@Tazim, there is an issue with the vivid screen as others have mentioned. I tested my S23 Ultra besides my S24 Ultra for a week, and I agree that the S24 Ultra is not as bright in general terms. If you switch adaptive brightness off and push the brightness of both all the way up and enable extra brightness, it is very evident that the S23 Ultra is brighter. Make sure both are set to "natural" as vivid doesn't currently do anything on the S24 Ultra (from recent responses from Samsung Chat and people in the known on Twitter etc have said this will be fixed in the February roll out).
This is most noticeable on the home screen and also on things like Netflix and YouTube. If you play the opening sequence from Our Planet with the moon, you will see a massive different in the brightness, along with the white pop up menu. Me, amongst others, have noticed the difference in brightness, so hopefully, this is also addressed at some point. I think the 2,600 not testing referred to is if you simulate a bright light on the sensor. This will push the brightness up. Apart from actual content, I tend to have mine on adaptive as the brightness of the S23 Ultra is blinding. From other tests I have seen online, the S24 Ultra was only about 100 nits brighter than the S23 Ultra in certain scenarios.
