so how good is the AF on the 1D mk. IV at f/8, for example with a 500mm and a 2.0 tc?
i'll probably buy a 500mm in the next couple of days and was researching for a successor for my 7D. either the 1D mk. IV or the 5D mk. III. as i figured, i can crop an image from the 5D3 to 16MP (it would have a cropfactor of 1.375) and would have about the same reach as the 1D4 (not including f/8 AF support).
i have not used the 1D4, neither the 5D3, but almost everywhere i read the IQ of the 5D3 beats that of the 1D4. so would a cropped 5D3 image be better (or equal) of that of a 1D4 image? i guess i should rent both camera's when i have the 500, to figure it all out
Technically speaking, you would have to crop out the center 13.3mp area (4464x2976 pixels) to have the same effective spatial area as the 1D IV. But your still going to be at a spatial resolution disadvantage...80lp/mm for the cropped 5D III area vs. 88lp/mm for the 1D IV's full sensor. To exactly match the 1D IV with a full-frame sensor when cropping, you would need a 26.6mp FF sensor, which would have the same 88lp/mm spatial resolution.
Keep in mind, raw IQ is not the only factor in "getting a good shot". Camera A may have stellar IQ, but only a 4fps frame rate and only 10 continuous frames, with limited reach (1.0x crop). That could significantly limit your ability to get a keeper in the first place. In that case, Camera B, which great IQ (but maybe not quite "stellar"), a 10fps frame rate and 30 continuous frames as well as extra reach (1.3x crop), is the much better camera. You have a higher chance of capturing that perfect moment that just "makes"
the photograph. I'd sacrifice a little bit of IQ any day for Camera B, as its a better tool for the kind of photography I'm doing.
I personally use the 7D, more because it was within budget than anything (and left me with enough extra money to buy accessories, extra batteries, bunches of CF cards, flash, etc. I would LOVE a 1D IV though...) I find it to be an awesome camera, despite the fact that it definitely does not have the best IQ. In high SNR areas (i.e. a bird itself), its IQ is great, but it has the tendency to speckle noise around in bokeh areas and it just looks terrible. I've learned, however, that that is a moot point. Noise can be cleaned up, by a variety of means (standard noise removal and a bit of quick masking and Gaussian blur in photoshop completely eliminate all noise from even an ISO 3200 7D shot.) Don't let potential IQ bottlenecks hold you back from getting the right tool for the job.
One area where the 5D III would definitely win out is in the high ISO category. Pretty much all Canon cameras perform the same at ISO 100-400 due to whatever limitation Canon has in their sensor design that prevents them from improving DR past ISO 400. If you regularly shoot birds in rather low-light situations, I would expect the 5D III to do better at ISO 6400-25600 (not to mention getting the extra stop of native ISO to boot.) I think I could have used ISO 6400 and maybe 12800 on some of the darkest days I've photographed birds.