Midtones adjustment

[Glenn]

Wondered if someone could do me a favour?

I am an Affinity Photo user and AP doesn't have a mid-tones adjustment slider. I would like to understand how the LR mid-tones slider works with a view to replicating its action with curves.I understand the tool is to be found in the colour grading panel.

Would somebody be able load the greyscale chart below into LR and send three screenshots of the histogram please? I would like one with the slider fully left, one with it central and one with it fully to the right.

Many thanks.

 

[Glenn]

 

[ian barber]

Left



Centre


Right

 

[Glenn]

Brilliant! Thanks Ian.

 

[ian barber]

Using Javascript, I re-mapped the values from the slider placed all the way to the left and it made this curve in Photoshop

 

[Glenn]

Thanks Ian. I was part way through doing the same thing in Excel. I suspected it would be an S-curve contrast adjustment with end points at X=33% and X=66% but it seems not. Instead it looks like a U-curve brightness adjustment extending further along the x axis in each direction. Interesting. So now I know.

 

[Glenn]

For the sake of completeness here is the excel version I made.

1. The range of control is from input luminance values of about 14% to about 90%
2. The slider shifts the brightness of the mid-tones by about +/-20% at the extreme ends of the adjustment range

 

[Glenn]

In case anybody is wondering about the practical application of the above, I now have two new Curves pre-sets in Affinity Photo, one for Mid-tones +20% and one for -20%. Adjusting to other values is simply a matter of dragging the central handle up and down.

If you want to make your own then add co-ordinates for three new handles as follows:

Lower handle (X=0.15, Y=0.15)​

Centre handles:​

• For +20% mid-tone (X=0.50, Y=0.60)
• For -20% mid-tone (X=0.50, Y=0.40)

Upper handle (X=0.85, Y=0.85)​

Once you have added the three handles the exact co-ordinates can be typed directly into the X: and Y: fields at the bottom left of the Curves panel.

The Add Preset field top right of the Curves panel will allow you to save the settings for later use.

 

[ian barber]

Again using Javascript to map, this is the curve produced from having the mid-tone slider to the right

 

[Glenn]

Thanks Ian. I appreciate the effort.

It looks to me like our two approaches give similar results in respect of the shape and key co-ordinates of the adjustment curves.