In aperture priority, you set the aperture and the camera automatically chooses the other exposure settings to reach what it thinks is a normal exposure. On your X100T, if you stay in aperture priority, changing ISO or shutter speed usually won’t make the photo brighter or darker by itself because the camera will compensate with the other setting.

To make the image brighter or darker while staying in aperture priority, use exposure compensation:

• positive EV = brighter
• negative EV = darker

That tells the camera to intentionally expose above or below its default meter reading.

ISO still matters for image quality and shutter-speed behavior, but not usually for changing final brightness in this mode if Auto exposure is active. For example, raising ISO may simply cause the camera to choose a faster shutter speed instead.

So the practical workflow is:

1. Choose the aperture for the depth of field you want.
2. Leave the camera to calculate the rest.
3. Use exposure compensation if the result looks too bright or too dark.

If you want direct control over aperture, shutter speed, and ISO without the camera compensating, switch to full manual exposure instead.