9, 11, 21 or 51? How to decide?

Started May 13, 2012 | Discussions thread
2wheel
Regular MemberPosts: 389
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Re: 9, 11, 21 or 51? How to decide?
In reply to ckon, May 14, 2012

Humbly offering a response...

I'm primarily an action shooter... Bicycle racing, motorcycle track days, snowboarding, adopted greyhounds at speed etc. So, my response is based on my experience in these environs and also from countless discussions and examinations with other sports shooters. If it's of consequence, I'm shooting with a D4 (And a D300.)

First, Nikons focus system is highly variable and dependent on a concert of settings including "Focus Tracking with Lock-On", "11 or 51 Focus Points" as well as aperture for DOF. etc. To utilize the system properly, one must balance several AF and other features. It's not just about how many/few points to use as a default and requires practice for your type of shooting or a particular situation.

Nikon's AF system in upper end DSLR's (D300 and above) utilizes a primary focus point for all focus modes except for the dynamic modes. (Which I'll exclude in the following examples) The number of points selected refer to the helper points around the primary or target focus point. The primary and helper points define an area of the frame to be considered in-focus for an exposure. i.e. 9 points will prioritize a smaller area of the frame than 21 points.

If one wished to have the entirety of the frame considered to be in-focus, one might choose 51 points. (And something above f/8) If one preferred to have only one area of the frame in focus and a narrow DOF, one might choose a single point and f/2.8.

Note that the above and below examples are illustrative only and I am not recommending that this is the only way to marry AF points and DOF.

Scenarios:

Consider a bicycle race where one might want to choose a particular rider in a pack of 50+. The pack may be moving directly at you at 30mph prior to entering a turn. Your planned exposure(s) will be mid-turn at which point, the pack will be moving from right to left. If you wish to capture a particular rider, upon initial acquisition, amongst the field of bright colors and high contrast options in the frame, you might only be able to see that riders face and nothing else. Thus, your initial target is very small and moving very quickly toward you. By choosing a single AF point, the AF system will acquire the subject rapidly and track that rider as they approach the turn and begin moving across the frame. The likelihood the that your chosen focus point will remain on that rider and continue to adjust focus as they both close distance and change direction is higher than if you were to utilize, say, a 21 (Or more) point array. In which case, you would be telling the AF system that it's OK to consider other near, but not precise, targets to factor into the exposure.

Conversely, if you were to desire that the entire pack be in focus at the same point in the turn, you might allow some flexibility in the AF system to select any rider or riders and choose 21 points. In this scenario, you might also chose a smaller aperture which is consistent with what you are looking for in the final image: A larger portion of the pack in-focus.

While I don't shoot BIF, I imagine it's not dissimilar to shooting a single snowboarder moving quickly and somewhat erratically on a mixed background of snow and trees. Typically, I'll chose 9 or 21 points as I may want more of the rider in focus than just his/her face. Again. I'll marry the broader AF point array with a smaller aperture.

I could envision favoring the 21 point as a birds head is a much smaller target than an entire human and, if I were to pan too quickly or to slowly, could allow the AF system to jump to another object in the frame.

Now, I have not experimented fully with 3D tracking and really can't comment on its effectiveness in either of the above examples but, I just can't imagine it as an effective tool for the bicycle race scenario. BIF and single snowboarder? Maybe.

Note that I have experienced the FPS of the D300 to outrun the AF system. For each exposure in say, a 6fps burst, the AF system might have to re-acquire the subject. If ones panning technique is off by just a small fraction, the AF system might jump to the tree line behind the rider for one exposure in the burst and then back to the rider once the panning has caught back up with the rider.

I've not yet had the chance to test the D4's superior AF system in the same situation.

One could go on and on with more examples but the above should suffice to make general point.

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