Are exposure and contrast lossy?

[IanSG]

This isn't an Affinity specific question, but I'm hoping someone here can shed some light on this...

This morning I watched a video in which the tutor stated that using the exposure and contrast sliders in RAW processors resulted in data being lost, and people should use curves and levels instead.  This has got my local U3A camera group scratching their heads - is it true, is it a throw back to older software...?

[Old Bruce]

59 minutes ago, IanSG said:

This has got my local U3A camera group scratching their heads

Got me scratching my head too. 

[HVDB Photography]

That is a big misunderstanding.
Affinity Photo's Develop Persona does not overwrite your RAW images. They remain untouched. The RAW data that is being read is processed virtually in memory.
If you save your settings as a preset, you can always open the RAW file again and start over from scratch our load the previous saved preset for further edit.

[carl123]

1 hour ago, IanSG said:

This morning I watched a video

Can you remember which one?

[IanSG]

2 hours ago, HVDB Photography said:

That is a big misunderstanding.
Affinity Photo's Develop Persona does not overwrite your RAW images.

That wasn't the issue - there was no suggestion that the RAW files were being overwritten.  The "problem" was that data would somehow be unavailable downstream in the workflow.

[IanSG]

2 hours ago, carl123 said:

Can you remember which one?

It was part of David Noton's "Chasing the light".

[v_kyr]

See for example "Loosing data and Limiting Options" in this ACR related book:  Real World Camera Raw with Adobe Photoshop

[paristo]

There are four assumptions here:

• That files itself that contains all the data are modified.
• The image data that is being processed in memory (files opened in editor) are lost.
• The process to change the image data values (contrat, brightness, saturation etc) are losing data.
• The specific process to change the image data value (curves vs levels vs exposure slide) are losing data while some not.

As everyone knows, the first one is lossless. No data is modified unless you rewrite the raw file (some editors can do that).

The second is question about how the application does the process. In the current way (1.6.x) the AP will convert the raw file to own form in RAM to process it, and that conversion is lossy for that session. So unless you reload the raw file and perform new conversion, you have lost the data. I am not sure about AP new 1.7 version does it allow return to raw conversion and adjust settings without reopening the file (lossless performance).

The third is just wrong if the adjustments are in own layers, like default in AP, and in Photoshop called "Smart layers". Meaning the adjustment only stores metadata that how the image is to be shown to the user, no image data is modified.

The fourth is then per application based, like there are plugins that requires that image editor will export the file first (it combines all the layers etc) and then outputs it as input to the plugin. And then plugin does perform its own edit to the image and outputs it back, either replacing the layer or getting added as new layer or even opening a new image. Like example in Affinity Photo using a DXO plugins, it is better to first stack all visible layers in AP and make copy of it, and then open that new layer in the plugin. You don't modify the existing layers than the new stacked layer. And it is lossy task as you can't go back later in the DXO plugin to readjust it, but you need to rerun the plugin from scratch to the copy layer you made and delete the previous one. 

Now, does the Exposure adjustment in AP perform a lossy adjustment to layer, instead being a adjustment layer modifying the view and being only applied on the export?

If it is not in the layer adjustment list, and become as adjustment layer among other layers, then it is destructive adjustment to the layer you apply it, but not the the original file but to that layer. 

But as you can see in Affinity Photo, Exposure adjustment in layers is non-destructive (lossless), so you can go back later and readjust it. 

But other question is, is it destructive (non-readjustable) when you convert raw file (open raw file) in to Affinity Photo? Likely once you hit "Develop", it is what it is.

 In some other applications, you might be able to just click the original layer and get the conversion settings again to modify it and then all your previous layers etc are applied normally again. 

[IanSG]

@v_kyr and @paristo -  thanks for the input!  I think the concern was that the exposure control could cause clipping.  The question's moot though - I've just watched a much more recent video (David Noton talking about post processing at the 2018 Photography Show) and he's now saying that using the exposure controls is fine!