Best "portable" options for a birding lens

I've not been around the forum for a while so a bit out of touch with new developments – spending too much time out in the field with the camera!!

I'm after some input on the best lenses for birding when 'portability' is the prime concern. I've read through the other threads on here for 'most affordable' lens etc but this question is very much about weight and compromises with the big primes.

I currently shoot with the 500mm F4 VR and the 800E (which I will upgrade to the 850/900 when it comes out). I also own the 80-400g which is a great lens but not the quality, reach or F stop of the 500mm prime. My bird photography is split into two areas, firstly I carry the camera when birding to get record shots – and help identify the birds. Secondly, when close enough I try to get the 'good' shots. This means I am walking a long way and seldom specifically target a bird to get that 'perfect' shot. I use a monopod which I find great with the 500mm F4.

The main issue now is that I find myself walking longer distances and recently have done a few vacations where I am walking 6-8 miles – and the weight of the 500mm F4 combo is taking its toll. - my knees are starting to say 'No' (I am approaching 65).

I like the quality I get from the 500mm prime (plus the speed of AF and the ability to shoot at F4 – which I do most of the time). Going down to the 80-400g reduces the weight big time but the IQ goes down a bit, I'm back to F5.6 and I have lost 20% of the reach. Walking 6 miles with the 80-400g is fine but it is a big compromise in the flexibility and ability of the combo.

Sanderling

One option is to change my shooting habits and not do all that walking. I will partly do that and likely aim to upgrade my 500mm F4 VR to the new 600mm F4 in the next 12 months so that I have that combo available when I am shooting close to the car. However, I enjoy birding and don't really want to compromise on the advantages of the prime when I am walking. What then are the options to minimise that compromise?

I know the 'gap' between the 80-400g zoom and the prime and it is not insignificant. I see the reports on the 200-500 nikon zoom in terms of IQ but it is significantly heavier than the 80-400g and I'm not sure if it would be as good wide open as the 80-400g. Can anyone who has had the two zooms and the prime give me an idea of where the 200-500 fits between the two. I also worry that the AF speed of the 200-500 is a bit slower than the other two.

Skylark

A year ago the 300mm F4 PF came out with a shower of great reviews. Is that an option with a variety of TC's? I dont like Tcs and tend to avoid using them with my Prime and zoom as I tend to find they are no advantage at long range.

Another area to explore is the new D500. I am firmly in the camp that sees the cropping ability of the D800E/D810 outweighs the extra reach of Dx formats (vis the 7200/7100) when iso is above 500 so have never moved to a 7200. Does the new D500 change that equation? I am not really seeing reports saying the IQ is that much better than the 7200 (I am not that interested in Fps).

Wheatear

Perhaps this will all come down to how much compromise I am prepared to make to be able to walk and I should just upgrade the prime to the new 500mm prime which is lighter – but that may be too heavy and I did want to go for the 600mm for 'car based' shots.

Andy

http://andyburnsphotography.zenfolio.com/portfolio/h73d60d0d#h73d60d0d