Is color filter detection on speedlights useful when shooting RAW?

Question

I am planning to upgrade my speedlight setup. Naturally, I stumbled upon Canon's Speedlite 600EX II-RT, which offers 2 color filters (both CTOs) that will be detected by the flash.

The speedlight's manual states on p. 48:

By attaching the provided color filter to the flash, automatic correction is made by the camera's white balance function so that both the subject and background can be shot with appropriate white balance.

I also found out that Nikon's SB-700 (and most likely other speedlights, too) supports this feature as "Color compensation filters", as described on p. E-21 of its manual:

When a color compensation filter is attached to the SB-700 while the camera's white balance is set to auto or flash, filter information is automatically transmitted to the camera, and the camera's optimum white balance is automatically adjusted to give the correct color temperature.

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To me, this seems to implicate that the speedlight will tell the camera¹ to not use the flash preset for white balance, but to use a lower color temperature. So while great for shooting JPEGs, this seems to have no impact on shooting RAW (other than changing the initial WB value, which is something I almost always ignore, anyways), meaning that any speedlight with a regular CTO-gel would do just as well (from this perspective).

¹ _{If the camera supports this speedlight-feature, of course.}

Is my assumption correct or is there more to this feature?

_{Note that I am aware that custom gels could melt and are harder to fix on the speedlight than OEM filters.}

Answer 1

The reason to use the filter is when two conditions hold:

1. The ambient light is a much lower color temperature than the flash, i.e. the ambient light is incandescent or 3000K LED. Or fluorescent, if you have a filter for that.
2. You are shooting such that the ambient light will be a significant contribution to the total light (i.e. 50% ambient 50% flash).

Without the filter, you'd have two colors of light illuminating the scene, which can produce odd-looking results, with shadows and highlights having very different colors (blue vs yellow).

The above is independent of whether you're shooting RAW or JPEG.

As to how this interacts with RAW vs JPEG shooting:

If shooting JPEG (or JPEG+RAW), the settings you have programmed into the camera, including the presence of the filter (if your camera and flash support it), are used to control the in-camera process that generates the JPEG from the raw data. You are correct, that if the filter is present and the camera can handle it, the automatic white balance is adjusted to compensate.

If shooting RAW, all the white balance information the camera is aware of is recorded ONLY as metadata in the raw file. It does not modify the raw data captured by the camera in any way.

In both RAW and JPEG, WB information is used to generate the preview you see on the camera's display but for RAW it does not modify the raw data.

When you import the RAW file into a metadata-aware program, such as Lightroom, the color balance is used to generate the initial display, but you still retain full control over the final WB.

To summarize, the filter has a purpose, which is to avoid a scene lit by two different colors of light. With the filter, both the ambient and flash lighting have similar color, at the cost of about 1½ stops loss of flash power.

But it has no effect on the raw image data captured.

It is useful to remember that RAW data is intended to be permanently READ-ONLY after the image is captured. No standard image-processing program will modify the image data in a RAW file.

Answer 2

The purpose of the CTO filter is to make the color of the flash the same as incandescent light. You would use it when you have incandescent light as the ambient light and you are lighting a subject with the flash. This makes the flash color the same as the ambient color. Without it you would have mixed lighting which may not be fixable in post processing.