Long Lens Options?

I am looking into replacing my Tamron 200-500mm. It has served me well, but I want something sharper, able to focus faster and possibly a little longer. I photograph a lot of birds and wildlife.

I use Nikon cameras and I am looking at the Nikon 500 f4 and 600 f4. Both are seriously expensive! I am wondering if there is any issues with image quality or autofocus when using a converter for Canon lenses? The Canon lenses are a good deal cheaper.

I don't want to start a Nikon/Canon war, but I am curious if anyone has experience with both companies long lenses and what they think of them?

I am also considering the Sigma 300-800 5.6. I realize it is huge. It is reportedly very sharp. Has anyone used it? How does it do with birds in flight? My Tamron 200-500 is rather poor with birds in flight.

I realize I am asking a lot of questions. I apreciate any help that you may offer!
Thanks,
Greg

I have used both companies as a professional down through the years. Both are great systems and both have top quality lenses. I’m using Nikon right now for a number of reasons but in the end your comparing apples to apples. I have not shopped long glass price in a while but I must say I’m surprised the prices are that different from the big two! Either way you slice it both Canon and Nikon long glass is going to lighten your wallet. I did a fast search on Amazon and the Nikkor 500 mm was 6 K and the Canon was 6700! I will say this switching systems is an expensive proposition and you will never get what you paid on gear already owned. Last and a parting thought, you are never going to get the same sharpness or contrast with a Sigma or a Tamron that you’re going to get with a Nikkor or a Canon lens.You say your looking at flight shots of birdsand all I can say is your precentages of sharp photos with off brand are going to be lower. Converters are nice and sharp as long as you use the brand name. The last thing Ill leave you with is look at used glass. There have been a lot of updates in the past couple of years with Nikon upgrading their glass with VR so maybe try looking at a non VR lens. I have access to both versions or the 500 and 600mm and I can tell you none of the photographers I work with at my paper ever turn the VR on! Just saying consider used.

Thanks Doug. I will keep watching Amazon.
I am certainly considering used glass. Thank you for the advice on the converters!

Greg, I come from a mixed marriage - I use Canon my husband uses Nikon. We both have respective 500mm lenses and have shot with each others gear and honestly they both are just as sharp. It used to be new Nikon lenses were more expensive than Canon but with the new IS II lenses that Canon is putting out that is no longer the case they've upped the price into the Nikon range. I agree with Douglas, you won't get the same sharpness with Sigma or a 3rd party brand or the same service after the fact, long or short lenses.

Gregg - I just "upgraded" from mid-range glass and from non-brand glass. After lots of research, I came to the conclusion that there simply isn't any third party glass that is really sharp at all apertures over the entire lens coverage. There is clearly a reason for the higher cost. You will buy this lens one time if you buy it right. If not, you will find yourself wishing you had. Buy the Nikon glass. I can't imagine that the Sigma zoom could come anywhere close to the Nikon 500 prime!