing the contrast again vs. doing all these inside 1 adjustment? What's the math behind it? Doesn't PS just calculate an average value for each pixel which is the same in both cases?

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Correct answer D Fosse

No, but further adjustments will be distributed over 32768 possible levels per channel, instead of 256 levels per channel.

When later reduced back to 8 bit, those levels will all be perfectly evenly distributed. What you avoid is cumulative banding.

So yes, it always pays to work in 16 bit.

Ivan ZajatsAuthor

Inspiring

If I have RAWs, I often work in 16 bits. Does it make any sense to switch from 8 to 16 bits while working with stock JPEGs (which are obviously 8 bits)? The information won't occur from nowhere, right?

D Fosse

Community Expert

Here's an illustration. Refreshed histograms showing true data, not cached:

Top is the result of a series of 8 bit adjustments.

Bottom is the same adjustments, but converted to 16 bit for the adjustments, then back to the original 8 bit.

D Fosse

Community Expert

Excellent example, Dag!

Thanks, Pierre-Etienne.

You really have to watch strict procedure to get a correct result from this exercise. I had to do it three times before I was convinced I got it right, with the histograms showing true and realistic data. I recorded the adjustments as an action on a 16 bit original, you can't do it the other way.

For a while the fashionable attitude was "histogram-schmistogram, what matters is what you see". Fair enough, but that misses what an extremely effective diagnostic tool the histogram is. If there's anything wrong with the file, the histogram will almost always show precisely what it is.

c.pfaffenbichler

Community Expert

What's the math behind it?

What do you mean exactly?

Adjustment Layers are applied in sequence, so the each Adjustment Layer operates on the result of the lower ones; and that means, as @D Fosse already mentioned, that the necassary rounding can accumulate to result in visible banding.

Ivan ZajatsAuthor

Inspiring

@c.pfaffenbichler read the last question in the post, that's what I mean.

2+(3+5) = 10 vs 2+8 = 10

any difference? no

doesn't PS work the same way?

if some error accumulates, where does it come from?

c.pfaffenbichler

Community Expert

I had read the question »Doesn't PS just calculate an average value for each pixel which is the same in both cases?« and did not understand what you meant.

Adjustment Layers can do more than apply simple addition. (Aside from the fact that, in 8bit, adding might push previously distinct values to the maximum possible value, 255, and cause clipping.)

if some error accumulates, where does it come from?

If a Curves Layer, for example, is applied to 8bit images the resulting values have to be fitted in 8bit – meaning there may still be 256 distinct values but there might also be less.

If a further Curves Layer is applied atop that the operation is applied on the degraded image that may not actually contain 256 distinct values anymore.

Edit: Note the histogram:

D Fosse

Community Expert

As long as you work in 16 bit color depth, this isn't a concern.

If you work in 8 bit color depth, any adjustment may potentially lead to banding and general degradation. There will only be 256 discrete steps per channel, and with several consecutive adjustments, those 256 steps may end up very unevenly distributed.

In 16 bit depth you have 32768 individual steps per channel. You won't be able to produce banding even if you try.

(Note that your display path will usually be 8 bit. On-screen banding does not necessarily mean there's banding in the data).

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