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Background : I am on the verge of going for a D800 and Sigma 500mm (I can't afford the Nikon 500mm) as an upgrade from my Pentax K5 (excellent camera) and DA*300 (also an excellent, very sharp, lens) + x1.7 TC. I have a few threads over on the Nature forum with this combination.
I can not afford a D4 or other cameras in that price bracket. The D800 is still stretching it for me and will no doubt cost me months of 'forced abstention' as the wife makes her feelings known ! We do what we must though
Studying the D800 for shooting birds with the 500mm, I made the supposition that those closer shots can be best managed with FF (with the Sigma 500mm that's ca. 335mm equiv on APS-C), a bird appears in the middle distance and a flick of a switch later and I am at x1.2 crop (24mp and ca. 400m APS-C / 600mm 35mm equiv. ) and then spotting a small bird 40 yds away and another flick the switch and I am now at x1.5 (APS-C) for 750mm (35mm equiv.) all with latest and greatest version of Nikon's AF. All that without having yet added a TC.
Of course people also use the Sigma 500/4.5 on K5s (to great effect indeed) however the D800 seems to give far greater flexibility, better FF files and IQ and it loses nothing over the K5 in mm or mp in crop mode whilst still retaining all the advanced features and benefits of the D800 and Nikon's AF and a larger, brighter 100% VF that will make it even easier to obtain critical focus when in MF.
Are my theories correct here ? Am I missing something ? Is there the major step up in IQ and performance from the Pentax K5 I'm assuming there will be ? Or am I just better off buying the same Sigma 500mm for the K5 - with it's inbuilt SR (vibration control) ? The DR range is a wash with the D800 so there's no advantage there, though I'm assuming the files from the D800, even in x1.5 crop mode, will still be more advanced ?
I'd really appreciate any insights / options / advice ! Thank you. Kevin.