There are many specifications that matter when it comes to VR, but refresh rate is often overlooked. While “field of view” or screen resolution are important in their own right, the headsets refresh rate can mean the difference between immersion and aversion.
The Basics of Refresh Rates
No matter what type of display device you’re using, it has a specific maximum refresh rate. This is the time it takes to completely redraw the on-screen image. Refresh rates are measured in Hz (Hertz) and so the number you see represents the total number of display redraws in one second.
The vast majority of displays out in the world are capable of at least 60Hz with newer displays hitting numbers like 120Hz, 240Hz, and even 360Hz! The higher the refresh rate, the smoother the motion that a screen can reproduce. For interactive media, such as video games, higher refresh rates also make the experiences more responsive and immediate.
The Difference Between Frame Rate and Refresh Rate
While a monitor’s refresh rate tells us the maximum number of fresh images it can display in one second, the content has to be there for this to mean anything. For example, your video game needs to render 60 frames every second if you want to see the full effect of a 60Hz screen.
If it renders less than this, you’ll only see the motion quality of the actual frames produced. Likewise, if you’re getting more frames in a second than the screen can display, you’re wasting those frames since you’ll never see them.
Frame Rate Is a Type of Resolution
To understand why refresh rate and frame rate are important, it’s helpful to think of them as a form of resolution. Usually, resolution in the context of displays refers to the number of pixels that are on display. A 4K image contains four times the pixels of a 1080p Full HD image. It’s, therefore, possible to see fine detail in the 4K image that simply doesn’t exist in the lower resolution image.
This is the spatial resolution of the image, a single moment frozen in time.
The frame rate is the temporal resolution. That is, the amount of detail in the image over time. Think of it this way, at 60 frames per second you are seeing 60 samples of time within the virtual world. Anything that happens in between those snapshots is invisible to you. If an object is moving within your view, you’re only seeing its current position update every 60th of a second. If you doubled the frame rate to 120 frames per second (and you’re using a 120Hz screen) you’d see twice as much information, because now you have twice as many samples of time every second. The end result is that motion appears smoother the higher the frame rate and refresh rate increase together.
This isn’t just a visual improvement either. As temporal resolution increases, the responsiveness of the world to your actions becomes faster as well. The time between you performing and action and seeing it reflect in the virtual world shrinks, which increases your sense of connection with it.
Refresh Rate, Frame Rate, and VR Presence
The real world has no refresh rate. Well, if we’re being pedantic surely a theoretical physicist will bring up something about string theory, vibrations, or some other related idea that’s beyond the scope of an article about VR. The point is that for our purposes, the real world happens in real-time.
Our perception of the real world isn’t quite in real-time and we don’t have the bandwidth to process all the detail that’s there, but likewise, for the sake of argument here, you perceive reality as a continuous flow of sensory information. Our vision is analog, not chopped up into digital slices the way a computer game is.
With the rise of modern VR, it became apparent that the concept of “presence” was important to make it a transformational experience. You achieve presence when you can fool the user’s brain into feeling as if they are present in the virtual world, rather than their real-world location.
After lots of research and experimentation, it turns out that there are a number of technical aspects that need to perform at certain minimum levels for presence to happen. For example, the horizontal field of view (essentially your central and peripheral vision) needs to be at least 90 degrees wide. Human vision is usually around 180 degrees wide and perhaps a little wider, but at 90 degrees presence becomes possible.
Latency is another major factor. The end-to-end latency of a VR system should be no more than 50ms and preferable less than 20ms. At least, these are the figures to aim for according to former Oculus Technology Chief John Carmack, also of ID Software fame.
Refresh rate (and frame rate) is a crucial part of the presence puzzle as well. For one thing, it’s directly related to latency, but also to the smoothness of motion. As the frame rate increases and latency decreases, the virtual world starts to look and feel more like the real world our brain expects.
How Important is Refresh Rate in VR?
As you’ve now seen, the refresh rate and the frame rates that it enables, are crucial for good VR that feels present and offers a comfortable user experience. How much refresh rate do we want? When work on the original Oculus Rift prototype was underway, the common wisdom was that 90Hz (and therefore 90 frames per second) was the minimum target for VR presence to work.
Since then, Oculus has actually released a headset with a lower refresh rate than this. The first Oculus Quest offered a “mere” 72Hz, but it turns out that this was a non-issue. At least for the Quest 1’s low-persistence OLED display which helped cut motion blur and latency to some extent.
That 72Hz product may only have been a blip, however, since the Quest 2 and every other major headset now offers 90Hz, with higher numbers set to become the norm in the future. In the end, refresh rate is very important to VR, but if you’re running at 90Hz already, you don’t have much to worry about, as long as you have enough horsepower to drive the frames required.