It may be small, but it sure packs a punch. The Zenfone 9's screen is under 6" diagonally - a true rarity these days. For that size, the 1080p+ resolution is perfectly adequate. In fact, as we always say, it would be so even for 'normal' sized devices, since we don't think anyone can tell the difference between that and QHD+ in real life at realistic distances from the eye. But for such a small phone, this is almost like QHD+ on a very large one. So that area is covered very well.
Next up, let's talk brightness. The Zenfone 9's screen goes up to over 800 nits, according to our testing, which is enough that it never feels illegible, even on a bright sunny day. That said, some competitors like the Galaxy S22 almost push 1,000, and if we're bringing in bigger handsets to the comparison, things look even worse for the Zenfone. So you won't actually have problems with it in day-to-day use, but with brightness, more is always better, so perhaps the Zenfone 10 can break past the 1,000-nit barrier when it arrives.
At the other end of the scale, the Zenfone 9's display does get reasonably dim at the minimum brightness level, but it's far from the dimmest we've seen. And yet, that possible shortcoming is fully mitigated by the presence of the Extra dim feature, which allows you to 'apply more dimness', as it were, with an intensity slider of its own. Sure, it's an added hassle to employ this standalone feature, but we'll take it over nothing any day of the week - and it does have its own Quick Settings toggle, which makes it easier to use.
With Extra dim, we don't think anyone would ever feel that the screen is too bright, even in pitch dark conditions. In fact, for this reviewer, the dimmest setting in Extra dim is way too dim, so there's clearly some leeway left even if you like your screen darker than we do. We wish all Android skins would implement this feature sooner, rather than later - it's a boon at night if you don't enjoy being fully lit up by your phone's screen like you're some kind of roadside attraction.
We're not sure if the Zenfone 9 has a second, rear-facing ambient light sensor as some other devices do, but if it does, then it's very bad at its job. So it's most likely there isn't one. Changes in ambient light taking place behind the phone are constantly ignored, and this would be okay for a mid-ranger, but the Zenfone 9 isn't one of those, so we expect nice little quality-of-life perks like this to be present.
Furthermore, the auto-brightness algorithm is pretty bad. It's not the worst we've ever encountered, but it does seem to get confused quite often, so you will find yourself with either a too bright or too dim screen at least a few times every day. Again, this isn't out of the ordinary performance for a mid-ranger, but from a flagship we expect more finely tuned algorithms, and this one still needs work.
The problem is compounded by the fact that when you do need to make manual adjustments, you have to swipe down twice (unlike in most heavier skins where only one swipe is required), since the brightness slider is at the bottom of the expanded Quick Settings view, and not in the notification pane.
The display is a high-quality one, we've had no complaints about that. The Splendid menu in Settings can take some getting used to, however, as it's quite confusing. It starts with the name - unless you've used an Asus computer or bought an Asus motherboard or graphics card, "Splendid" just won't mean anything to you, but this is in fact where you go to tweak colors.
Standard mode is the one that's tailored to cover the sRGB color space most accurately, and it does a decent job, but we've definitely seen better screen calibration in other devices. We stuck with Standard since most stuff on the internet is in the sRGB space still, but if you want to go P3, then Cinematic mode is for you. Optimal and Natural are just going for the vivid look and aren't really calibrated to anything.
In day-to-day use, the Zenfone 9's screen doesn't seem lacking in quality or colors, but if you are a connoisseur, you'll know that there are better panels out there. We just think it's fair to say most people wouldn't tell the difference in real life unless holding devices side to side - and who does that on a daily basis aside from us reviewers and all-out geeks?
The Zenfone 9's screen goes up to 120 Hz, as it should in this day and age, and while the panel is a bit flexible about refresh rates, this is no LTPO 2.0 or 3.0. It basically switches between 120 Hz, 90 Hz, and 60 Hz, while touch sampling is handled at 240 Hz. As always, we manually selected 120 Hz in Settings - after all, why wouldn't we? Refresh rate influences the perception of smoothness in a huge way and we like our smartphones as smooth as they can be.
Picking 120 Hz results in you being exposed to that exact rate at all times while you interact with the phone. It drops to 60 Hz when idle, unless the screen is on low brightness, in which case it might stay at 120 Hz 100% of the time. If you choose 60 Hz or 90 Hz, you'll be locked into that specific setting, which isn't surprising for the former but is for the latter. Finally, Auto mode tries to do its best to pick and choose a rate based on what you're doing, so there are a lot of intricacies to Asus' handling of refresh rates on this phone.
And yet, if you're looking for quick advice - just set it to 120 Hz and never think about it again. That is, if the battery life you then get is enough for you. If you desperately need more endurance, it might pay to choose a different mode - perhaps start with Auto and see if that improves things on the battery life front, and if not, go down to 90 Hz or even 60 Hz.
You will, unfortunately, have to set the latter if you need DC dimming, as it's only available when the screen is locked to 60 Hz. So if you're sensitive to pulse-width modulation dimming, either skip this handset or make sure you're okay with a non-high refresh rate. We wish you didn't have to make this compromise, but it is what it is.
The Asus Zenfone 9 technically has an always-on display, which ZenUI is calling "Always-on Panel", but it's one of the most barebones implementations we've seen in quite some time. You get to choose between a grand total of four clock styles, and that's it.
The only other thing that's customizable is whether you want it to be truly always-on, operate on a schedule, or only show for 10 seconds after each interaction with the phone. We appreciate the presence of this functionality, as we always do, but if you're coming from a more customizes Android skin by another manufacturer, you might be shocked at how little customizability there is.
Similarly, the Night Light blue light filter exists and comes with an intensity slider and the ability to automatically turn on at sunset and off at sunrise, or operate on a custom schedule. If you're looking for textures and desaturated colors and stuff like that, stick with MIUI devices, since at the moment those are unique in having such extra functions built into their blue light filters.