Options for getting up to 600mm ish with Canon

[Thunderace]

Currently have 1Dx, 5D2 and a 40D in the back of a cupboard somewhere. Longest lens is 400mm F5.6L prime.

Would like to get up to 600mm ish but can't currently justify the 600mm L.

Options seem to be buying the new Sigma 150-600mm Sport or perhaps considering using a crop body with the 400mm.

Any thoughts on quality comparisons between the 150-600mm on the full frame bodies or the 400mm on a crop (either dusting off the 40D or buying a 7D2), particularly at 1600-3200 ISOs?

Would a 7D2 be significantly better than a 40D? Will I find a 7D2 significantly poorer than a 1Dx?

Main use would be wildlife, size and weight aren't an issue.

[toasty]

I might be stating the bleeding obvious but what about the 1.4x converter? I use one with the 100-400 and it seems pretty good to me. Certainly worth looking at.

[Thunderace]

I have a 1.4 mk2 and happily use it on the 70-200mm F2.8 but find it soft on the 100-400mm and 400mm prime.

[LongQ]

Thunderace said:

I have a 1.4 mk2 and happily use it on the 70-200mm F2.8 but find it soft on the 100-400mm and 400mm prime.

I use a 2x extender MkII with a 70-200 f2.8 IS II and it is usually be very sharp given half a chance using my 1D3. I would expect better from a 1DX.

A cheap alternative might be an old FD mount 600mm if you can find one. Cracking results when things work out. Not such a good hit rate in challenging conditions but quite a lot of fun no matter what happens - and not expensive if you strike lucky on the price.

The down side is it's fully manual ....

[troc]

The 7d2 is vastly superior to the 40D at higher ISO. I never got anything decent above 1600 with the 40D whereas I'm happy to use the 7D2 at 3200.

From what I've read, the (new) 100-400 with a 1.4x is better that any of the sigma or tamron zooms at the long end.

[RobDickinson]

The sigma 150-600 contemporary is a pretty good option in addition to the sport. lighter but doesnt look compromised on optics.

7d2 will get you 'closer' than the 1dx by a fair way ( 7mp at 1.6 crop or something vs 20mp), 5dr would get you the same pixel density plus the 5d3 af tracking and 5fps?

[Thunderace]

Went for a 7D2 in the end, thought for a while it might be a backward step from FF but think I now have the best of both worlds