Compared to conventional diesel combustion (CDC), isobaric combustion can achieve a similar or higher indicated efficiency, lower heat transfer losses, reduced nitrogen oxides (NOx) emissions; however, with a penalty of soot emissions. While the engine performance and exhaust emissions of isobaric combustion are well known, the overall flame development, in particular, the flow-field details within the flames are unclear. In this study, the performance analysis of CDC and two isobaric combustion cases was conducted, followed by high-speed imaging of Mie-scattering and soot luminosity in an optically accessible, single-cylinder heavy-duty diesel engine. From the soot luminosity imaging, qualitative flow-fields were obtained using flame image velocimetry (FIV). The peak motoring pressure (PMP) and peak cylinder pressure (PCP) of CDC are kept fixed at 50 and 70 bar, respectively. The two isobaric combustion cases, achieved using multiple injections, are maintained at the CDC PMP level of 50 bar for the low-pressure case (IsoL) and CDC PCP level of 70 bar for the high-pressure case (IsoH). For each operating condition, soot luminosity signals are captured at a frame rate of 20 kHz, and a semi-quantitative velocity flow-field is obtained from FIV post-processing. Consistent with previous metal engine experiments, isobaric combustion - in particular IsoH, resulted in similar gross indicated efficiency, lower heat losses but higher exhaust losses, compared to CDC. The soot luminosity images of CDC show initial signals originated close to the bowl-wall for certain jets while for the isobaric combustion, the flames corresponding to each jet are clearly distinguished during the earlier flame development process. The vector field distribution within the flames shows the transition of flame-wall impingement to flame-flame interaction regions between the neighboring jets for each combustion mode. Furthermore, higher flame-flame interaction regions and uniform distribution of signals around the combustion chamber for isobaric combustion, justifying higher soot formation and lower heat transfer losses, respectively, compared to CDC.