The eq
filter adjusts the color channels relative to their current state, that is, increasing or decreasing their intensity (like a "volume" knob in sound). To achieve your target color temperature with eq
you'd have to calculate the current color temperature of each region in the photo and then modify it - something for which you need a frame server such as AviSynth. But all this is not really needed, because you don't really want to fully control your white balance, you just want to apply a fixed filter that will result in a specific color tone - in this case sepia. For that you should use instead the colorchannelmixer filter which provides a way to manage the white balance of the pixels relative to each other. The filter documentation has a specific example for sepia:
colorchannelmixer=.393:.769:.189:0:.349:.686:.168:0:.272:.534:.131
How it works (very simplified explanation)
The colorchannelmixer
filter describes the image as if it has 4 color channels - called Red, Green, Blue, and Alpha (the "mask" channel). By default, every channel represents the intensity of the color after which it is named as 1.0. So, the "Red" channel represents the portion of the image which is red times 1.0, the portion which is green times 0.0, the portion which is blue as 0.0., and the portion which is alpha times 0.0. Likewise, each other channel has a value of 1.0 for "its" color and 0.0 for all others. Now the filter enables you to "steal" a color from its channel and inject its "energy" to another channel. For example, you could increase the value of red in the "Green" channel to 1.0, and change the value of red in the "Red" channel to 0.0, so now the intensity of green will increase in every pixel by the original intensity of red, without leaving any red at all in the image, and without changing the relative intensity of blue and alpha. When applying this concept on all 16 color combinations you get a matrix that defines the resulting color intensity of each pixel as a product of all the original color intensities. The result is then normalized, and you get a simple way of expressing the color transformation curve.
The above is somewhat simplistic, and to understand how it applies to your specific question you can read more about Color Temperature theory. Note though that with the colorchannelmixer
filter you can do also other stuff such as generate color negatives, reduce color depths to achieve comics-like effects, and much more.
Hope this works well for you!