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The iPhone 5S may be the "most forward thinking" smartphone ever, as Apple (AAPL) likes to call it. But you'll be forgiven if you can't tell the difference between it and its predecessor at first glance -- or even after playing around with it for a while.

That's because the changes Apple made between last year's iPhone 5 and the new phone are either subtle or under the hood. If you were hoping for something radically different from Apple, this isn't it. And if you were hoping that the company was going to address some of the long-standing complaints about the iPhone, such as its relatively small screen size or its insufficient battery life, you'll have to wait until next year -- or beyond.

That said, the iPhone 5S remains among the best smartphones on the market. But its most significant new features will take time to realize their full potential.

Take, for example, the new phone's fingerprint sensor, its hallmark new feature that is built into its home button. Once you train the sensor, which Apple calls Touch ID, to recognize your fingerprint, you can use it in place of a passcode to secure your phone.

It takes a couple of minutes to teach Touch ID to recognize a particular fingerprint. You can configure it to recognize up to 10.

In my tests, the sensor worked fairly well. Although it seemed to recognize my thumbprint only about half the time at first, it improved over time. But even when it failed, it wasn't too annoying -- the sensor takes only a second or two to read your print, so it doesn't take very long to retry, and I found it usually got it right the second time around.

The bigger problem with the Touch ID is that unlocking your phone is one of only two functions it has right now. The other is that you can also use it instead of a password to authorize your purchases in Apple's books, music and app stores.

It's not hard to imagine that Touch ID might be used in the future to verify purchases in stores other than Apple's or to log into your financial or social media accounts. But right now, that is just a potential a use of the Touch ID.

Another big change with the iPhone 5S is an internal one: Apple has replaced the 32-bit A6 processor that's in the iPhone 5 with a speedier 64-bit chip it dubs the A7.

At least in benchmark tests, the A7 is plenty fast. It blew away not only my iPhone 5, but also Motorola's brand new Moto X.

Diana Nguyn holds her new iPhone 5S during the opening day of new iPhone sales in Richmond, Va., Friday, Sept. 20, 2013. (AP Photo/Steve Helber)
Diana Nguyn holds her new iPhone 5S during the opening day of new iPhone sales in Richmond, Va., Friday, Sept. 20, 2013. (AP Photo/Steve Helber) (Steve Helber)

But in real-world usage, its speed wasn't readily apparent. The new iPhone didn't launch apps or load Web pages significantly faster than last year's model. In fact, the only time I really noticed a difference in speed was when I exported a movie I edited in iMovie to my camera roll, but that's not something that most people are going to do every day. However, it's likely that developers will take advantage of the faster chip with new, more sophisticated apps.

The iPhone 5S also has several cool new camera features, among them a slow-motion video mode and a burst shooting mode. The burst mode in particular is great, because it will keep shooting 10 pictures a second until you stop pressing the button or you hit 999 shots. I wish Apple had added more features to the burst mode -- such as the ability users have on Samsung's phones to combine photos to create one with the best facial expressions -- but the much greater number of burst shots you can make with the iPhone and the sheer speed of the burst mode are both pretty impressive.

The camera comes with a bigger sensor that is supposed to be better able to shoot in dark environments. I found the difference to be minor. Apple has also included with the iPhone 5S a new two-tone flash that's supposed to more naturally illuminate subjects. I found it worked moderately well; skin tones were a little truer to life than in flash pictures taken on the iPhone 5.

Like last year's model, the iPhone 5S looks sleek and feels solid. It retains the same four-inch screen that Apple introduced with last year's iPhone 5.

The new phone ships with iOS 7, Apple's revamped operating system, which offers a much cleaner, refreshed look and delivers some long-awaited features.

Overall, the iPhone 5S offers modest changes, although some may become more important in the future. If you have an iPhone 4S or earlier, it represents a great upgrade. If you already have an iPhone 5, though, I'd wait for the iPhone 6.

Contact Troy Wolverton at 408-840-4285 or twolverton@mercurynews.com. Follow him at www.mercurynews.com/troy-wolverton or Twitter.com/troywolv.

Troy's
RATING

8.0

(Out of 10)

What: Apple iPhone 5S
Likes: Bright, vivid screen; new Touch ID sensor quickly reads fingerprints to unlock device; new camera has superfast burst mode; slow-motion video mode and a two-tone flash that offers more true-to-life colors in flash photos; new processor
Dislikes: Battery life remains the same as last year's model; several new features, including the new processor, offer more theoretical than practical benefits; screen is small compared to Android rivals; Apple charges a pricey premium for extra storage space.
Specs: 64-bit, dual-core processor; 1.2 megapixel front and 8 megapixel rear cameras; four-inch, 1136 x 640 display.
Price: With a two-year contract, $200 for 16GB model, $300 for 32GB model, $400 for 64GB model
Web: www.apple.com