Is there a way to set exposure in RT with a grey card?

Later today I’m going to take some real estate photos for a friend of mine. I haven’t done this sort of photography before but after watching some youtube tutorials, I plan to take 3-5 frames for each picture and then composite them together in GIMP.

The reason for multiple shots is that I will adjust the strobe power in between frames to keep a consistent exposure across the room. I’ll have a grey card reference for each frame that I want to use.

I know I can use the eyedropper in RT to white balance on the grey card. Is there a way to also use the card for setting the exposure? I know in Gimp, I could do this in the levels or curves dialog with an eyedropper sample tool, but I’m surprised that I can’t do this in RT since RT seems to have tools for just about everything else.

Or maybe the way to do it in RT is to first white balance off the grey card, then put a lockable color picker spot on the card, and adjust exposure until the V in HSV is 50? Does a V of 50 correspond to 18% grey?

This isn’t what you asked but Darktable can do both exposure and color matching…you can sample one frame and match the others to it or set the spot exposure to 50% and do it on all the grey cards.

and from the dev…

Note not experimental now…its in the current release…

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I would do it this way, yes. There is no automated way to set for middle grey exposure. That could be a nice feature for the future. I am not 100% sure V = 50 equals 18% grey. However, that shouldn’t really matter for your shots: as long as the exposure is equal. Right?

You mention using a grey card. These have specific values and, if you are lucky, these values were provided with the card.

If I look at the ColorChecker that I use it has these values for neutral grey:

RGB → 122 122 121
L*a*b → 50.867 -0,153 -0.27

Using the Colour picker set to L*a*b to set the L value to, in the above given 50,8 and then use the WB picker, on the same spot, to auto set the WB should get you in a very good ballpark. You might need to go back-and-forth to fine-tune and most often I need to slightly adjust the temperature slider to fine-tune the a and/or b channel.

If your card/checker did not come with a data-sheet I do think that the afore mentioned L*a*b numbers are rather universal (assuming you have a good grey card).

How about using normal procedure:

  • white card for white balance, and
  • gray card for exposure?

Have fun!
Claes in Lund, Sweden

Hmm, I was under the impression that you could pick any neutral grey tone to use for the white balance, so grey and white would both work OK. At least, that’s how I have been doing it for a while now.

LOL my grey card is the $9.99 ebay special. Though I think your RGB values will be close enough to get me in the ballpark. Thanks!

I think consistency over all your images is more important then (very) exact numbers, so your grey card and the aforementioned method should indeed work nicely in this case.

About the white vs grey card: I use a white card if I do in camera White Balancing (preset). The grey card can be present in the first (and last) shot to do the white balancing afterwards. Both methods have their use.



Later today I’m going to take some real estate photos

How did the photos turn out?

Have fun!
Claes in Lund, Sweden

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This might help! Andy gets into WB in RT