Pavel, nikon can use old lenses, canon as well. But when come to pro grade and new lenses other brands are rare or very expensive.
If you have time you can play woth old manual lenses, but when timing is important MF cant beat good AF.
Sigmas are not that good as brand flashes, special when comes to canon and nikon.( i had sigma for pentax) For amateur they are fine.
Show me zooms for pentax witch can stand 70-200 f2.8, 80-200 f2.8, 18-200 vr, 24-70 f2.8 or canon L series.
Nikon below x00 series doesn't meter with old Ai or Ai-S lenses. But I can't afford such a body only to use my old Nikon MF lenses, some of them from old Kiev-19. I am no profi, just a lame amateur. I need an SLR mostly because I hate point-and-shoot lag, can't take "genre" pictures. And I am definitely not rich for Canon L series, or 80-200 f2.8.
Is there any way to use old FD-mount lenses on modern Canon DSLR?
Can anyone tell my why Nikon DSLRs can't use old TTL flashes? F80 could, D70 can't... Maybe just to make you buy an SB-800? I have a 1981 Nikon FG with TTL flash, 50mm 1.8, 100mm 2.8 Kaleinar-N and 28-200 Soligor zoom (surprisingly good). Now my co-worker bought a used D200 and uses my 50mm and 100mm a lot, but a body cost him (even used) two times more then I paid for Samsung kit plus Sigma 70-300APO. I think of buying something like 18-200 or 18-250 now, or maybe just a Sigma 17-70 2.8-5.6, and my bag will still be cheaper then his body alone. Does an Ai metering lever cost like $300? If I only could buy something like D70 with Ai lens and TTL flash support - I'd never even think of buying anything but Nikon. But now I can use Zenitar 50mm 1.7 with a $10 adapter ring, plus all K-mount lenses.
As for manual focusing - sometimes it's easier for me to focus manually or even by scale (my Samsung failed to focus fast only at night at the campfire when I was shooting through the smoke). Auto-focusing a 18mm lens at 10m is a nonsense, isn't it?