Sensor and Performance

The Finalmouse UltralightX is equipped with the PixArt PAW3395. According to specifications, the 3395 is capable of up to 26,000 CPI, as well as a maximum tracking speed of 650 IPS, which equals 16.51 m/s. Out of the box, five pre-defined CPI steps are available, though these can only be switched within the software: 400, 800, 1600, 3200, and 6400.

All testing was done on the latest firmware (2.1.0). As such, results obtained on earlier firmware versions may differ from those presented hereafter.

CPI Accuracy

"CPI" (short for counts per inch) describes the number of counts registered by the mouse if it is moved exactly an inch. There are several factors (firmware, mounting height of the sensor not meeting specifications, mouse feet thickness, mousing surface, among others) which may contribute to nominal CPI not matching actual CPI. It is impossible to always achieve a perfect match, but ideally, nominal and actual CPI should differ as little as possible. In this test, I'm determining whether this is the case or not. However, please keep in mind that said variance will still differ from unit to unit, so your mileage may vary.


I've restricted my testing to the four most common CPI steps, which are 400, 800, 1600, and 3200. As you can see, deviation is consistently positive, consistent, and large, which is an average result overall.

Motion Delay

"Motion delay" encompasses all kinds of sensor lag. Any further sources of input delay will not be recorded in this test. The main thing I'll be looking for in this test is sensor smoothing, which describes an averaging of motion data across several capture frames, in order to reduce jitter at higher CPI values, increasing motion delay along with it. The goal here is to have as little smoothing as possible. As there is no way to accurately measure motion delay absolutely without special equipment, it is done by comparison with a control subject that has been determined to have consistent and low motion delay. In this case, the control subject is a Logitech G403, whose PMW3366 sensor has no visible smoothing across the entire CPI range. Note that the G403 is moved first and thus receives a slight head start.

Wired testing

First, I'm looking at two xCounts plots—generated at 1600 and 6400 CPI—to quickly gauge whether there is any smoothing, which would be indicated by any visible "kinks." As you can see, neither plot shows any, which strongly suggests there not being any smoothing.


Furthermore, the UltralightX allows enabling MotionSync, which reduces SPI timing jitter.


In order to determine motion delay, I'm looking at xSum plots generated at 1600 and 6400 CPI. The line further to the left denotes the sensor with less motion delay. There is no motion delay differential at either step, which further confirms there not being smoothing across the entire CPI range.


Upon enabling MotionSync, motion delay is increased by a bit more than 0.5 ms.

Wireless testing

Not much changes when running the UltralightX in wireless mode as SPI timing jitter and general tracking are virtually on the same level as when wired, both without (first plot) and with (second plot) MotionSync.


1600 CPI is tested, both without (first plot) and with (second plot) MotionSync. Without MotionSync, I can measure a motion delay differential of a bit more than 0.6 ms in favor of the UltralightX. With MotionSync, this motion delay differential is reduced to zero.

Speed-related Accuracy Variance (SRAV)

What people typically mean when they talk about "acceleration" is speed-related accuracy variance (or SRAV). It's not about the mouse having a set amount of inherent positive or negative acceleration, but about the cursor not traveling the same distance if the mouse is moved the same physical distance at different speeds. The easiest way to test this is by comparison with a control subject that is known to have very low SRAV, which in this case is the G403. As you can see from the plot, no displacement between the two cursor paths can be observed, which confirms that SRAV is very low.

Perfect Control Speed

Perfect Control Speed (or PCS for short) is the maximum speed up to which the mouse and its sensor can be moved without the sensor malfunctioning in any way. I've only managed to hit a measly 5 m/s, which is within the proclaimed PCS range and causes no observable sensor malfunction.

Polling Rate Stability

Considering the UltralightX is usable as a wired mouse as well, I'll be testing polling rate stability for both wired and wireless use.

Wired testing

All the available polling rates (500 and 1000 Hz) look and perform fine.

Wireless testing
For wired mice, polling rate stability merely concerns the wired connection between the mouse (SPI communication) and USB. For wireless mice, another device that needs to be kept in sync between the first two is added to the mix: the wireless dongle/wireless receiver. I'm unable to measure all stages of the entire end-to-end signal chain individually, so testing polling-rate stability at the endpoint (the USB) has to suffice here.


First, I'm testing whether SPI, wireless, and USB communication are synchronized. Any of these being out of sync would be indicated by at least one 2 ms report, which would be the result of any desynchronization drift accumulated over time. There are no off-period polls that would be indicative of a desynchronization drift.


Second, I'm testing the general polling-rate stability of the individual polling rates in wireless mode. Running the UltralightX at a lower polling rate can have the benefit of extending battery life. Both 500 and 1000 Hz look and perform fine, though occasionally, bouts of elevated noise show up:


I'm unable to reproduce this consistently.

Paint Test

This test is used to indicate any potential issues with angle snapping (non-native straightening of linear motion) and jitter, along with any sensor lens rattle. As you can see, no issues with angle snapping can be observed. There is no jitter visible at 1600 and 3200 CPI. 6400 CPI shows minor jitter. This is in line with what to expect from a sensor lacking any smoothing. Lastly, there is minor lens movement at 6400 CPI.

Lift-off Distance

The UltralightX offers two pre-defined LOD levels. At the "1 mm" setting, the sensor does not track at a height of one DVD (<1.2 mm). Using the "2 mm" setting, the sensor does track at a height of one DVD (1.2 mm<x<2.4 mm, with x being LOD height), but not at a height of two DVDs. Keep in mind that LOD may vary slightly depending on the mousing surface (pad) it is being used on.

Click Latency

In most computer mice, debouncing is required to avoid double clicks, slam-clicks, or other unintended effects of switch bouncing. Debouncing typically adds a delay, which, along with any potential processing delay, shall be referred to as click latency. In order to measure click latency, the mouse has been interfaced with an NVIDIA LDAT (Latency Display Analysis Tool). Many thanks go to NVIDIA for providing an LDAT device. More specifically, the LDAT measures the time between the electrical activation of the left main button and the OS receiving the button-down message. Unless noted otherwise, the values presented in the graph refer to the lowest click latency possible on the mouse in question. If a comparison mouse is capable of both wired and wireless operation, only the result for wireless (2.4 GHz) operation will be listed. All the listed values have been gathered without any sensor motion. If latency differs between no motion and motion, it will be noted as such.

In wired mode, click latency has been measured to be 0.5 ms, with standard deviation being 0.18 ms. In wireless mode and at a polling rate of 1000 Hz, click latency has been measured to be 0.4 (0.39) ms, with standard deviation being 0.07 ms. In wireless mode and at a polling rate of 2000 Hz, click latency has been measured to be 0.4 (0.43) ms, with standard deviation being 0.07 ms. In wireless mode and at a polling rate of 4000 Hz, click latency has been measured to be 0.5 (0.46) ms, with standard deviation being 0.06 ms.

The main button switches were measured to be running at 1.895 V. I'm not aware of the voltage specifications of the Omron D2FC-F-7N (20 M) (OF) (China) switches used, but find this voltage to be rather low.