Community Matters

Here’s an interesting observation. Under otherwise identical settings, autofocus feels much faster and reliable with a faster lens. Sounds embarrassingly obvious, right? But I think it’s something a lot of people do not take into account when buying lenses!

I did some experiments with the Nikon D300 (which has a very fast and accurate autofocus system - 51 points, different tracking modes, the works) and two great lenses of different maximum aperture (which is conversationally called “speed” for lenses). The f/2.8 lens not only allows you faster shutter speeds compared to an f/5.6, it also does a better job at quickly acquiring correct focus.

I used a Nikon AF-S 80-200mm f/2.8D and a Nikon AF-S DX VR 18-200mm f/3.5-5.6G IF-ED for comparison. Both are AF-S lenses with fast focus motor built into the lens. I used both at 200mm, focussing on the same sequence of subjects from about 10 feet away to several hundred feet away in otherwise identical circumstances. Lighting was reasonable but not extremely bright (ISO 200, f/2.8, 1/60s-1/125s, depending on the subject).

While I wasn’t able to use a stop watch to get absolute times (not enough hands, I guess), there’s a noticeable difference in the time it took the camera to be in focus. It’s hard to put a ratio to it, but my guess would be somewhere between 50% and 200% slower on the f/5.6, depending on the subject. There’s also a more than twice as high likelihood that the first attempt at focus is incorrect (the camera thinks it acquired focus, but it actually didn’t). I did 20 attempts switching between subjects close by and further away; I twice had to refocus with the f/2.8 lens, five times(!) with the f/5.6 lens.

The explanation seems easy enough. Focus acquisition is always at maximum aperture - so the f/2.8 lens gets four times the amount of light to the sensor than the f/5.6 lens. And that clearly makes a lot of difference.

Don’t get me wrong, the 18-200 is a wonderful all-round lens (I bought it explicitly to take with me when going to a park or doing something else in bright sunlight); I was simply curious to see if there was a downside to the reduced amount of light available when focusing. And it turns out there is.

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