Review:http://www.martinzimelka.com/homepage/Blog/Blog.html

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Auto Focus:

The D800 and D4 feature a revised and greatly improved auto-focus system, featuring the same Multi-CAM 3500 FX AF module found in the D3, D3s and D4 cameras. Reading the spec sheets, it seems nothing much has changed, but in reality a lot has.


The new AF cam is sensitised to even lower light levels than before, boasting a sensitivity of -2EV. Thats more sensitive than the human eye! Together with the new and much more powerful Expeed 3 processor, the new 91 000 pixel RGB Colour Matrix Meter III sensor, and a revised set of algorithms, the Multi-CAM 3500 FX offers a much faster subject lock-on with greatly improved 3D tracking capabilities!


I’ve hardly made use of the various AF tracking modes offered in my Nikon cameras. From the F5 to the D700, I never felt that it was reliable enough for my unpractised approach to shoot action. However, the the D800’s AF performance feels as if it’s super simple and not easily fooled. Better yet, focusing with AF-D lenses seems smoother, quicker, and more accurate. This is a big plus for me!

One Saturday, I made a stop at the Killarney race track to look at some fast and loud cars ;) I had little intention to shoot, but the D800, the Zeiss 35/2 and the AF-D 85/1.4 were in the bag. Out of curiosity, I stood at the pit-lane wall and started photographing the cars racing down the finish straight. Traveling at what must have been close to 200 km/h, I hardly ever missed focus! I was quite surprised, since the AF-D 85/1.4 usually stutters into focus on the D700. The D800 purred by comparison.

My first D800 had the venerable AF issue, where both left and right AF points were back-focusing, with the left points showing a larger error than the right. The centre point was spot on, needing no AF fine tune.


My first camera was replaced by Nikon, first with a loan unit and then by the actual replacement. The loan unit also had this AF issue, but the replacement camera did not. Frustratingly enough, the replacement had a slight front focus, even though Nikon calibrated the AF points prior to sending me the camera. The above images were taken with the replacement camera, but set with a global AF fine tune value of +6.


The image on the left represents the degree of back focus error I was experiencing with the left AF points on my original camera. Not even a -20 micro AF adjust was sufficient to correct the error. Click the image to see it larger.

The centre AF points were consistently accurate after an AF fine tune was done. The reliability of the centre AF point far outweighs that of the outer AF points. I hardly ever stray away from the centre AF, but that doesn’t make it acceptable for outer AF points to be inaccurate. After all, if the subject conditions are right, they have to be accurate.


The autofocus performance of the D800 is very responsive, and very accurate in most cases. However, there are times it gets fooled, just like all the other cameras before it... Shooting under various light sources can alter the point of focus slightly, but this usually only happens with fast lenses.


For example, when I shoot events or functions, I have the AF fine-tune menu available for quick access under My Menu. I know, due to repeated use, that many of my fast lenses will cause a slight back-focus under tungsten lit scenes. I experience this most often with my AF-D 85/1.4, and this lens will also slightly front focus under very cool light sources.

My experience with this has been consistent with the D2x, D700 and D800 cameras, across various fast lenses.


I’m not sure why this happens, but it’s probably because the AF sensor might be properly calibrated for green light, since it’s the median colour of the visible spectrum. When shooting under tungsten light, an AF error will occur if the AF cam assumes the light is green. After all, red has a different wavelength to green. It would be great to get some technical feedback as to why this happens. The various Nikon technicians I’ve asked didn’t know this happened, and also couldn’t explain why it would.

 

Live view AF feels greatly improved over the D700, but it has little practical benefit for the work I do. I’m not convinced that live view focus should always be considered the benchmark for proper focus. Live view focuses using a contrast detect method, and when using fast lenses like the AF-D 85/1.4, the Live view focus and manual focus results are sometimes slightly different. In certain high contrast situations, the live view AF will focus the lens on the subject where it displays the least aberrations, but this isn’t necessarily the optimal focus position.


Focusing manually in live view is sometimes annoyingly tricky, since line skipping is very evident in the live view feed.

Not only does this lower effective resolution of the displayed image section, it also introduces artefacts, making proper focus difficult to confirm at times. Because line skipping samples every second or third pixel row off the sensor, less than half the light that reached the sensor is being represented in the live view feed. This hampers live view performance in dark areas, and induces more noise when the image is brightened for the LCD display. The Canon 5DMIII is far better in live view mode than the D800.