Thanks Bruce for your input. I'm very sorry to hear of your health issues and hope they stabilise for you.
I'm currently in Talbot rehab hospital, just 2 weeks till l go home, after a fall early Nov 17, fracturing C3 & C4, had decompressive and stabilisation surgery, was classed as incomplete quad but now recovering from central cord syndrome: have regained the use of my legs and arms. My upper arms are problematic as I have limited lift above my shoulders and their weight bearing and carrying capacity is minimal. My right arm is still quite numb and my other arm, and legs, have patches of numbness and pins & needles sensation. It will take 12 months to see what gains I can make on this. So very lucky I'm not a total quad!
Your lightweight combination sounds great, esp if you changed to the 300mm f4 with the X1.4 TC for a total weight of approx 1.8kg and 650mm equivalent. I assume on this set up with the TC you would loose X1 light stop giving f/5.6? This combination is definitely food for thought for my situation.
Thanks and Cheers Andrew

Geez us Andrew, you don't do things by halves do you? Note to myself: "Never complain about back injury again!"
Talbot, as in just up the road from Ballarat talbot? I live in Wendouree mate, so I'll drop up for a chin wag and check out the nurses while I'm there. . .
I was out along a local track this morning using my D500/200-500mm F5.6 and I reckon that that combination is only just do-able as I walk around unit, as a matter of fact there was a lady there today whom I have gotten to know recently and she carries the same outfit as me but she has to wear one of those 'Cotton' (brand name) harness systems to support the carry in between shooting, otherwise it's too heavy for her.
I was thinking about your situation while I was walking around this morning and I reckon that your best bet would be the new Nikon D7500 DX coupled up with the 300mm F4 PF lens and converter when needed. Those D7500 are getting really good reviews and they have the best of the D7200 and some of the D500 guts in them and that would save you a little more weight.
Bruce

Thanks Bruce. Good day to be outside...l have a busy day with physio, OT and weights.
Thanks for the offer, but I'm at Royal Talbot Rehab Centre, part of the Austin hospital network, on Yarra Bvd Kew!
With your suggestions more reading and research on the internet.
Again many thanks

Hello Andrew,
I am just back from a five-day family get-together in Tasmania and was stunned to read about your terrible accident. Jill and I would like to add our best wishes to those of other members and hope you make a good recovery.

I have not had much time to think about the best way to reduce the size and weight of your rig but a cropped sensor body and your very capable 400 mm f/5.6 looks like a good option. Both the Canon 90D and 7DIII are the subject of persistent rumours for release this year though I am doubtful that the we will see the latter in 2018. Glenn Pure takes amazing pictured with the 80D so I have no doubt that its successor will be a serious camera. I can understand your desire to stay within the Canon ecosystem if possible, especially as you are now starting to use DPP4. The more I learn about DPP4 the more I realize what a great gift we Canon shooters have been given and I would not want to move to any other RAW adjustment and conversion software.

Very best wishes from me and Jill, warm regards, Ian.

Hello Andrew,

I had a think about your requirements last night. If you stay in the Canon ecosystem then I think the best option is a cropped sensor body like the 80D or 7DII and your existing 400 mm f/5.6. The 80D weighs 730 gm with battery, the 7DII weighs 930 gm and your 400 mm lens weighs approx. 1250 gm. So we are looking at a rig with a total weight of 1980 gm or 2180 gm. This compares with 4870 gm for your 5DIII + 600 mm f/4 II.

The downside of the 400 mm f/5.6 is that it is not an image stabilized lens so for hand-held operation you may be restricted to shooting in good light with a reasonable shutter speed like 1/800 sec or faster.

The cropped sensor bodies have a smaller field of view than your full-frame rig but when you take account of the shorter focal length proposed (400 mm), there is not much difference between your full-size rig and the cropped sensor with 400 mm lens. The resolution is much the same as well. The easiest way to compare the resolution is using the angular resolution measurement. This is the resolution of one pixel in the sensor array and can be calculated as follows:

Angular resolution = sensor pixel size (mm)/lens focal length (mm)

The angular resolution is in radian (1 radian = 57.3 degrees). The number you get for a telephoto lens will be very small and it is convenient to convert to microradians. It will typically be about 10 microradian = 0.000573 degrees. So for the 80D with pixel size 3.72 micrometre and with a 400 mm lens the angular resolution will be 0.00372/400 = 0.0000093 = 9.3 microradian. Similarly, with the 7DII and 400 mm lens the angular resolution is 10.2 microradian. This is almost the same as your 5DIII + 600 mm which has an angular resolution of 10.4 microradian. This shows that the cropped sensor bodies with 400 mm lens will resolve about the same amount of detail as your big rig. In this discussion I have not included the effects of diffraction, residual lens aberrations or the optical low-pass filter but I can assure you that the inclusion of these second order effects does not change the main conclusion.

The downside of going to a cropped sensor body will be a little more noise in low light and the AF system with the f/5.6 lens may also struggle in low light. However, in good light there will be very little difference in performance between your big rig and the cropped sensor body with 400 mm lens. In fact, you may find the 400 mm lens is faster to lock onto flying birds than your big rig.

If you think you can manage a little more weight, then the Canon 100-400 mm f/4.5-5.6 II would be well worth considering. It weighs about 1380 gm but has the latest image stabilization which would open up more options for photography in poor light with a shutter speed of say 1/100 sec which would probably be out of the question with the older non-image stabilized 400 mm lens.

I hope these thoughts are of some help. I guess it is hard to tell how much load you will be able to handle and how much agility you will have in a year’s time. If you have mobility issues then perhaps you could think about working with set-ups such as Warren Wilson uses in his garden.

Ian,

I think that you should be sitting in a very comfy chair and residing in a cave.
That way we could all visit to worship you and find out all the answers that we need to ask to follow out chosen lifestyle. . .

Humble disciple,
Bruce