Q. Has the speed of the autofocus system improved, specifically the time it takes to acquire focus when first engaged or when rapidly switching to a new target at a different distance? For example, when shifting from the quarterback throwing the football to the receiver catching it, the D3S' AF system is a bit too slow to always bring the receiver into focus in time to get him making the catch.

Yes, it is faster. First, the D4 is able to autofocus at 10fps, vs the D3S at 9fps, so that's one improvement. Second, in situations where the D3S might have taken until the third frame in a burst to bring the subject into focus, the D4 will get it on the first frame. Third, some professional photographers in Japan, ones who told us the D3S was a little bit slow to acquire focus before the first frame, have tried the D4 and said it is improved, said it is faster.

(Akagi then showed on his computer an example sequence of a significantly defocused subject coming into focus by the fourth frame with the D3S, and by the second frame with the D4. The photos appeared to have been taken with a long lens, and showed exactly the sort of AF system speedup that should, in our football example, lead to more properly-focused pictures of the receiver receiving the ball.)

The autofocus calculation is done in the camera's main CPU, which is faster in the D4 than the D3S and which contributes to the AF system's better speed.

Q. Has tracking accuracy been improved in sunlight vs lower light? This is a quirk we've experienced with supertelephotos such as the 400mm f/2.8 when photographing the same team at a night game and then at a day game: the in-focus percentage is consistently somewhat higher at the night game (as long as the field lighting isn't completely terrible).

This has not been reported to us as a problem.

(Nikon USA's Lindsay Silverman spoke up to say that, at Nikon USA, they have received some reports of this phenomenon.)

The new 91,000-pixel RGB metering sensor, which also plays a role in autofocus tracking, we believe will improve autofocus accuracy generally. This is true not just when the AF system is set to Auto-Area AF or 3D tracking, but also, for example, with 9-point Dynamic AF too.

Q. How would you compare the high ISO image quality of the D4 to the D3S?

Overall, D4 high ISO noise levels are very similar to the D3S, though photos shot with the D4 will have reduced colour noise (thanks mainly to noise reduction improvements in the D4's EXPEED 3 processing hardware). The main difference you'll see in pictures from the two cameras at high sensitivities is in the fine detail: D4 photos are noticeably better in this way.

(Akagi then showed several 13 x 19-inch prints of the same scene, captured as NEFs with the D3S and D4 at ISO 12,800, in which the overall colour, tonality, visibility of noise and shadow rendering couldn't have been more similar. But, the D4 photos looked sharper and more detailed. The D4's crispness advantage in Nikon's comparisons was significant, suggesting that we were seeing the effect of more than just the new camera's higher pixel count.)

Q. Among the image quality improvements cited for the D4 is better skin tones. What difference would we notice in a portrait shot with the D3S and then the D4?

You will notice better tonality. We have reshaped the tone curve to give more natural-looking shadows and shadow transitions in faces.

(This change presumably will be apparent in photos processed to JPEG and TIFF in the camera, as well as NEFs converted to finished files in Capture NX2.)

Q. Have there been any changes to the optical low pass filter?

The D3S has a resolution of 12MP, and the D4 has a resolution of 16MP, so we had to make the filter thinner in the D4, to match its finer pixel pitch. But, the effect of the filter in the D3S and D4 is the same.

Q. Have there been any changes to the self-cleaning mechanism?

No, it's comparable to the D3S.

Q. Are there any changes to the long exposure noise reduction, include a shortening of the delay between frames?

No, the dark frame subtraction works the same as the D3S, and takes the same amount of processing time.

Q. Why did you choose the XQD format for one of the D4's card slots?

The main reason is the XQD format is faster than CompactFlash. We worked with Sony (the maker of the first XQD cards) to get better write speed in the D4. Sony says their first XQD cards will be capable of 125MB/s, and the D4 will be able to take advantage of that level of performance. In fact, the camera's XQD slot can go much faster than that, well over 200MB/s, as higher-speed XQD cards come out in the future.

This is faster than CompactFlash cards today, though the D4 will be able to take advantage of fast UDMA 7 CompactFlash cards as well.

Q. Both SanDisk and Lexar have told us that XQD is not on their product roadmap currently. Have any companies other than Sony told you they will be making XQD cards?

At this moment we don't know which companies will be making XQD, other than Sony.

Q. Why does the D4's EN-EL18 battery have a lower capacity than the D3S' EN-EL4a?

(The answer to this question is in its own, separate article.)

Q. The Nikon 1 J1 and V1 demonstrate that a camera without a mirror can be made to autofocus quickly and accurately, and the ultra-quiet operation of these cameras is really useful in certain situations. Do you see a mirrorless design eventually coming to Nikon's F-mount digital cameras, including flagship models?

We cannot comment. What do you suggest we do to improve our cameras in the future?