I took a number of pictures inside my office (without flash and with tripod) and depending on my position in respect to the large windows, picture luminosity varies a lot.
Before publishing them, what's the best workflow to balance their luminosity?
My camera is a Canon 100D. Thanks.
...What's the best workflow to balance their luminosity?
That kind of depends on whether you mean you want to balance the luminosity of the view through the window with the interior of your office or whether you mean you want to make the inside of the office the same brightness in all of the shots.
For the first case, you probably can't, since it is very likely the difference between the indoor and outdoor parts of your photos exceed the dynamic range capability of your (or any other digital) camera. This is particularly the case if you saved the images as JPEG files rather than saving the raw data as CR2 files.
For the second case, you can increase brightness of the dimmer images and decrease brightness of the brighter ones. This will be much easier to accomplish and give better results if you are working with the raw image data that if all you saved were jpeg images.
Most raw convertors will allow you to increase exposure by 2-3 stops fairly easily. Some will allow even more in the raw development module. You can also increase the brightness more using the "curves" tools in many raw convertors.
With jpeg images it is usually difficult to increase brightness much more than a stop or so before the appearance of the image really starts to suffer.
You might also need to increase the amount of noise reduction applied with large boosts to exposure/brightness.
Going forward when shooting in such conditions:
Of course the obvious strategy is, "Plan your shots so the windows aren't in the frame." But this is not always possible.
Varying lighting conditions such as described in the question is one reason to use Manual exposure mode. Set exposure for the office interior and then let the widows blow out, regardless of their position in the frame. As you have discovered, the camera is trying to guess what you want to be properly exposed. When the total brightness range of the scene exceeds your camera's range, it leads to inconsistent results.
Particularly with 'Evaluative' or 'Center-weighted Averaging' metering, the camera will change exposure significantly when a bright light source (such as a bright window) is closer to the center of the frame compared to when it is nearer the edge of a frame or not in a frame at all. If 'Highlight Tone Priority' is enabled, that will also cause the camera to reduce exposure to prevent blowing out very bright light sources in an otherwise dark scene. You can try using 'partial' or 'Spot' metering (I'm not sure which the Rebel SL1/100D offers), but you must be careful not to let the metering area point at a window. You'll also need to account for unusually dark (business suit?) or light (linen dress?) things if you are metering them. The camera assumes you want whatever you meter, no matter if it is a black cat in a coal mine or a white cat in the snow, to turn out medium gray.
In a comment, you say
I mean that simply camera auto settings change luminosity a lot depending on where you point the camera. In particular, I'm shooting the office furniture
Right, so... don't do that. Get a meter reading that looks right, and then put your camera on manual mode with those same settings so nothing changes. If needed, adjust shutter speed up and down by hand to balance things out. You're using a tripod and your subject is stationary (unless it's exceptionally exciting office furniture!) so you can adjust shutter speed basically freely.
If these are for serious purpose of any kind, you probably will want to take better control of the light — adding off-camera wireless flash will make a world of difference. But simply owning the exposure will be a good start.
jpegso far, I can took more in raw file format if necessary. \$\endgroup\$