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Thursday, November 13, 2014

Nikon's Fastest Focusing Lens (and why You Don't Need It)

Confess: you came here trying to find which is the fastest focusing Nikkor lens for your Nikon camera, right? Or, other possible queries might have been: "Best Nikon Lens for sports", "Best Nikon Lens for birds in flight", "fastest Nikkor autofocus", and stuff like that. Give me 5 minutes of your time, it might change the way you think about photography - not to mention, it might save you money.

Taken with a slow, AF (screw-driven) consumer tele. So what? The focus is spot-on perfect
When people think of "lens performance", the first thing they think of is optical quality (usually sharpness). The second thing is almost always focusing speed. But let's take a moment to think about that, shall we?

You see, there are two ways to talk about photography: a) in charts, diagrams, and cold numbers; b) in real-life terms. AmateurNikon is about real-life and photography as art. You might have thought that focusing speed is something pretty easily measurable (heck, speed is speed, right?). Well, wrong. It's easy to make a lens that focuses fast. It's harder to make a lens that focuses fast and accurately. It's even harder to make a lens that focuses fast, accurately, and has a short minimum focusing distance.

I often see video reviews of lenses that portray how supposedly slow it is to focus from minimum focus to infinity. Well, let me ask you this: Who cares? Who cares if it takes 1.2 vs 0.8 seconds to focus from a flower in macro range to a plane flying over the next village hill? What exactly is the real-life application here?

"Hang on!", you'll say, "This is not about focusing from macro to infinity; it's a more general sign of autofocus speed". OK, I'll answer. Let me ask you this: How many shots have you, personally, missed because the lens didn't focus fast enough from 7m/20ft to 9m/27ft? The slowest autofocus I can imagine would need 0.2 sec for that distance shift. Try it yourself if you don't believe me. In real-life applications, the change is practically instantaneous. Focusing from this doggy's tail onto his head is instantaneous:

You see, it's all about planning, vision, and anticipation. If you're afraid you might miss the shot of a lifetime because your lens focuses 0.4 sec slower than some other lens from macro to infinity, your photography has much bigger issues to tackle before focusing speed. Let's talk about focusing accuracy and reliability, shall we?

- Focus Speed
- Focus Accuracy
- Focus Reliability

Forget speed. It's not about that. They're all fast enough for real-life use. Worry instead about accuracy and reliability. By focusing accuracy, we mean whether the focus is really where you intended it to be - some camera/lens combos might give a false positive (i.e. they think focus is acquired and the shutter is tripped, while in reality it's not accurate).
Psst, hint: the faster a lens focuses, the greater the chance for a focus miss

By 'Focus Reliability', I refer to how certain the camera is that focus has been acquired. Some camera/lens combos might hunt a bit back and forth, before focus is positively identified. Of course, this causes a way longer delay than any focusing speed difference between lenses.

What is, therefore, more important? Focus accuracy/reliability or speed?

So, perhaps you want me to answer instead this question: "which is Nikon's most accurately focusing lens"? Sorry, there is no practical answer there. One reason is that it's not so much a lens or camera thing, but a lens/camera combo - the same lens might be a bit better with some cameras than others. But here are some tips to keep in mind:

  1. A smaller max aperture lens is more forgiving; generally speaking, it's only with f/2 or faster lenses that you will start noticing focusing errors
  2. Conversely, with low light, smaller max aperture lenses will hunt more, thus causing delays.
  3. AF or AF-S? No difference. Although AF-S lenses (having focus motors) are more silent, they are not necessarily faster. Often they can even be slower compared to an AF-D counterpart.
  4. Don't worry about speed; accuracy is your concern. To get the best possible accuracy/reliability from your camera/lens combo, you must also help it yourself. As I said here, you must learn to anticipate, regardless of what you are photographing, and you must learn to adapt to the shooting conditions. If you try to shoot snowflakes falling against a washed-out sky, you can't possibly blame your lens for not focusing properly.

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