Which camera are you referring to above in your experience?
I regularly shoot a D4/D800E combo and my anecdotal impressions are:
- D4 achieves initial focus faster I believe and handles low contrast subjects better also.
- Once the D800E obtains initial AF, I have occasions where the D800E tracked better than the D4 (although that was before the D4 1.04 firmware). I have this weird feeling that 1.02 D4 tracking was excellent, 1.03 went backwards to very good, and hopefully 1.04 will be exceptional again but I haven't tried 1.04 in the field yet as there were no suitable subjects yesterday, so this 1.02/1.03 feeling isn't a factual conclusion I could make. It's just that up until 1.03 I never noticed tracking errors with the D4 so I thought it was infallible, then I did notice some tracking fails with 1.03 but that may just have been some low-contrast situations or me failing to hold steady on the target, so really hard to say for sure!
- I feel like the slow 4fps of the D800 allows a lot of mirror-down time making for good tracking on BIF. So once the D800 is locked on, I feel it is very good.
- I actually intend to try my D4 in 7 or 8fps mode to see if that improves its tracking vs. 10fps. In so doing I also hope to improve the operator error that seems to occur when 10fps is going full out and I accidentally chop off portions of my subject I have also now installed D4 1.04 so I will be testing 2 variables that have changed. Anyhow I still intend, situation permitting, to compare some 10fps flybys with some 7fps flybys. I know part of the problem could be the operator.
- I am still trying to form a solid impression of D4 vs. D800E tracking myself, and they must be fairly close, as I am having a hard time deciding. I need to shoot both cameras in the same situation, which is hard to reproduce in the wild, especially as my shooting buddy's can be impatient and always ready to move on to search for better opportunities. Like you, I would definitively like to know! I intend to keep trying this comparison as I feel it is vital to know when it is best to use each.
- I use teleconverters a lot, on an f/4 500VR, in oftentimes low contrast situations and D800 has trouble achieving first focus. This is TC14E f/5.6 and TC17E f/6.7. Overall I don't feel like the D4 hesitates pretty much ever, but I have rarely had an opp to try D4 in exact same light/target so my caution about anecdotal comparisons. But for sure the D800E can and does hesitate, where sometimes I must point the rig onto a higher contrast target at similar distance, point back at my subject and then achieve focus. Mostly I think this occurs with TCs and low contrast in order to provoke this behaviour. If one was never using TCs, I am not sure if I would notice this, but it is noticeable with TCs on an f/4 lens.
- In low-light theatre, I obtain good focus with both cameras.
- In low-light theatre at ISOs 6400 and 8000 I prefer the D4 results, maybe an even battle at 3200, and prefer the D800E at lower ISOs for that high res crystal clarity. My use of D4 with 70-200 f/2.8 and D800E with 24-70 in theatre is working out to be a very good balance I think, that optimizes the strength of each camera.
- My budding impression is I think I prefer the D4 for portraits as the D800E is too high res for normal human beings. Absolutely, on a photo-shoot, with professional make up staff and professional models, I'm sure the D800 is the better portrait tool, but on real people I spend too much time with NR in post removing blemishes etc. Now if one is showing off fabric and hair etc. the D800 is the better tool, but I feel it must be matched with some selective skin softening. Anyhow, I intend to use the D4 for a head-shot today and not pull out the D800E.
I should close with they are both great cameras and I could probably make do with either just fine. Or I could for those who like to shoot two identical cameras, two of either would be great too! Because I have both I am actively seeking out where each is better for me and for what I do.
If you are interested, I shoot both with video too, and they both have their strengths, favouring the D800 for wider shots in FX mode in good light and the D4 for telephoto work in CX mode in lower light.