There are already some threads discussing how "bigma's" compare to the DFA 150-450mm:
Pentax 150-450 vs Sigma 150-500 questions - PentaxForums.com Pentax 150-450 vs Sigma 50-500 OS - PentaxForums.com DFA 150-450 vs Sigma 150-500 - PentaxForums.com
Since I have the DFA 150-450mm and have just acquired a non-OS example of the Sigma DG 150-500mm APO HSM I have been able to take comparison images at different focal lengths.
150mm, the tower on the Telford railway bridge. I thought both lenses were really good at 150mm, both sharp from wide open. The sigma perhaps showed a little more fringing/purpling on some backlit twigs, and the DFA seemed a bit better at coping with the dynamic range from the shady side to the sunlit side. Both lenses were good across the frame, so I haven't bothered with edge crops.
UPDATE: another point of comparison, the sigma apo 100-300mm f4 at 150mm. Very similar iq.
300mm, one of the turrets on Conwy Castle. Unfortunately I couldn't get the tourist in in both sets, so the DFA shots are just of the same point of the turret. Again I was impressed with both lenses, both lenses looked softer at f11 but that I think was due to some shutter shake creeping in at ~1/200th shutter speeds, or perhaps my not letting the lens settle for long enough after changing the aperture on camera before firing the shutter with the cable release.

800px
450mm (+ sigma @ 500mm), another turret.
Sigma @ 500mm and DFA @ 450mm. 
dfa 450mm

sigma 500mm
Distant Focus. The house in the crop on the Great Orme is about 3km away (sorry about the pole, best I could do from the driveway). Atmospherics can be the main limiter of IQ in these cases, but this was a favourable, still, clear frosty morning. Neither lens managed to resolve the car number plates! The sigma pics at 500mm highlight a drop off in iq from 440mm. IQ was again good across the frame with no particularly significant falloff visible with either lens in this case.
Bird Focus. I did look at the possibility of acquiring a taxidermy specimen for authenticity but good ones are a fair wack, so I picked the kingfisher ornament up for 99p and the owl for a bit more. This portmanteau was set up at a distance of just over 5m, this was just right for frame coverage with both lenses (the kingfisher ornament is 16.2cm high). Note that the sigma at 500mm has no advantage over the D FA at 450mm due to focus breathing. If you think you might go with the sigma because you get a bit more at 500mm, well at this range, you don't!
Light was so so, clag over the hills blocking the low sun, but I was ok with that as an opportunity to look for fringing. I had to do a fair bit of spot removal on these pics, and tweaking of exposure + a bit of fill light. Two crops. Not much to choose between them on the crop of the kingfisher head apart from the higher level of fringing of the sigma (curiously the fringing gets worse as the f stop increases. A trick of the light perhaps, although I have another lens that does that - the Canon FD 400mm f4.5). The fine detail of the owl fluff is slightly but distinctly clearer with the D FA (less so actually when comparing the whole pics side by side in fastroom).


Close focus. The DFA focuses to 2m, the Sigma to 2.3m. Focus distance here was approx 2.5m. It was immediately noticeable that the sigma focus breathes much more than the DFA. I aimed to cover the width of the banknote. The sigma was at 440mm, the DFA at 330mm (though I didn't exactly equalise the field of view, the DFA is a bit wider). My first efforts were sub standard - shutter shake influencing results with both lenses. I didn't use any extra lighting, just the sunlight filtering into the conservatory, so shutter speeds were sub 1/100th. Corrected second go using mirror lock-up and higher (400) ASA. These test pics most clearly distinguished between the lenses, the DFA distinctly better.
Last edited by marcusBMG; 02-01-2020 at 10:08 AM.