For process stability in laser powder bed fusion (LPBF) additive manufacturing (AM), control of melt pool dimensions is imperative. In order to control melt pool dimensions in real time, sampling frequencies in excess of 10 kHz may be required, which presents a challenge for many thermal and optical monitoring systems. The National Institute of Standards and Technology (NIST) is currently developing the Additive Manufacturing Metrology Testbed (AMMT), which replicates a metal based laser powder bed fusion AM process while providing open architecture for control, sensing, and calibration sources. The system is outfitted with a coaxially aligned, near-infrared (NIR) high speed melt pool monitoring (MPM) system. Similar monitoring systems are incorporated into LPBF research testbeds, and appearing on commercial machines, but at lower available frame rates, which may limit observation of higher frequency events such as spatter or size fluctuations. This paper presents an investigation of the coaxial imaging systems of the AMMT to capture the process dynamics, and quantify the effects of dynamic fluctuations on melt pool size measurements. Analysis is carried out on a baseline experiment with no powder material added, melt pool size measurements collected in-situ are compared to ex-situ measurements, and results are discussed in terms of temporal bandwidth. Findings will show that, even at the frame rate and resolution presented, challenges in relating in-situ video signals to the ex-situ measurement analysis remain.