Friday, September 01, 2006

Focus Tracking Tests With The Nikon D50 And Older AF Lenses

(The photographs shown here are uncropped and uncorrected. I've posted them on my Flickr account at full size, so you can download them to examine if you wish.)

After being held up by work, I'm finally getting round to writing up the tests I did with my D50 and my telephoto lenses. The D50 has a useful focus mode called "AF-A" - this defaults to single-shot AF, but will switch automatically to continuous focus (focus tracking, in fact) if the subject it's locked on to starts moving continuously towards or away from the camera.

I was a bit wary of this setting, but it works like a dream; it doesn't seem to cause focus lock problems in single-shot mode, and the transition to continuous focussing is pretty reliable (though it does work better the longer the focal length of the lens - it seems happiest at 85mm upwards). I now leave my D50 on this setting all the time.

Lenses and settings

28-85mm @85mm
85mm prime lens
80-200mm @ 80mm
80-200mm @ 200mm (3 sequences)

All are pre-D Nikkors, the 85mm and the 80-200 zoom are the old type with the locking pin on the aperture ring.

AF was set to use the central focus point. Mode: AF-A.

The D50 controls aperture via a dial on the camera body, so the aperture ring on the lens has to be locked for all modes, even aperture priority and manual.

Details of the focus tracking tests

On our local high street, I tried tracking approaching cars moving at 20-30 mph from a set of road markings about 100m distant, until they passed me.

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Sample first and last frame from a test sequence taken with the 28-85mm zoom at 85mm (click on images to see full size version)

With each lens, the D50 managed about 7 shots in the same distance. I couldn't time the tests and also shoot, but I'm guessing that the camera was shooting a little below its maximum burst rate - what you'd expect, given it was focus tracking at the same time. Focussing at 80mm (FoV 120mm) was accurate and also consistent, despite the differences in maximum aperture and the physical bulk of the three lenses. The D50 has no trouble driving the big 80-200mm zoom.
At the 200mm (FoV 300mm) setting, accuracy started to suffer - of the 25 shots I took at 200mm, 5 were unsharp, one unusably so (the camera had focussed some distance behind the target car). But to put that in context, 58 out of 59 shots were as good as or better than I'd have expected to manage by the only other technique available - pre-focussing on a spot in the road and waiting for the vehicle to pass.

More problematic was the tendency for the shutter speed in Aperture Priority to drop as the approaching car filled the frame; sequences starting at 1/350sec would drop to as little as 1/60 sec by the last frame, resulting in camera shake. I'd expected the matrix metering to compensate for this, and it's possible that a D-type lense might cope better. It's worth noting that it 's better to use manual exposure mode for this type of shooting.

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Sample shots affected by camera shake - but if you look at the seam line on the bonnet, they're still in focus (click on images to see full version)

It still has to be said, these results are significantly better than I expected, and better than anything I've achieved with my film SLRs. I never considered focus tracking to be a serious tool before, but I'll now be using it, particularly with that conventient AF-A mode.

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Sample shot from 80-200 zoom, this time without camera shake

The fact that the 1gb SD card on the D50 will routinely hold 360 images at JPEG Fine (83 more than the counter suggests when the card is empty) makes me willing to use the "motor drive" much more than I ever would have when shooting film, though I'm not sure if that's actually a good thing or not.

Recently I had a chance to repeat the tests with an AF Nikkor 70-300mm f4-5.6 D lens; the D specification had no effect on exposure, though it did routinely manage one more frame per sequence than any of the non-D lenses. While it's no surprise that it could focus faster than the bulky 80-200 zoom, I'm surprised at it out-performing the fast 85mm prime. At the 200mm setting, the D lens focussed more accurately than the 80-200mm, the lighter construction presumably offsetting any disadvantage from the smaller aperture.

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70-300D@200 28.JPG

End shots from two sequences taken with the AF Nikkor 70-300mm f4-5.6 D Zoom - note the camera shake still present in the first shot due to shooting in Aperture Priority mode with a dark-coloured car filling the frame.
The second shot is was taken in Manual exposure mode with a shutter speed of 1/800sec - this was the last shot from a prodigious run of 13 consecutive frames, all in focus.
(Click on images to see full size version)

I thought this would be where any cracks in the D50's performance would appear, and I was happy to be proved wrong. I'm not saying that the D50 will give the ultimate in performance when fitted with an old AF Nikkor lens - I'm sure a new G-type zoom would give better results (as, for that matter would a D2X) - but the fact remains that if, as I am, you're trading up from an early-90's film SLR and you don't want to ditch your old AF lenses, you can be confident that the D50 will give at least as good a performance as your old camera, if not slightly better. What's more, the D50's FoV conversion factor of 1.5 means that your telephoto lenses all effectively become longer - so my 80-200mm zoom now gives the same field of view as a 120-300mm zoom on a 35mm camera, and my 70-300 zoom acts like a whopping 105-450 zoom. Not bad for a budget solution.

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