Can Lightroom apply a precise numeric gamma correction, such as converting linear scans from gamma 1.0 to 2.2?

Question

I’m working with scanned images that are effectively linear (gamma 1.0) and need an accurate gamma conversion to something like gamma 2.2. In Photoshop this can be done numerically with Levels/Exposure-style adjustments, but in Lightroom I only have tools like Tone Curve.

I’m looking for a reproducible, preferably numeric method in Lightroom that gives the same result as a true gamma correction, not just an approximate visual tweak. The goal is to keep everything non-destructive in Lightroom using virtual copies, without creating duplicate corrected files.

user38978 · Answer

One notion.

We know that midpoint of 128 linear data when raised to gamma 2.2 goes to 186, or to about 73%. (0.5^1/2.2)

We know RGB images normally consist of gamma data (histograms show gamma data), but you have gamma 1.0 data that does not. So raising the 50% input point in the Curve Tool straight up from 128 output to 186 output (50% to 73%) ought to convert 1.0 to 2.2.

You probably could work up a formula for any exponent, but this should do 1.0 to 2.2.

EDIT: removed incorrect statement about PS Exposure.

The PS Levels middle slider is gamma too.

EDIT:

Pondering the gamma multiplier a bit, here's a chart of various multipliers:

128 linear (0.5) with gamma 2.2 multiplier of

Mult 0.2x, Exp 0.4, value 53
Mult 0.3x, Exp 0.7, value 89
Mult 0.4x, Exp 0.9, value 116
Mult 0.5x, Exp 1.1, value 136
Mult 0.6x, Exp 1.3, value 151
Mult 0.7x, Exp 1.5, value 163
Mult 0.8x, Exp 1.8, value 172
Mult 0.9x, Exp 2.0, value 180
Mult 1.0x, Exp 2.2, value 186
Mult 1.1x, Exp 2.4, value 191
Mult 1.2x, Exp 2.6, value 196
Mult 1.3x, Exp 2.9, value 200
Mult 1.4x, Exp 3.1, value 204
Mult 1.5x, Exp 3.3, value 207
Mult 1.6x, Exp 3.5, value 209
Mult 1.7x, Exp 3.7, value 212
Mult 1.8x, Exp 4.0, value 214
Mult 1.9x, Exp 4.2, value 216
Mult 2.0x, Exp 4.4, value 218
Mult 2.1x, Exp 4.6, value 219
Mult 2.2x, Exp 4.8, value 221
Mult 2.3x, Exp 5.1, value 222
Mult 2.4x, Exp 5.3, value 224
Mult 2.5x, Exp 5.5, value 225
Mult 2.6x, Exp 5.7, value 226
Mult 2.7x, Exp 5.9, value 227
Mult 2.8x, Exp 6.2, value 228
Mult 2.9x, Exp 6.4, value 229
Mult 3.0x, Exp 6.6, value 230
Mult 3.1x, Exp 6.8, value 230
Mult 3.2x, Exp 7.0, value 231
Mult 3.3x, Exp 7.3, value 232
Mult 3.4x, Exp 7.5, value 232
Mult 3.5x, Exp 7.7, value 233
Mult 3.6x, Exp 7.9, value 234
Mult 3.7x, Exp 8.1, value 234
Mult 3.8x, Exp 8.4, value 235
Mult 3.9x, Exp 8.6, value 235
Mult 4.0x, Exp 8.8, value 236

Can Lightroom apply a precise numeric gamma correction, such as converting linear scans from gamma 1.0 to 2.2?

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