Setting Exposure

[Fedevan10]

Hi, I'm a dentistry student and I'm doing a project where I need to use photoshop. I have made a pic of a mouth with near a grey/white balance card which I know exactly be 79.0.0. (CIE LAB Values). I need to set the pic's exposure based on this value. How can I make it without trying many times until I've reached the correct value? Like a command or specific function

P.S. I'm attaching also the pic so you understand what I'm talking about 

 

[Bojan Živković]

If I understand correctly, you're asking to go to Image > Mode > Lab Color, place a Color Sampler point on the card, then create a Curves adjustment layer and correct the Lab values to the exact numbers, right?

[Fedevan10]

Thanks a lot! Even for the fast reply💪💪💪

[Lumigraphics]

Add a color fill layer set to L*A*B values of 79, 0, 0. Set it to Difference mode.

Then add a curves adjustment layer. Use the black point eyedropper and click on an area of the card. This will set the card to black, which is what Difference give you if two layers are identical. Now turn off the color fill layer.

Please note that illumination on your image is not even! So the grey card has different values depending on where sampled. You'll have to decide which is accurate.

[Fedevan10]

Thanks. Do you think it's more accurate than the other solution? 

[Lumigraphics]

Its exact. But as I said, your card is not lit evenly.

[Fedevan10]

Thanks. However this is just a test. The real pics will be taken with a polarized filter so that I'll take down the reflection problems and the sample has to be down within the two circles. Thanks very much 

[Lumigraphics]

Medical photos are often shot with a ring light, which helps give even illumination.

You can automate this with a Colorchecker and calibration of your display and a camera/lighting profile.

https://calibrite.com/us/product/colorchecker-passport-photo-2/

https://calibrite.com/us/product-category/photographers/

[Davide_Barranca]

Hi @Lumigraphics,

I have tried out of curiosity, but what you suggested (if I haven't misinterpreted it) doesn't seem to work; the readings are way off, so it's not rounding errors or a misplaced reading—see the attached screencast.

I think the Curve should be added below the Solid Color layer (not above), and tweaked until the reading of the composite (with the Solid color layer on top, Difference mode) is 0,0,0. But then, I don't see how to automate the process with a single eyedropper click... Am I missing something?

[Lumigraphics]

I haven't done any testing. Maybe if you invert the curve? or reorder the layers. But the principle should work. The composite should be black.

[Davide_Barranca]

The principle works if you keep the Curves below the Solid Color; then, by definition of the Difference mode, the reading is black. The theory checks.

And when you remove the Solid Color, you hit the Lab target.

But there is no way to automate the curves with a single Shadow/Highlights eyedropper click. One can do it manually, though, channel by channel. It took me like half a minute.

Alternatively (and this is even faster) @Fedevan10 can just operate this way:

• disregard the solid color thing: just add a Curves adjustment layer.
• select the gray point eyedropper in the Curves (the middle one) and click on the target in the image; this neutralizes the white balance card moving each R, G, B channels curves middle point.
• Set the readings in the Info palette as Lab. The a,b channels will be zeroed, but the L value will likely be off. 
• Add one point to the composite curve, and move it until the L is right.

The attached screencast shows the whole thing; when you get the hang of it, it's really fast. If you have a thousand pictures to correct, maybe annoying, but... 😅

[Fedevan10]

Ciao Davide, immagino tu sia italiano. Ma secondo te la prima soluzione non andava bene? Ossia quella di prendere il LAB prima di fare qualsiasi modifica nel punto di interesse (che io considero tra i due cerchietti), poi creare una curva di aggiustamento e inserire i valori sotto la curva i valori vecchi e quelli nuovi per L, a e b. Secondo te? Mi sembrava piuttosto veloce come sistema.